PLRsearch: Initial implementation and suites

Missing bits:
- Add up-to-date .rst document (in child Change).
- Prepare for releasing to PyPI.org (in child Change):
-- Either copy dependencies from MLRsearch, or list in requirements.
-- Perhaps move common dependencies to separate package
   for both search to depend on.
-- All the other metadata stuff.

Jira: CSIT-1276

Change-Id: I277efdb63dbb90b30e11f4e30a82e2130ac8efc3
Signed-off-by: Vratko Polak <vrpolak@cisco.com>

8 files changed, 1202 insertions, 3 deletions

diff --git a/resources/libraries/python/MLRsearch/__init__.py b/resources/libraries/python/MLRsearch/__init__.py
index 4cc8158397..70c713eaa0 100644
--- a/resources/libraries/python/MLRsearch/__init__.py
+++ b/resources/libraries/python/MLRsearch/__init__.py
@@ -12,5 +12,5 @@
 # limitations under the License.
 
 """
-__init__ file for Python package "MLRsearch"
+__init__ file for Python package "MLRsearch".
 """
diff --git a/resources/libraries/python/PLRsearch/Integrator.py b/resources/libraries/python/PLRsearch/Integrator.py
new file mode 100644
index 0000000000..f8846fdf62
--- /dev/null
+++ b/resources/libraries/python/PLRsearch/Integrator.py
@@ -0,0 +1,378 @@
+# Copyright (c) 2019 Cisco and/or its affiliates.
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at:
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Module for numerical integration, tightly coupled to PLRsearch algorithm.
+
+See log_plus for an explanation why None acts as a special case "float" number.
+
+TODO: Separate optimizations specific to PLRsearch
+      and distribute as a standalone package so other projects may reuse.
+"""
+
+import math
+import traceback
+
+import dill
+import numpy
+
+# TODO: The preferred way to consume this code is via a pip package.
+# If your project copies code instead, make sure your pylint check does not
+# require these imports to be absolute and descending from your superpackage.
+from log_plus import log_plus
+
+
+def try_estimate_nd(communication_pipe, scale_coeff=10.0, trace_enabled=False):
+    """Call estimate_nd but catch any exception and send traceback."""
+    try:
+        return estimate_nd(communication_pipe, scale_coeff, trace_enabled)
+    except BaseException:
+        # Any subclass could have caused estimate_nd to stop before sending,
+        # so we have to catch them all.
+        traceback_string = traceback.format_exc()
+        communication_pipe.send(traceback_string)
+        # After sendig, re-raise, so usages other than "one process per call"
+        # keep behaving correctly.
+        raise
+
+
+# FIXME: Pylint reports multiple complexity violations.
+# Refactor the code, using less (but structured) variables
+# and function calls for (relatively) loosly coupled code blocks.
+def estimate_nd(communication_pipe, scale_coeff=10.0, trace_enabled=False):
+    """Use Bayesian inference from control queue, put result to result queue.
+
+    TODO: Use a logging framework that works in a user friendly way.
+    (Note that multiprocessing_logging does not work well with robot
+    and robotbackgroundlogger only works for threads, not processes.
+    Or, wait for https://github.com/robotframework/robotframework/pull/2182
+    Anyway, the current implementation with trace_enabled looks ugly.)
+
+    The result is average and standard deviation for posterior distribution
+    of a single dependent (positive scalar) value.
+    The prior is assumed to be uniform on (-1, 1) for every parameter.
+    Number of parameters and the function for computing
+    the dependent value and likelihood both come from input.
+
+    The likelihood is assumed to be extremely uneven (but never zero),
+    so the function should return the logarithm of the likelihood.
+    The integration method is basically a Monte Carlo
+    (TODO: Add links to notions used here.),
+    but importance sampling is used in order to focus
+    on the part of parameter space with (relatively) non-negligible likelihood.
+
+    Multivariate Gauss distribution is used for focusing,
+    so only unimodal posterior distributions are handled correctly.
+    Initial samples are mostly used for shaping (and shifting)
+    the Gaussian distribution, later samples will probably dominate.
+    Thus, initially the algorithm behavior resembles more "find the maximum",
+    as opposed to "reliably integrate". As for later iterations of PLRsearch,
+    it is assumed that the distribution position does not change rapidly;
+    thus integration algorithm returns also the distribution data,
+    to be used as initial focus in next iteration.
+
+    After some number of samples (depends on dimension),
+    the algorithm starts tracking few most likely samples to base the Gaussian
+    distribution around, mixing with estimates from observed samples.
+    The idea is that even when (usually) one of the samples dominates,
+    first few are treated as if equally likely, to get reasonable focus.
+    During the "find the maximum" phase, this set of best samples
+    frequently takes a wrong shape (compared to observed samples
+    in equilibrium). Therefore scale_coeff argument is left for humans to tweak,
+    so the convergence is reliable and quick.
+    Any data (other than correctly weighted samples) used to keep
+    distribution shape reasonable is called "bias", regardles of
+    whether it comes from input hint, or from tracking top samples.
+
+    Until the distribution locates itself roughly around
+    the maximum likeligood point, the integration results are probably wrong.
+    That means some minimal time is needed for the result to become reliable.
+    The reported standard distribution attempts to signal inconsistence
+    (when one sample has dominating weight compared to the rest of samples),
+    but some human supervision is strongly encouraged.
+
+    To facilitate running in worker processes, arguments and results
+    are communicated via pipe. The computation does not start
+    until arguments appear in the pipe, the computation stops
+    when another item (stop object) is detected in the pipe
+    (and result is put to pipe).
+
+    TODO: Create classes for arguments and results,
+          so their fields are documented (and code perhaps more readable).
+
+    Input/argument object (received from pipe)
+    is a 4-tuple of the following fields:
+    - dimension: Integer, number of parameters to consider.
+    - dilled_function: Function (serialized using dill), which:
+    - - Takes the dimension number of float parameters from (-1, 1).
+    - - Returns float 2-tuple of dependent value and parameter log-likelihood.
+    - param_hint_avg: Dimension-tuple of floats to start searching around.
+    - param_hint_cov: Covariance matrix defining initial focus shape.
+
+    Output/result object (sent to pipe queue)
+    is a 6-tuple of the following fields:
+    - value_avg: Float estimate of posterior average dependent value.
+    - value_stdev: Float estimate of posterior standard deviation of the value.
+    - param_importance_avg: Float tuple, center of Gaussian to use next.
+    - param_importance_cov: Float covariance matrix of the Gaussian to use next.
+    - debug_list: List of debug strings to log at main process.
+    - trace_list: List of trace strings to pass to main process if enabled.
+    Trace strings are very verbose, it is not recommended to enable them.
+    In they are not enabled, trace_list will be empty.
+    It is recommended to edit some lines manually to debug_list if needed.
+
+    :param communication_pipe: Pipe to comunicate with boss process.
+    :param scale_coeff: Float number to tweak convergence speed with.
+    :param trace_enabled: Whether trace list should be populated at all.
+        Default: False
+    :type communication_pipe: multiprocessing.Connection (or compatible)
+    :type scale_coeff: float
+    :type trace_enabled: boolean
+    :raises OverflowError: If one sample dominates the rest too much.
+        Or if value_logweight_function does not handle
+        some part of parameter space carefully enough.
+    :raises numpy.linalg.LinAlgError: If the focus shape gets singular
+        (due to rounding errors). Try changing scale_coeff.
+    """
+
+    # Block until input object appears.
+    dimension, dilled_function, param_hint_avg, param_hint_cov = (
+        communication_pipe.recv())
+    debug_list = list()
+    trace_list = list()
+    def trace(name, value):
+        """
+        Add a variable (name and value) to trace list (if enabled).
+
+        This is a closure (not a pure function),
+        as it accesses trace_list and trace_enabled
+        (without any of them being an explicit argument).
+
+        :param name: Any string identifying the value.
+        :param value: Any object to log repr of.
+        :type name: str
+        :type value: object
+        """
+        if trace_enabled:
+            trace_list.append(name + " " + repr(value))
+    value_logweight_function = dill.loads(dilled_function)
+    len_top = (dimension + 2) * (dimension + 1) / 2
+    top_weight_param = list()
+    samples = 0
+    log_sum_weight = None
+    # Importance sampling produces samples of higher weight (important)
+    # more frequently, and corrects that by adding weight bonus
+    # for the less frequently (unimportant) samples.
+    # But "corrected_weight" is too close to "weight" to be readable,
+    # so "importance" is used instead, even if it runs contrary to what
+    # important region is.
+    log_sum_importance = None
+    log_importance_best = None
+    value_avg = 0.0
+    # 1x1 dimensional covariance matrix is just variance.
+    # As variance is never negative, we can track logarithm.
+    value_log_variance = None
+    # Here "secondary" means "excluding the weightest sample".
+    log_secondary_sum_importance = None
+    value_secondary_avg = 0.0
+    value_log_secondary_variance = None
+    param_sampled_avg = [0.0 for first in range(dimension)]
+    # TODO: Examine whether we can gain speed by tracking triangle only.
+    # Covariance matrix can contain negative element (off-diagonal),
+    # so no logarithm here. This can lead to zeroes on diagonal,
+    # but we have biasing to make sure it does not hurt much.
+    param_sampled_cov = [[0.0 for first in range(dimension)]
+                         for second in range(dimension)]
+    # The next two variables do NOT need to be initialized here,
+    # but pylint is not a mathematician enough to understand that.
+    param_top_avg = [0.0 for first in range(dimension)]
+    param_top_cov = [[0.0 for first in range(dimension)]
+                     for second in range(dimension)]
+    if not (param_hint_avg and param_hint_cov):
+        # First call has Nones instead of useful hints.
+        param_hint_avg = [0.0 for first in range(dimension)]
+        param_hint_cov = [
+            [1.0 if first == second else 0.0 for first in range(dimension)]
+            for second in range(dimension)]
+    while not communication_pipe.poll():
+        # Compute focus data.
+        if len(top_weight_param) < len_top:
+            # Not enough samples for reasonable top, use hint bias.
+            param_focus_avg = param_hint_avg
+            param_focus_cov = param_hint_cov
+        else:
+            # We have both top samples and overall samples.
+            # Mix them according to how much the weightest sample dominates.
+            log_top_weight = top_weight_param[0][0]
+            log_weight_norm = log_plus(log_sum_weight, log_top_weight)
+            top_ratio = math.exp(log_top_weight - log_weight_norm)
+            sampled_ratio = math.exp(log_sum_weight - log_weight_norm)
+            trace("log_top_weight", log_top_weight)
+            trace("log_sum_weight", log_sum_weight)
+            trace("top_ratio", top_ratio)
+            trace("sampled_ratio", sampled_ratio)
+            param_focus_avg = [
+                sampled_ratio * param_sampled_avg[first]
+                + top_ratio * param_top_avg[first]
+                for first in range(dimension)]
+            param_focus_cov = [[
+                scale_coeff * (
+                    sampled_ratio * param_sampled_cov[first][second]
+                    + top_ratio * param_top_cov[first][second])
+                for first in range(dimension)] for second in range(dimension)]
+        trace("param_focus_avg", param_focus_avg)
+        trace("param_focus_cov", param_focus_cov)
+        # Generate next sample.
+        while 1:
+            # TODO: Inform pylint that correct version of numpy is available.
+            sample_point = numpy.random.multivariate_normal(
+                param_focus_avg, param_focus_cov, 1)[0]
+            # Multivariate Gauss can fall outside (-1, 1) interval
+            for first in range(dimension):
+                sample_coordinate = sample_point[first]
+                if sample_coordinate <= -1.0 or sample_coordinate >= 1.0:
+                    break
+            else:  # These two breaks implement "level two continue".
+                break
+        trace("sample_point", sample_point)
+        samples += 1
+        value, log_weight = value_logweight_function(*sample_point)
+        trace("value", value)
+        trace("log_weight", log_weight)
+        # Update bias related statistics.
+        log_sum_weight = log_plus(log_sum_weight, log_weight)
+        if len(top_weight_param) < len_top:
+            top_weight_param.append((log_weight, sample_point))
+        # Hack: top_weight_param[-1] is either the smallest,
+        # or the just appended to len_top-1 item list.
+        if (len(top_weight_param) >= len_top
+                and log_weight >= top_weight_param[-1][0]):
+            top_weight_param = top_weight_param[:-1]
+            top_weight_param.append((log_weight, sample_point))
+            top_weight_param.sort(key=lambda item: -item[0])
+            trace("top_weight_param", top_weight_param)
+            # top_weight_param has changed, recompute biases.
+            param_top_avg = top_weight_param[0][1]
+            param_top_cov = [[0.0 for first in range(dimension)]
+                             for second in range(dimension)]
+            top_item_count = 1
+            for _, near_top_param in top_weight_param[1:]:
+                top_item_count += 1
+                next_item_ratio = 1.0 / top_item_count
+                previous_items_ratio = 1.0 - next_item_ratio
+                param_shift = [
+                    near_top_param[first] - param_top_avg[first]
+                    for first in range(dimension)]
+                # Do not move center from the weightest sample.
+                for second in range(dimension):
+                    for first in range(dimension):
+                        param_top_cov[first][second] += (
+                            param_shift[first] * param_shift[second]
+                            * next_item_ratio)
+                        param_top_cov[first][second] *= previous_items_ratio
+            trace("param_top_avg", param_top_avg)
+            trace("param_top_cov", param_top_cov)
+        # The code above looked at weight (not importance).
+        # The code below looks at importance (not weight).
+        param_shift = [sample_point[first] - param_focus_avg[first]
+                       for first in range(dimension)]
+        rarity_gradient = numpy.linalg.solve(param_focus_cov, param_shift)
+        rarity_step = numpy.vdot(param_shift, rarity_gradient)
+        log_rarity = rarity_step / 2.0
+        trace("log_rarity", log_rarity)
+        trace("samples", samples)
+        log_importance = log_weight + log_rarity
+        trace("log_importance", log_importance)
+        # Update sampled statistics.
+        old_log_sum_importance = log_sum_importance
+        log_sum_importance = log_plus(old_log_sum_importance, log_importance)
+        trace("new log_sum_weight", log_sum_weight)
+        trace("log_sum_importance", log_sum_importance)
+        if old_log_sum_importance is None:
+            param_sampled_avg = list(sample_point)
+            value_avg = value
+            # Other value related quantities stay None.
+            continue
+        previous_samples_ratio = math.exp(
+            old_log_sum_importance - log_sum_importance)
+        new_sample_ratio = math.exp(log_importance - log_sum_importance)
+        param_shift = [sample_point[first] - param_sampled_avg[first]
+                       for first in range(dimension)]
+        value_shift = value - value_avg
+        for first in range(dimension):
+            param_sampled_avg[first] += param_shift[first] * new_sample_ratio
+        old_value_avg = value_avg
+        value_avg += value_shift * new_sample_ratio
+        value_absolute_shift = abs(value_shift)
+        for second in range(dimension):
+            for first in range(dimension):
+                param_sampled_cov[first][second] += (
+                    param_shift[first] * param_shift[second] * new_sample_ratio)
+                param_sampled_cov[first][second] *= previous_samples_ratio
+        trace("param_sampled_avg", param_sampled_avg)
+        trace("param_sampled_cov", param_sampled_cov)
+        update_secondary_stats = True
+        if log_importance_best is None or log_importance > log_importance_best:
+            log_importance_best = log_importance
+            log_secondary_sum_importance = old_log_sum_importance
+            value_secondary_avg = old_value_avg
+            value_log_secondary_variance = value_log_variance
+            update_secondary_stats = False
+            # TODO: Update all primary quantities before secondary ones.
+            # (As opposed to current hybrid code.)
+        if value_absolute_shift > 0.0:
+            value_log_variance = log_plus(
+                value_log_variance, 2 * math.log(value_absolute_shift)
+                + log_importance - log_sum_importance)
+        if value_log_variance is not None:
+            value_log_variance -= log_sum_importance - old_log_sum_importance
+        if not update_secondary_stats:
+            continue
+        # TODO: Pylint says the following variable name is bad.
+        # Make sure the refactor uses shorter names.
+        old_log_secondary_sum_importance = log_secondary_sum_importance
+        log_secondary_sum_importance = log_plus(
+            old_log_secondary_sum_importance, log_importance)
+        if old_log_secondary_sum_importance is None:
+            value_secondary_avg = value
+            continue
+        new_sample_secondary_ratio = math.exp(
+            log_importance - log_secondary_sum_importance)
+        # TODO: No would-be variable named old_value_secondary_avg
+        # appears in subsequent computations. Probably means there is a bug.
+        value_secondary_shift = value - value_secondary_avg
+        value_secondary_absolute_shift = abs(value_secondary_shift)
+        value_secondary_avg += (
+            value_secondary_shift * new_sample_secondary_ratio)
+        if value_secondary_absolute_shift > 0.0:
+            value_log_secondary_variance = log_plus(
+                value_log_secondary_variance, (
+                    2 * math.log(value_secondary_absolute_shift)
+                    + log_importance - log_secondary_sum_importance))
+        if value_log_secondary_variance is not None:
+            value_log_secondary_variance -= (
+                log_secondary_sum_importance - old_log_secondary_sum_importance)
+    debug_list.append("integrator used " + str(samples) + " samples")
+    debug_list.append(
+        "value_avg " + str(value_avg)
+        + " param_sampled_avg " + repr(param_sampled_avg)
+        + " param_sampled_cov " + repr(param_sampled_cov)
+        + " value_log_variance " + str(value_log_variance)
+        + " value_log_secondary_variance " + str(value_log_secondary_variance))
+    value_stdev = math.exp(
+        (2 * value_log_variance - value_log_secondary_variance) / 2.0)
+    debug_list.append("top_weight_param[0] " + repr(top_weight_param[0]))
+    # Intentionally returning param_focus_avg and param_focus_cov,
+    # instead of possibly hyper-focused bias or sampled.
+    communication_pipe.send(
+        (value_avg, value_stdev, param_focus_avg, param_focus_cov, debug_list,
+         trace_list))
diff --git a/resources/libraries/python/PLRsearch/PLRsearch.py b/resources/libraries/python/PLRsearch/PLRsearch.py
new file mode 100644
index 0000000000..f1b3f740c2
--- /dev/null
+++ b/resources/libraries/python/PLRsearch/PLRsearch.py
@@ -0,0 +1,596 @@
+# Copyright (c) 2019 Cisco and/or its affiliates.
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at:
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Module holding PLRsearch class."""
+
+import logging
+import math
+import multiprocessing
+import time
+
+import dill
+# TODO: Inform pylint about scipy (of correct version) being available.
+from scipy.special import erfcx, erfc
+
+# TODO: The preferred way to consume this code is via a pip package.
+# If your project copies code instead, make sure your pylint check does not
+# require these imports to be absolute and descending from your superpackage.
+import Integrator
+from log_plus import log_plus, log_minus
+
+
+class PLRsearch(object):
+    """A class to encapsulate data relevant for search method.
+
+    The context is performance testing of packet processing systems.
+    The system, when being offered a steady stream of packets,
+    can process some of them successfully, other are considered "lost".
+
+    See docstring of the search method for algorithm description.
+
+    Two constants are stored as class fields for speed.
+
+    Method othed than search (and than __init__)
+    are just internal code structure.
+    TODO: Those method names should start with underscore then.
+
+    TODO: Figure out how to replace #print with logging
+    without slowing down too much.
+    """
+
+    xerfcx_limit = math.pow(math.acos(0), -0.5)
+    log_xerfcx_10 = math.log(xerfcx_limit - math.exp(10) * erfcx(math.exp(10)))
+
+    def __init__(
+            self, measurer, trial_duration_per_trial, packet_loss_ratio_target,
+            trial_number_offset=0, timeout=60.0):
+        """Store rate measurer and additional parameters.
+
+        Also declare packet_loss_per_second_target field (float),
+        to be initialized later.
+
+        TODO: Copy AbstractMeasurer from MLRsearch.
+
+        :param measurer: The measurer to call when searching.
+        :param trial_duration_per_trial: Each trial has larger duration
+            than the previous trial. This is the increment, in seconds.
+        :param packet_loss_ratio_target: The algorithm tries to estimate
+            the offered load leading to this ratio on average.
+            Trial ratio is number of packets lost divided by packets offered.
+        :param trial_number_offset: The "first" trial number will be 1+this.
+            Use this to ensure first iterations have enough time to compute
+            reasonable estimates for later trials to use.
+        :param timeout: The search ends if it lasts more than this many seconds.
+        :type measurer: MLRsearch.AbstractMeasurer
+        :type trial_duration_per_trial: float
+        :type packet_loss_ratio_target: float
+        :type trial_number_offset: int
+        :type timeout: float
+        """
+        self.measurer = measurer
+        self.trial_duration_per_trial = trial_duration_per_trial
+        self.packet_loss_ratio_target = packet_loss_ratio_target
+        self.trial_number_offset = trial_number_offset
+        self.timeout = timeout
+
+    def search(self, min_rate, max_rate):
+        """Perform the search, return average and stdev for throughput estimate.
+
+        Considering measurer and packet_loss_ratio_target (see __init__),
+        find such an offered load (called critical load) that is expected
+        to hit the target loss ratio in the limit of very long trial duration.
+        As the system is probabilistic (and test duration is finite),
+        the critical ratio is only estimated.
+        Return the average and standard deviation of the estimate.
+
+        In principle, this algorithm performs trial measurements,
+        each with varied offered load (which is constant during the trial).
+        During each measurement, Bayesian inference is performed
+        on all the measurement results so far.
+        When timeout is up, the last estimate is returned,
+        else another trial is performed.
+
+        It is assumed that the system under test, even though not deterministic,
+        still follows the rule of large numbers. In another words,
+        any growing set of measurements at a particular offered load
+        will converge towards unique (for the given load) packet loss ratio.
+        This means there is a deterministic (but unknown) function
+        mapping the offered load to average loss ratio.
+        This function is called loss function.
+        This also assumes the average loss ratio
+        does not depend on trial duration.
+
+        The actual probability distribution of loss counts, achieving
+        the average ratio on trials of various duration
+        can be complicated (and can depend on offered load), but simply assuming
+        Poisson distribution will make the algorithm converge.
+        Binomial distribution would be more precise,
+        but Poisson is more practical, as it effectively gives
+        less information content to high ratio results.
+
+        Even when applying other assumptions on the loss function
+        (increasing function, limit zero ratio when load goes to zero,
+        global upper limit on rate of packets processed), there are still
+        too many different shapes of possible loss functions,
+        which makes full Bayesian reasoning intractable.
+
+        This implementation radically simplifies things by examining
+        only two shapes, each with finitely many (in this case just two)
+        parameters. In other words, two fitting functions
+        (each with two parameters and one argument).
+        When restricting model space to one of the two fitting functions,
+        the Bayesian inference becomes tractable (even though it needs
+        numerical integration from Integrator class).
+
+        The first measurement is done at min_rate to help with convergence
+        if max_rate measurements give loss below target.
+        FIXME: Fix overflow error and really use min_rate.
+        The second measurement is done at max_rate, next few measurements
+        have offered load of previous load minus excess loss ratio.
+        This simple rule is found to be good when offered loads
+        so far are way above the critical rate. After few measurements,
+        inference from fitting functions converges faster that the initial
+        "optimistic" procedure.
+
+        Offered loads close to (limiting) critical rate are the most useful,
+        as linear approximation of the fitting function
+        becomes good enough there (thus reducing the impact
+        of the overall shape of fitting function).
+        After several trials, usually one of the fitting functions
+        has better predictions than the other one, but the algorithm
+        does not track that. Simply, it uses the estimate average,
+        alternating between the functions.
+
+        The returned average and stdev is a combination of the two fitting
+        estimates.
+
+        TODO: If measurement at max_rate is already below target loss rate,
+        the algorithm always measures at max_rate, and likelihood
+        no longer has single maximum, leading to "random" estimates.
+        Find a way to avoid that.
+
+        :param min_rate: Avoid measuring at offered loads below this,
+            in packets per second.
+        :param max_rate: Avoid measuring at offered loads above this,
+            in packets per second.
+        :type min_rate: float
+        :type max_rate: float
+        :returns: Average and stdev of critical load estimate.
+        :rtype: 2-tuple of floats
+        """
+        stop_time = time.time() + self.timeout
+        min_rate = float(min_rate)
+        max_rate = float(max_rate)
+        trial_result_list = list()
+        trial_number = self.trial_number_offset
+        integrator_data = (None, None, None, None)
+        message = "Trial %(number)r computed avg %(avg)r stdev %(stdev)r"
+        message += " stretch %(a1)r erf %(a2)r difference %(diff)r"
+        transmit_rate = (min_rate + max_rate) / 2.0
+        while 1:
+            trial_number += 1
+            trial_duration = trial_number * self.trial_duration_per_trial
+            results = self.measure_and_compute(
+                trial_duration, transmit_rate, trial_result_list, max_rate,
+                integrator_data)
+            measurement, average, stdev, avg1, avg2, integrator_data = results
+            logging.info(message, {
+                "number": trial_number, "avg": average, "stdev": stdev,
+                "a1": avg1, "a2": avg2, "diff": avg2 - avg1})
+            if stop_time <= time.time():
+                return average, stdev
+            trial_result_list.append(measurement)
+            if (trial_number - self.trial_number_offset) <= 1:
+                next_load = max_rate
+            elif (trial_number - self.trial_number_offset) <= 3:
+                next_load = (measurement.receive_rate
+                             / (1.0 - self.packet_loss_ratio_target))
+            else:
+                next_load = avg1 if trial_number % 2 else avg2
+            transmit_rate = min(max_rate, max(min_rate, next_load))
+
+    @staticmethod
+    def lfit_stretch(load, mrr, spread):
+        """Stretch-based fitting function.
+
+        Return the logarithm of average packet loss per second
+        when the load (argument) is offered to a system with given
+        mrr and spread (parameters).
+        Stretch function is 1/(1+Exp[-x]). The average itself is definite
+        integral from zero to load, of shifted and x-scaled stretch function.
+        As the integrator is sensitive to discontinuities,
+        and it calls this function at large areas of parameter space,
+        the implementation has to avoid rounding errors, overflows,
+        and correctly approximate underflows.
+
+        TODO: Explain how the high-level description
+        has been converted into an implementation full of ifs.
+
+        :param load: Offered load (positive), in packets per second.
+        :param mrr: Parameter of this fitting function, equal to limiting
+            (positive) average number of packets received (as opposed to lost)
+            when offered load is many spreads more than mrr.
+        :param spread: The x-scaling parameter (positive). No nice semantics,
+            roughly corresponds to size of "tail" for loads below mrr.
+        :type load: float
+        :type mrr: float
+        :type spread: float
+        :returns: Logarithm of average number of packets lost per second.
+        :rtype: float
+        """
+        # TODO: chi is from https://en.wikipedia.org/wiki/Nondimensionalization
+        chi = (load - mrr) / spread
+        chi0 = -mrr / spread
+#        print "load", load, "mrr", mrr, "spread", spread, "chi", chi
+        if chi > 0:
+            log_lps = math.log(
+                load - mrr + (log_plus(0, -chi) - log_plus(0, chi0)) * spread)
+#            print "big loss direct log_lps", log_lps
+        else:
+            approx = (math.exp(chi) - math.exp(2 * chi) / 2) * spread
+            if approx == 0.0:
+                log_lps = chi
+#                print "small loss crude log_lps", log_lps
+                return log_lps
+            third = math.exp(3 * chi) / 3 * spread
+            if approx + third != approx + 2 * third:
+                log_lps = math.log(
+                    (log_plus(0, chi) - log_plus(0, chi0)) * spread)
+#                print "small loss direct log_lps", log_lps
+            else:
+                log_lps = math.log(
+                    approx - (math.exp(chi0) - math.exp(2 * chi0)) * spread)
+#                print "small loss approx log_lps", log_lps
+        return log_lps
+
+    @staticmethod
+    def lfit_erf(load, mrr, spread):
+        """Erf-based fitting function.
+
+        Return the logarithm of average packet loss per second
+        when the load (argument) is offered to a system with given
+        mrr and spread (parameters).
+        Erf function is Primitive function to normal distribution density.
+        The average itself is definite integral from zero to load,
+        of shifted and x-scaled erf function.
+        As the integrator is sensitive to discontinuities,
+        and it calls this function at large areas of parameter space,
+        the implementation has to avoid rounding errors, overflows,
+        and correctly approximate underflows.
+
+        TODO: Explain how the high-level description
+        has been converted into an implementation full of ifs.
+
+        :param load: Offered load (positive), in packets per second.
+        :param mrr: Parameter of this fitting function, equal to limiting
+            (positive) average number of packets received (as opposed to lost)
+            when offered load is many spreads more than mrr.
+        :param spread: The x-scaling parameter (positive). No nice semantics,
+            roughly corresponds to size of "tail" for loads below mrr.
+        :type load: float
+        :type mrr: float
+        :type spread: float
+        :returns: Logarithm of average number of packets lost per second.
+        :rtype: float
+        """
+        # Beware, this chi has the sign opposite to the stretch function chi.
+        # TODO: The stretch sign is just to have less minuses. Worth changing?
+        chi = (mrr - load) / spread
+        chi0 = mrr / spread
+#        print "load", load, "mrr", mrr, "spread", spread,
+#        print "chi", chi, "chi0", chi0
+        if chi >= -1.0:
+#            print "positive, b ~> m"
+            if chi > math.exp(10):
+                first = PLRsearch.log_xerfcx_10 + 2 * (math.log(chi) - 10)
+#                print "approximated"
+            else:
+                first = math.log(PLRsearch.xerfcx_limit - chi * erfcx(chi))
+#                print "exact"
+            first -= chi * chi
+            second = math.log(PLRsearch.xerfcx_limit - chi * erfcx(chi0))
+            second -= chi0 * chi0
+            intermediate = log_minus(first, second)
+#            print "first", first, "second", second,
+#            print "intermediate", intermediate
+        else:
+#            print "negative, b ~< m"
+            exp_first = PLRsearch.xerfcx_limit + chi * erfcx(-chi)
+            exp_first *= math.exp(-chi * chi)
+            exp_first -= 2 * chi
+            # TODO: Why has the following line chi there (as opposed to chi0)?
+            # In general the functions would be more readable if they explicitly
+            #     return math.log(func(chi) - func(chi0))
+            # for some function "func", at least for some branches.
+            second = math.log(PLRsearch.xerfcx_limit - chi * erfcx(chi0))
+            second -= chi0 * chi0
+            intermediate = math.log(exp_first - math.exp(second))
+#            print "exp_first", exp_first, "second", second,
+#            print "intermediate", intermediate
+        result = intermediate + math.log(spread) - math.log(erfc(-chi0))
+#        print "lfit erf result", result
+        return result
+
+    @staticmethod
+    def find_critical_rate(lfit_func, log_lps_target, mrr, spread):
+        """Given lps target and parameters, return the achieving offered load.
+
+        This is basically an inverse function to lfit_func
+        when parameters are fixed.
+        Instead of implementing effective implementation
+        of the inverse function, this implementation uses
+        brute force binary search.
+        The search starts at an arbitrary offered load,
+        uses exponential search to find the other bound
+        and then bisecting until the load is found (or interval
+        becoming degenerate).
+
+        TODO: Use at least some method with faster convergence.
+
+        :param lfit_func: Fitting function, typically lfit_spread or lfit_erf.
+        :param log_lps_target: Fitting function should return this
+            at the returned load and parameters.
+        :param mrr: The mrr parameter for the fitting function.
+        :param spread: The spread parameter for the fittinmg function.
+        :type lfit_func: Function from 3 floats to float.
+        :type log_lps_target: float
+        :type mrr: float
+        :type spread: float
+        :returns: Load [pps] which achieves the target with given parameters.
+        :rtype: float
+        """
+        # TODO: Should we make the initial rate configurable?
+        rate = 10000000.0
+        log_loss = lfit_func(rate, mrr, spread)
+        if log_loss == log_lps_target:
+            return rate
+        # Exponential search part.
+        if log_loss > log_lps_target:
+            rate_hi = rate
+            while 1:
+                rate_lo = rate_hi / 2.0
+                log_loss = lfit_func(rate_lo, mrr, spread)
+                if log_loss > log_lps_target:
+                    rate_hi = rate_lo
+                    continue
+                if log_loss == log_lps_target:
+                    return rate_lo
+                break
+        else:
+            rate_lo = rate
+            while 1:
+                rate_hi = rate_lo * 2.0
+                log_loss = lfit_func(rate_hi, mrr, spread)
+                if log_loss < log_lps_target:
+                    rate_lo = rate_hi
+                    continue
+                if log_loss == log_lps_target:
+                    return rate_hi
+                break
+        # Binary search part.
+        while rate_hi != rate_lo:
+            rate = (rate_hi + rate_lo) / 2.0
+            log_loss = lfit_func(rate, mrr, spread)
+            if rate == rate_hi or rate == rate_lo or log_loss == log_lps_target:
+#                print "found", rate
+                return rate
+            if log_loss > log_lps_target:
+                rate_hi = rate
+            else:
+                rate_lo = rate
+
+    @staticmethod
+    def log_weight(lfit_func, trial_result_list, mrr, spread):
+        """Return log of weight of trial results by the function and parameters.
+
+        Integrator assumes uniform distribution, but over different parameters.
+        Weight and likelihood are used interchangeably here anyway.
+
+        Each trial has an offered load, a duration and a loss count.
+        Fitting function is used to compute the average loss per second.
+        Poisson distribution (with average loss per trial) is used
+        to get likelihood of one trial result, the overal likelihood is product.
+        As likelihoods can be extremely small, logarithms are tracked instead.
+
+        TODO: Copy ReceiveRateMeasurement from MLRsearch.
+
+        :param lfit_func: Fitting function, typically lfit_spread or lfit_erf.
+        :param result_list: List of trial measurement results.
+        :param mrr: The mrr parameter for the fitting function.
+        :param spread: The spread parameter for the fittinmg function.
+        :type lfit_func: Function from 3 floats to float.
+        :type result_list: list of MLRsearch.ReceiveRateMeasurement
+        :type mrr: float
+        :type spread: float
+        :returns: Logarithm of result weight for given function and parameters.
+        :rtype: float
+        """
+        log_likelihood = 0.0
+        for result in trial_result_list:
+#            print "DEBUG for tr", result.target_tr,
+#            print "lc", result.loss_count, "d", result.duration
+            log_avg_loss_per_second = lfit_func(result.target_tr, mrr, spread)
+            log_avg_loss_per_trial = (
+                log_avg_loss_per_second + math.log(result.duration))
+            # Poisson probability computation works nice for logarithms.
+            log_trial_likelihood = (
+                result.loss_count * log_avg_loss_per_trial
+                - math.exp(log_avg_loss_per_trial))
+            log_trial_likelihood -= math.lgamma(1 + result.loss_count)
+            log_likelihood += log_trial_likelihood
+#            print "log_avg_loss_per_second", log_avg_loss_per_second
+#            print "log_avg_loss_per_trial", log_avg_loss_per_trial
+#            print "avg_loss_per_trial", math.exp(log_avg_loss_per_trial)
+#            print "log_trial_likelihood", log_trial_likelihood
+#            print "log_likelihood", log_likelihood
+#        print "returning log_likelihood", log_likelihood
+        return log_likelihood
+
+    # FIXME: Refactor (somehow) so pylint stops complaining about
+    # too many local variables.
+    # Some work already done in subsequent changes.
+    def measure_and_compute(
+            self, trial_duration, transmit_rate,
+            trial_result_list, max_rate, integrator_data):
+        """Perform both measurement and computation at once.
+
+        High level steps: Prepare and launch computation worker processes,
+        perform the measurement, stop computation and combine results.
+
+        Integrator needs a specific function to process (-1, 1) parameters.
+        As our fitting functions use dimensional parameters,
+        so a transformation is performed, resulting in a specific prior
+        distribution over the dimensional parameters.
+        Maximal rate (line rate) is needed for that transformation.
+
+        Two fitting functions are used, computation is started
+        on temporary worker process per fitting function. After the measurement,
+        Average and stdev of the critical rate (not log) of each worker
+        are combined and returned. Raw averages are also returned,
+        offered load for next iteration is chosen from them.
+        The idea is that one fitting function might be fitting much better,
+        measurements at its avg are best for relevant results (for both),
+        but we do not know which fitting function it is.
+
+        TODO: Define class for integrator data, so that fields are documented.
+        TODO: Define class for result object, so that fields are documented.
+        TODO: More processes to the slower fitting function?
+        TODO: Re-use processes, instead creating on each computation.
+        TODO: As only one result is needed fresh, figure out a way
+        how to keep the other worker running. This will alow shorter
+        duration per trial. Special handling at first and last measurement
+        will be needed (to properly initialize and to properly combine results).
+
+        :param trial_duration: Length of the measurement in seconds.
+        :param transmit_rate: Offered load in packets per second.
+        :param trial_result_list: Results of previous measurements.
+        :param max_rate: Theoretic maximum of possible ofered load.
+        :param integrator_data: Hints to speed up the numeric computation.
+        :type trial_duration: float
+        :type transmit_rate: float
+        :type trial_result_list: list of MLRsearch.ReceiveRateMeasurement
+        :type max_rate: float
+        :type integrator_data: 4-tuple of gaussian positions and covariances
+        :returns: Measurement and computation results.
+        :rtype: 6-tuple: ReceiveRateMeasurement, floats, integrator data.
+        """
+        # Preparation phase.
+        dimension = 2
+        stretch_bias_avg, erf_bias_avg, stretch_bias_cov, erf_bias_cov = (
+            integrator_data)
+        packet_loss_per_second_target = (
+            transmit_rate * self.packet_loss_ratio_target)
+        def start_computing(fitting_function, bias_avg, bias_cov):
+            """Just a block of code to be used for each fitting function.
+
+            Define function for integrator, create process and pipe ends,
+            start computation, return the boss pipe end.
+
+            :param fitting_function: lfit_erf or lfit_stretch.
+            :param bias_avg: Tuple of floats to start searching around.
+            :param bias_cov: Covariance matrix defining initial focus shape.
+            :type fitting_function: Function from 3 floats to float.
+            :type bias_avg: 2-tuple of floats
+            :type bias_cov: 2-tuple of 2-tuples of floats
+            :returns: Boss end of communication pipe.
+            :rtype: multiprocessing.Connection
+            """
+            def value_logweight_func(x_mrr, x_spread):
+                """Return log of critical rate and log of likelihood.
+
+                This is a closure. The ancestor function got
+                trial_result_list as a parameter, and we are accessing it.
+                As integrator has strict conditions on function signature,
+                trial_result_list cannot be an explicit argument
+                of the current function.
+                This is also why we have to define this closure
+                at each invocation of the ancestor function anew.
+
+                The dimensional spread parameter is the (dimensional) mrr
+                raised to the power of x_spread scaled to interval (0, 1).
+                The dimensional mrr parameter distribution has shape of
+                1/(1+x^2), but x==1 corresponds to max_rate
+                and 1.0 pps is added to avoid numerical problems in fitting
+                functions.
+
+                TODO: x^-2 (for x>1.0) might be simpler/nicer prior.
+
+                :param x_mrr: The first dimensionless param
+                    from (-1, 1) interval.
+                :param x_spread: The second dimensionless param
+                    from (-1, 1) interval.
+                :returns: Log of critical rate [pps] and log of likelihood.
+                :rtype: 2-tuple of float
+                """
+                mrr = max_rate * (1.0 / (x_mrr + 1.0) - 0.5) + 1.0
+                spread = math.exp((x_spread + 1.0) / 2.0 * math.log(mrr))
+#                print "mrr", mrr, "spread", spread
+                logweight = self.log_weight(
+                    fitting_function, trial_result_list, mrr, spread)
+                value = math.log(self.find_critical_rate(
+                    fitting_function,
+                    math.log(packet_loss_per_second_target), mrr, spread))
+                return value, logweight
+            dilled_function = dill.dumps(value_logweight_func)
+            boss_pipe_end, worker_pipe_end = multiprocessing.Pipe()
+            boss_pipe_end.send(
+                (dimension, dilled_function, bias_avg, bias_cov))
+            worker = multiprocessing.Process(
+                target=Integrator.try_estimate_nd, args=(worker_pipe_end,))
+            worker.daemon = True
+            worker.start()
+            return boss_pipe_end
+        erf_pipe = start_computing(
+            self.lfit_erf, erf_bias_avg, erf_bias_cov)
+        stretch_pipe = start_computing(
+            self.lfit_stretch, stretch_bias_avg, stretch_bias_cov)
+        # Measurement phase.
+        measurement = self.measurer.measure(trial_duration, transmit_rate)
+        # Processing phase.
+        erf_pipe.send(None)
+        stretch_pipe.send(None)
+        if not stretch_pipe.poll(1.0):
+            raise RuntimeError("Stretch worker did not finish!")
+        result_or_traceback = stretch_pipe.recv()
+        try:
+            (stretch_avg, stretch_stdev, stretch_bias_avg,
+             stretch_bias_cov, debug_list, _) = result_or_traceback
+        except ValueError:
+            raise RuntimeError(
+                "Stretch worker failed with the following traceback:\n{tr}"
+                .format(tr=result_or_traceback))
+        logging.info("Logs from stretch worker:")
+        for message in debug_list:
+            logging.debug(message)
+        if not erf_pipe.poll(1.0):
+            raise RuntimeError("Erf worker did not finish!")
+        result_or_traceback = erf_pipe.recv()
+        try:
+            (erf_avg, erf_stdev, erf_bias_avg,
+             erf_bias_cov, debug_list, _) = result_or_traceback
+        except ValueError:
+            raise RuntimeError(
+                "Erf worker failed with the following traceback:\n{tr}"
+                .format(tr=result_or_traceback))
+        logging.info("Logs from erf worker:")
+        for message in debug_list:
+            logging.debug(message)
+        avg = math.exp((stretch_avg + erf_avg) / 2.0)
+        var = (stretch_stdev * stretch_stdev + erf_stdev * erf_stdev) / 2.0
+        var += (stretch_avg - erf_avg) * (stretch_avg - erf_avg) / 4.0
+        stdev = avg * math.sqrt(var)
+        integrator_data = (
+            stretch_bias_avg, erf_bias_avg, stretch_bias_cov, erf_bias_cov)
+        return (
+            measurement, avg, stdev, math.exp(stretch_avg),
+            math.exp(erf_avg), integrator_data)
diff --git a/resources/libraries/python/PLRsearch/__init__.py b/resources/libraries/python/PLRsearch/__init__.py
new file mode 100644
index 0000000000..bce703803c
--- /dev/null
+++ b/resources/libraries/python/PLRsearch/__init__.py
@@ -0,0 +1,16 @@
+# Copyright (c) 2018 Cisco and/or its affiliates.
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at:
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""
+__init__ file for Python package "PLRsearch".
+"""
diff --git a/resources/libraries/python/PLRsearch/log_plus.py b/resources/libraries/python/PLRsearch/log_plus.py
new file mode 100644
index 0000000000..3f21cc78d7
--- /dev/null
+++ b/resources/libraries/python/PLRsearch/log_plus.py
@@ -0,0 +1,89 @@
+# Copyright (c) 2018 Cisco and/or its affiliates.
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at:
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Module holding functions for avoiding rounding underflows.
+
+Some applications wish to manipulate non-negative numbers
+which are many orders of magnitude apart.
+In those circumstances, it is useful to store
+the logarithm of the intended number.
+
+As math.log(0.0) raises an exception (instead returning -inf or nan),
+and 0.0 might be a result of computation caused only by rounding error,
+functions of this module use None as -inf.
+
+TODO: Figure out a more performant way of handling -inf.
+
+The functions handle the common task of adding or substracting
+two numbers where both operands and the result is given in logarithm form.
+There are conditionals to make sure overflow does not happen (if possible)
+during the computation."""
+
+import math
+
+
+def log_plus(first, second):
+    """Return logarithm of the sum of two exponentials.
+
+    Basically math.log(math.exp(first) + math.exp(second))
+    which avoids overflow and uses None as math.log(0.0).
+
+    TODO: replace with scipy.special.logsumexp ? Test it.
+
+    :param first: Logarithm of the first number to add (or None if zero).
+    :param second: Logarithm of the second number to add (or None if zero).
+    :type first: float
+    :type second: float
+    :returns: Logarithm of the sum (or None if zero).
+    :rtype: float
+    """
+
+    if first is None:
+        return second
+    if second is None:
+        return first
+    if second > first:
+        return second + math.log(1.0 + math.exp(first - second))
+    else:
+        return first + math.log(1.0 + math.exp(second - first))
+
+
+def log_minus(first, second):
+    """Return logarithm of the difference of two exponentials.
+
+    Basically math.log(math.exp(first) - math.exp(second))
+    which avoids overflow and uses None as math.log(0.0).
+
+    TODO: Support zero difference?
+    TODO: replace with scipy.special.logsumexp ? Test it.
+
+    :param first: Logarithm of the number to subtract from (or None if zero).
+    :param second: Logarithm of the number to subtract (or None if zero).
+    :type first: float
+    :type second: float
+    :returns: Logarithm of the difference.
+    :rtype: float
+    :raises RuntimeError: If the difference would be non-positive.
+    """
+
+    if first is None:
+        raise RuntimeError("log_minus: does not suport None first")
+    if second is None:
+        return first
+    if second >= first:
+        raise RuntimeError("log_minus: first has to be bigger than second")
+    factor = -math.expm1(second - first)
+    if factor <= 0.0:
+        raise RuntimeError("log_minus: non-positive number to log")
+    else:
+        return first + math.log(factor)
diff --git a/resources/libraries/python/TrafficGenerator.py b/resources/libraries/python/TrafficGenerator.py
index b5558e663d..60915117fc 100644
--- a/resources/libraries/python/TrafficGenerator.py
+++ b/resources/libraries/python/TrafficGenerator.py
@@ -25,6 +25,7 @@ from .topology import Topology
 from .MLRsearch.AbstractMeasurer import AbstractMeasurer
 from .MLRsearch.MultipleLossRatioSearch import MultipleLossRatioSearch
 from .MLRsearch.ReceiveRateMeasurement import ReceiveRateMeasurement
+from .PLRsearch.PLRsearch import PLRsearch
 
 __all__ = ['TGDropRateSearchImpl', 'TrafficGenerator', 'OptimizedSearch']
 
@@ -757,3 +758,46 @@ class OptimizedSearch(object):
         result = algorithm.narrow_down_ndr_and_pdr(
             minimum_transmit_rate, maximum_transmit_rate, packet_loss_ratio)
         return result
+
+    @staticmethod
+    def perform_soak_search(
+            frame_size, traffic_type, minimum_transmit_rate,
+            maximum_transmit_rate, plr_target=1e-7, tdpt=0.2,
+            initial_count=50, timeout=1800.0):
+        """Setup initialized TG, perform soak search, return avg and stdev.
+
+        :param frame_size: Frame size identifier or value [B].
+        :param traffic_type: Module name as a traffic type identifier.
+            See resources/traffic_profiles/trex for implemented modules.
+        :param minimum_transmit_rate: Minimal bidirectional
+            target transmit rate [pps].
+        :param maximum_transmit_rate: Maximal bidirectional
+            target transmit rate [pps].
+        :param plr_target: Fraction of packets lost to achieve [1].
+        :param tdpt: Trial duration per trial.
+            The algorithm linearly increases trial duration with trial number,
+            this is the increment between succesive trials, in seconds.
+        :param initial_count: Offset to apply before the first trial.
+            For example initial_count=50 makes first trial to be 51*tdpt long.
+            This is needed because initial "search" phase of integrator
+            takes significant time even without any trial results.
+        :param timeout: The search will stop after this overall time [s].
+        :type frame_size: str or int
+        :type traffic_type: str
+        :type minimum_transmit_rate: float
+        :type maximum_transmit_rate: float
+        :type plr_target: float
+        :type initial_count: int
+        :type timeout: float
+        :returns: Average and stdev of estimated bidirectional rate giving PLR.
+        :rtype: 2-tuple of float
+        """
+        tg_instance = BuiltIn().get_library_instance(
+            'resources.libraries.python.TrafficGenerator')
+        tg_instance.set_rate_provider_defaults(frame_size, traffic_type)
+        algorithm = PLRsearch(
+            measurer=tg_instance, trial_duration_per_trial=tdpt,
+            packet_loss_ratio_target=plr_target,
+            trial_number_offset=initial_count, timeout=timeout)
+        result = algorithm.search(minimum_transmit_rate, maximum_transmit_rate)
+        return result
diff --git a/resources/libraries/python/autogen/Regenerator.py b/resources/libraries/python/autogen/Regenerator.py
index c0937df4e8..8bfe054e5e 100644
--- a/resources/libraries/python/autogen/Regenerator.py
+++ b/resources/libraries/python/autogen/Regenerator.py
@@ -101,6 +101,11 @@ class Regenerator(object):
                 # Not supported by AVF driver.
                 # https://git.fd.io/vpp/tree/src/plugins/avf/README.md
                 pass
+            elif ("soak" in suite_id and
+                  (kwargs["phy_cores"] != 1
+                   or kwargs["framesize"] in (1518, 9000, "IMIX_v4_1"))):
+                # Soak test take too long, do not risk other than tc01.
+                pass
             else:
                 file_out.write(testcase.generate(num=num, **kwargs))
             return num + 1
diff --git a/resources/libraries/robot/performance/performance_utils.robot b/resources/libraries/robot/performance/performance_utils.robot
index 66fd307451..b98d103beb 100644
--- a/resources/libraries/robot/performance/performance_utils.robot
+++ b/resources/libraries/robot/performance/performance_utils.robot
@@ -220,6 +220,42 @@
 | | Perform additional measurements based on NDRPDR result
 | | ... | ${result} | ${frame_size} | ${topology_type}
 
+| Find critical load using PLRsearch
+| | [Documentation]
+| | ... | Find boundaries for troughput (of given target loss ratio)
+| | ... | using PLRsearch algorithm.
+| | ... | Display results as formatted test message.
+| | ... | Fail if computed lower bound is below minimal rate.
+| | ... | Input rates are understood as uni-directional,
+| | ... | reported result contains bi-directional rates.
+| | ... | TODO: Any additional measurements for debug purposes?
+| | ...
+| | ... | *Arguments:*
+| | ... | - frame_size - L2 Frame Size [B] or IMIX string. Type: int or str
+| | ... | - topology_type - Topology type. Type: string
+| | ... | - minimum_transmit_rate - Lower limit of search [pps]. Type: float
+| | ... | - maximum_transmit_rate - Upper limit of search [pps]. Type: float
+| | ... | - packet_loss_ratio - Accepted loss during search. Type: float
+| | ... | - timeout - Stop when search duration is longer [s]. Type: float
+| | ...
+| | ... | *Example:*
+| | ...
+| | ... | \| Find critical load usingPLR search \| \${64} \| \
+| | ... | 3-node-IPv4 \| \${100000} \| \${14880952} \| \${1e-7} \| \${1800} \
+| | ...
+| | [Arguments] | ${frame_size} | ${topology_type} | ${minimum_transmit_rate}
+| | ... | ${maximum_transmit_rate} | ${packet_loss_ratio}=${1e-7}
+| | ... | ${timeout}=${1800.0}
+| | ...
+| | ${min_rate} = | Set Variable | ${minimum_transmit_rate*2}
+| | ${average} | ${stdev} = | Perform soak search | ${frame_size}
+| | ... | ${topology_type} | ${min_rate} | ${maximum_transmit_rate*2}
+| | ... | ${packet_loss_ratio} | timeout=${timeout}
+| | ${lower} | ${upper} = | Display result of soak search
+| | ... | ${average} | ${stdev} | ${frame_size}
+| | Should Not Be True | ${lower} < ${min_rate}
+| | ... | Lower bound ${lower} is below bidirectional minimum ${min_rate}.
+
 | Display single bound
 | | [Documentation]
 | | ... | Display one bound of NDR+PDR search,
@@ -244,7 +280,7 @@
 | | ...
 | | [Arguments] | ${text} | ${rate_total} | ${framesize} | ${latency}=${EMPTY}
 | | ...
-| | ${bandwidth_total}= | Evaluate | ${rate_total}*(${framesize}+20)*8/(10**9)
+| | ${bandwidth_total} = | Evaluate | ${rate_total} * (${framesize}+20)*8 / 1e9
 | | Set Test Message | ${\n}${text}: ${rate_total} pps, | append=yes
 | | Set Test Message | ${bandwidth_total} Gbps (untagged) | append=yes
 | | Return From Keyword If | not """${latency}"""
@@ -272,7 +308,7 @@
 | | ...
 | | [Arguments] | ${result} | ${framesize}
 | | ...
-| | ${framesize}= | Get Frame Size | ${framesize}
+| | ${framesize} = | Get Frame Size | ${framesize}
 | | Display single bound | NDR_LOWER
 | | ... | ${result.ndr_interval.measured_low.transmit_rate} | ${framesize}
 | | ... | ${result.ndr_interval.measured_low.latency}
@@ -284,6 +320,41 @@
 | | Display single bound | PDR_UPPER
 | | ... | ${result.pdr_interval.measured_high.transmit_rate} | ${framesize}
 
+| Display result of soak search
+| | [Documentation]
+| | ... | Display result of soak search, avg+-stdev, as upper/lower bounds,
+| | ... | in packet per seconds (total and per stream)
+| | ... | and Gbps total bandwidth with untagged packet.
+| | ... | Througput is calculated as:
+| | ... | Measured rate per stream * Total number of streams
+| | ... | Bandwidth is calculated as:
+| | ... | (Throughput * (L2 Frame Size + IPG) * 8) / Max bitrate of NIC
+| | ... | TODO: Do we want to report some latency data,
+| | ... | even if not measured at the reported bounds?.
+| | ...
+| | ... | *Arguments:*
+| | ... | - avg - Estimated average critical load [pps]. Type: float
+| | ... | - stdev - Standard deviation of critical load [pps]. Type: float
+| | ... | - framesize - L2 Frame Size [B]. Type: integer
+| | ...
+| | ... | *Returns:*
+| | ... | - Lower and upper bound of critical load [pps]. Type: 2-tuple of float
+| | ...
+| | ... | *Example:*
+| | ...
+| | ... | \| Display result of soak search \| \${100000} \| \${100} \| \${64} \|
+| | ...
+| | [Arguments] | ${avg} | ${stdev} | ${framesize}
+| | ...
+| | ${framesize} = | Get Frame Size | ${framesize}
+| | ${avg} = | Convert To Number | ${avg}
+| | ${stdev} = | Convert To Number | ${stdev}
+| | ${lower} = | Evaluate | ${avg} - ${stdev}
+| | ${upper} = | Evaluate | ${avg} + ${stdev}
+| | Display single bound | PLRsearch lower bound: | ${lower} | ${framesize}
+| | Display single bound | PLRsearch upper bound: | ${upper} | ${framesize}
+| | Return From Keyword | ${lower} | ${upper}
+
 | Check NDRPDR interval validity
 | | [Documentation]
 | | ... | Extract loss ratio of lower bound of the interval.