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* [glibc] malloc: Improve MAP_HUGETLB with glibc.malloc.hugetlb=2
@ 2023-11-29 12:31 Adhemerval Zanella
  0 siblings, 0 replies; only message in thread
From: Adhemerval Zanella @ 2023-11-29 12:31 UTC (permalink / raw)
  To: glibc-cvs;h=bc6d79f4ae99206e7ec7d6a8c5abf26cdefc8bff

commit bc6d79f4ae99206e7ec7d6a8c5abf26cdefc8bff
Author: Adhemerval Zanella <>
Date:   Thu Nov 23 14:29:15 2023 -0300

    malloc: Improve MAP_HUGETLB with glibc.malloc.hugetlb=2
    Even for explicit large page support, allocation might use mmap without
    the hugepage bit set if the requested size is smaller than
    mmap_threshold.  For this case where mmap is issued, MAP_HUGETLB is set
    iff the allocation size is larger than the used large page.
    To force such allocations to use large pages, also tune the mmap_threhold
    (if it is not explicit set by a tunable).  This forces allocation to
    follow the sbrk path, which will fall back to mmap (which will try large
    pages before galling back to default mmap).
    Checked on x86_64-linux-gnu.
    Reviewed-by: DJ Delorie <>
    Tested-by: Zhangfei Gao <>

 malloc/arena.c | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/malloc/arena.c b/malloc/arena.c
index a1a75e5a2b..c73f68890d 100644
--- a/malloc/arena.c
+++ b/malloc/arena.c
@@ -312,10 +312,17 @@ ptmalloc_init (void)
 # endif
   TUNABLE_GET (mxfast, size_t, TUNABLE_CALLBACK (set_mxfast));
   TUNABLE_GET (hugetlb, size_t, TUNABLE_CALLBACK (set_hugetlb));
   if (mp_.hp_pagesize > 0)
-    /* Force mmap for main arena instead of sbrk, so hugepages are explicitly
-       used.  */
-    __always_fail_morecore = true;
+    {
+      /* Force mmap for main arena instead of sbrk, so MAP_HUGETLB is always
+         tried.  Also tune the mmap threshold, so allocation smaller than the
+	 large page will also try to use large pages by falling back
+	 to sysmalloc_mmap_fallback on sysmalloc.  */
+      if (!TUNABLE_IS_INITIALIZED (mmap_threshold))
+	do_set_mmap_threshold (mp_.hp_pagesize);
+      __always_fail_morecore = true;
+    }
 /* Managing heaps and arenas (for concurrent threads) */

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