From: Christophe Thiéry <christopho128@gmail.com>
Date: Wed, 7 Dec 2011 16:57:19 +0000 (+0100)
Subject: Parmap: make maestro also do some work.
X-Git-Tag: exp_20120216~240^2
X-Git-Url: http://info.iut-bm.univ-fcomte.fr/pub/gitweb/simgrid.git/commitdiff_plain/e6d9cc62d3fa395f1111d3af226afe91cc91f3b1?ds=sidebyside

Parmap: make maestro also do some work.

The gain looks much bigger than I expected (Chord 10k, constant
network: 55s -> 20s and 160,000 syscalls -> 88,000 syscalls).
Not sure to understand why.
---

diff --git a/src/xbt/parmap.c b/src/xbt/parmap.c
index b9db0756a4..4e79b71bb1 100644
--- a/src/xbt/parmap.c
+++ b/src/xbt/parmap.c
@@ -49,8 +49,7 @@ typedef struct s_xbt_parmap {
   int work;                        /* index of the current round (1 is the first) */
   int done;                        /* number of rounds already done */
   unsigned int thread_counter;     /* number of threads currently working */
-  unsigned int num_workers;        /* total number of worker threads */
-  unsigned int workers_max_id;     /* id of the next worker thread to create */
+  unsigned int num_workers;        /* total number of worker threads including the controller */
   void_f_pvoid_t fun;              /* function to run in parallel on each element of data */
   xbt_dynar_t data;                /* parameters to pass to fun in parallel */
   unsigned int index;              /* index of the next element of data to pick */
@@ -75,7 +74,7 @@ xbt_parmap_t xbt_parmap_new(unsigned int num_workers)
   parmap->status = PARMAP_WORK;
 
   /* Create the pool of worker threads */
-  for (i = 0; i < num_workers; i++) {
+  for (i = 0; i < num_workers - 1; i++) {
     worker = xbt_os_thread_create(NULL, xbt_parmap_worker_main, parmap, NULL);
     xbt_os_thread_detach(worker);
   }
@@ -184,6 +183,7 @@ static void xbt_parmap_start(xbt_parmap_t parmap)
 {
 #ifdef HAVE_FUTEX_H
   int myflag = parmap->done;
+  __sync_fetch_and_add(&parmap->thread_counter, 1);
   if (parmap->thread_counter < parmap->num_workers) {
     /* wait for all workers to be ready */
     futex_wait(&parmap->done, myflag);
@@ -208,8 +208,20 @@ static void xbt_parmap_signal(xbt_parmap_t parmap)
   /* wake all workers */
   futex_wake(&parmap->work, parmap->num_workers);
 
-  /* wait for all of them to finish */
-  futex_wait(&parmap->done, myflag);
+  if (parmap->status == PARMAP_WORK) {
+    /* also work myself */
+    void* work = xbt_parmap_next(parmap);
+    if (work != NULL) {
+      parmap->fun(work);
+    }
+  }
+
+  unsigned int mycount = __sync_add_and_fetch(&parmap->thread_counter, 1);
+  if (mycount < parmap->num_workers) {
+    /* some workers have not finished yet */
+    futex_wait(&parmap->done, myflag);
+  }
+
 #endif
 }