perfmon plugin: 2-way parallel stat collection

As a FUD reduction measure, this patch implements 2-way parallel
counter collection. Synthetic stat component counter pairs run at the
same time. Running two counters (of any kind) at the same time
naturally reduces the aggregate time required by an approximate
factor-of-2, depending on whether an even or odd number of stats have
been requested.

I don't completely buy the argument that computing synthetic stats
such as instructions-per-clock will be inaccurate if component counter
values are collected sequentially. Given uniform traffic pattern, it
must make no difference.

As the collection interval increases, the difference between serial
and parallel component counter collection will approach zero, see also
the Central Limit theorem.

Change-Id: I36ebdcf125e8882cca8a1929ec58f17fba1ad8f1
Signed-off-by: Dave Barach <dave@barachs.net>

1 files changed, 2 insertions, 9 deletions

diff --git a/src/vlibapi/node_serialize.c b/src/vlibapi/node_serialize.c
index 0774eea251e..b50d79e2922 100644
--- a/src/vlibapi/node_serialize.c
+++ b/src/vlibapi/node_serialize.c
@@ -57,7 +57,7 @@ vlib_node_serialize (vlib_main_t * vm, vlib_node_t *** node_dups, u8 * vector,
   u8 *namep;
   u32 name_bytes;
   uword i, j, k;
-  u64 l, v, c, d, pmc;
+  u64 l, v, c, d;
   state_string_enum_t state_code;
 
   serialize_open_vector (sm, vector);
@@ -77,8 +77,6 @@ vlib_node_serialize (vlib_main_t * vm, vlib_node_t *** node_dups, u8 * vector,
 	  v = n->stats_total.vectors - n->stats_last_clear.vectors;
 	  c = n->stats_total.calls - n->stats_last_clear.calls;
 	  d = n->stats_total.suspends - n->stats_last_clear.suspends;
-	  pmc = n->stats_total.perf_counter_ticks
-	    - n->stats_last_clear.perf_counter_ticks;
 
 	  state_code = STATE_INTERNAL;
 
@@ -151,8 +149,6 @@ vlib_node_serialize (vlib_main_t * vm, vlib_node_t *** node_dups, u8 * vector,
 	      serialize_integer (sm, v, 8);
 	      /* Total suspends */
 	      serialize_integer (sm, d, 8);
-	      /* PMC counter */
-	      serialize_integer (sm, pmc, 8);
 	    }
 	  else			/* no stats */
 	    serialize_likely_small_unsigned_integer (sm, 0);
@@ -171,7 +167,7 @@ vlib_node_unserialize (u8 * vector)
   vlib_node_t **nodes;
   vlib_node_t ***nodes_by_thread = 0;
   int i, j, k;
-  u64 l, v, c, d, pmc;
+  u64 l, v, c, d;
   state_string_enum_t state_code;
   int stats_present;
 
@@ -229,9 +225,6 @@ vlib_node_unserialize (u8 * vector)
 	      /* Total suspends */
 	      unserialize_integer (sm, &d, 8);
 	      node->stats_total.suspends = d;
-	      /* PMC counter */
-	      unserialize_integer (sm, &pmc, 8);
-	      node->stats_total.perf_counter_ticks = pmc;
 	    }
 	}
     }