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diff --git a/doc/guides/prog_guide/thread_safety_dpdk_functions.rst b/doc/guides/prog_guide/thread_safety_dpdk_functions.rst new file mode 100644 index 00000000..403e5fc6 --- /dev/null +++ b/doc/guides/prog_guide/thread_safety_dpdk_functions.rst @@ -0,0 +1,102 @@ +.. BSD LICENSE + Copyright(c) 2010-2014 Intel Corporation. All rights reserved. + All rights reserved. + + Redistribution and use in source and binary forms, with or without + modification, are permitted provided that the following conditions + are met: + + * Redistributions of source code must retain the above copyright + notice, this list of conditions and the following disclaimer. + * Redistributions in binary form must reproduce the above copyright + notice, this list of conditions and the following disclaimer in + the documentation and/or other materials provided with the + distribution. + * Neither the name of Intel Corporation nor the names of its + contributors may be used to endorse or promote products derived + from this software without specific prior written permission. + + THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS + "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT + LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR + A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT + OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, + SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT + LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, + DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY + THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT + (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE + OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. + +Thread Safety of DPDK Functions +=============================== + +The DPDK is comprised of several libraries. +Some of the functions in these libraries can be safely called from multiple threads simultaneously, while others cannot. +This section allows the developer to take these issues into account when building their own application. + +The run-time environment of the DPDK is typically a single thread per logical core. +In some cases, it is not only multi-threaded, but multi-process. +Typically, it is best to avoid sharing data structures between threads and/or processes where possible. +Where this is not possible, then the execution blocks must access the data in a thread- safe manner. +Mechanisms such as atomics or locking can be used that will allow execution blocks to operate serially. +However, this can have an effect on the performance of the application. + +Fast-Path APIs +-------------- + +Applications operating in the data plane are performance sensitive but +certain functions within those libraries may not be safe to call from multiple threads simultaneously. +The hash, LPM and mempool libraries and RX/TX in the PMD are examples of this. + +The hash and LPM libraries are, by design, thread unsafe in order to maintain performance. +However, if required the developer can add layers on top of these libraries to provide thread safety. +Locking is not needed in all situations, and in both the hash and LPM libraries, +lookups of values can be performed in parallel in multiple threads. +Adding, removing or modifying values, however, +cannot be done in multiple threads without using locking when a single hash or LPM table is accessed. +Another alternative to locking would be to create multiple instances of these tables allowing each thread its own copy. + +The RX and TX of the PMD are the most critical aspects of a DPDK application +and it is recommended that no locking be used as it will impact performance. +Note, however, that these functions can safely be used from multiple threads +when each thread is performing I/O on a different NIC queue. +If multiple threads are to use the same hardware queue on the same NIC port, +then locking, or some other form of mutual exclusion, is necessary. + +The ring library is based on a lockless ring-buffer algorithm that maintains its original design for thread safety. +Moreover, it provides high performance for either multi- or single-consumer/producer enqueue/dequeue operations. +The mempool library is based on the DPDK lockless ring library and therefore is also multi-thread safe. + +Performance Insensitive API +--------------------------- + +Outside of the performance sensitive areas described in Section 25.1, +the DPDK provides a thread-safe API for most other libraries. +For example, malloc and memzone functions are safe for use in multi-threaded and multi-process environments. + +The setup and configuration of the PMD is not performance sensitive, but is not thread safe either. +It is possible that the multiple read/writes during PMD setup and configuration could be corrupted in a multi-thread environment. +Since this is not performance sensitive, the developer can choose to add their own layer to provide thread-safe setup and configuration. +It is expected that, in most applications, the initial configuration of the network ports would be done by a single thread at startup. + +Library Initialization +---------------------- + +It is recommended that DPDK libraries are initialized in the main thread at application startup +rather than subsequently in the forwarding threads. +However, the DPDK performs checks to ensure that libraries are only initialized once. +If initialization is attempted more than once, an error is returned. + +In the multi-process case, the configuration information of shared memory will only be initialized by the master process. +Thereafter, both master and secondary processes can allocate/release any objects of memory that finally rely on rte_malloc or memzones. + +Interrupt Thread +---------------- + +The DPDK works almost entirely in Linux user space in polling mode. +For certain infrequent operations, such as receiving a PMD link status change notification, +callbacks may be called in an additional thread outside the main DPDK processing threads. +These function callbacks should avoid manipulating DPDK objects that are also managed by the normal DPDK threads, +and if they need to do so, +it is up to the application to provide the appropriate locking or mutual exclusion restrictions around those objects. |