malloc: Reduce maximum arenas

The default maximum arenas is 8 times the number of cores in a 64-bit system.
Since modern CPUs have many cores and big servers have 256 cores, this results
in excessive number of arenas, which wastes memory.  Limit the number of arenas
to max (8, ncores) which is less extreme.  In the future the limit should be
lowered further for large systems.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

diff --git a/malloc/arena.c b/malloc/arena.c
index 023cb3ba067d0de097a687b73c81f5d860393413..1ed419a30569f59e169c5c3138e38a2bdb9e5de2 100644 (file)
--- a/malloc/arena.c
+++ b/malloc/arena.c
@@ -799,16 +799,11 @@ arena_get2 (size_t size, mstate avoid_arena)
         {
           if (mp_.arena_max != 0)
             narenas_limit = mp_.arena_max;
-          else if (narenas > mp_.arena_test)
+          else if (narenas >= mp_.arena_test)
             {
-              int n = __get_nprocs ();
-
-              if (n >= 1)
-                narenas_limit = NARENAS_FROM_NCORES (n);
-              else
-                /* We have no information about the system.  Assume two
-                   cores.  */
-                narenas_limit = NARENAS_FROM_NCORES (2);
+             narenas_limit = __get_nprocs ();
+             if (narenas_limit < mp_.arena_test)
+               narenas_limit = mp_.arena_test;
             }
         }
     repeat:;

diff --git a/malloc/malloc.c b/malloc/malloc.c
index 6ce76cb4df88930b2bd65e35e4f83bee0c635c27..e121fad35a1a9c4dd6915f1be06d7c6e7df2d1fb 100644 (file)
--- a/malloc/malloc.c
+++ b/malloc/malloc.c
@@ -1650,8 +1650,7 @@ static struct malloc_par mp_ =
   .n_mmaps_max = DEFAULT_MMAP_MAX,
   .mmap_threshold = DEFAULT_MMAP_THRESHOLD,
   .trim_threshold = DEFAULT_TRIM_THRESHOLD,
-#define NARENAS_FROM_NCORES(n) ((n) * (sizeof (long) == 4 ? 2 : 8))
-  .arena_test = NARENAS_FROM_NCORES (1),
+  .arena_test = sizeof (long) == 4 ? 2 : 8,
   .thp_mode = thp_mode_not_supported
 #if USE_TCACHE
   ,

diff --git a/manual/memory.texi b/manual/memory.texi
index f8b6be6ee781cafda1774598585b50524d6cdca0..744ea6ac64a52ec512d86c1c7a23733fffd08785 100644 (file)
--- a/manual/memory.texi
+++ b/manual/memory.texi
@@ -1388,11 +1388,8 @@ environment variable @env{MALLOC_ARENA_TEST} to the desired value.
 This parameter sets the number of arenas to use regardless of the number of
 cores in the system.
 
-The default value of this tunable is @code{0}, meaning that the limit on the
-number of arenas is determined by the number of CPU cores online. For 32-bit
-systems the limit is twice the number of cores online and on 64-bit systems, it
-is eight times the number of cores online.  Note that the default value is not
-derived from the default value of M_ARENA_TEST and is computed independently.
+The default value of this tunable is @code{0}, meaning that the limit is the
+number of CPU cores online reported by get_nprocs().
 
 This parameter can also be set for the process at startup by setting the
 environment variable @env{MALLOC_ARENA_MAX} to the desired value.

diff --git a/manual/tunables.texi b/manual/tunables.texi
index 05f5e65a8901b67301a9d8b1da9369e2e5395e28..e0f141077b133a6b6d5f5f76028e65025c7047b6 100644 (file)
--- a/manual/tunables.texi
+++ b/manual/tunables.texi
@@ -220,10 +220,8 @@ identical in features.
 This tunable sets the number of arenas to use in a process regardless of the
 number of cores in the system.
 
-The default value of this tunable is @code{0}, meaning that the limit on the
-number of arenas is determined by the number of CPU cores online.  For 32-bit
-systems the limit is twice the number of cores online and on 64-bit systems, it
-is 8 times the number of cores online.
+The default value of this tunable is @code{0}, meaning that the limit is the
+number of CPU cores online reported by get_nprocs().
 @end deftp
 
 @deftp Tunable glibc.malloc.tcache_max