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[PATCH v2] Allow shrinking of arena heaps using mmap based onovercommit settings when available
On Mon, 13 Aug 2012 02:32:36 -0400, KOSAKI wrote:
> (Again, MADV_DONTNEED has good multi thread performance than
> PROT_NONE.)
Right, so I did another program[1] that uses malloc/free rather than the
system calls directly to get an idea of the kind of performance
improvement we would see in a multithreaded scenario and as it turns
out, there is a significant difference as you said.
The program I wrote, spawns 8 threads (conservative number for my 4
core box), each of them allocating 64MB in splits of 16MB
(MMAP_THRESHOLD adjusted to put these on arenas) and freeing them in
reverse order, resulting in a heap trim at each free call. Each of
those threads did 1024 such iterations, giving me 4096 samples. The
results were as follows:
With madvise:
RUNS=4096, TOTAL=9264596.000000, AVG=2261.864258
RUNS=4096, TOTAL=9541428.000000, AVG=2329.450195
RUNS=4096, TOTAL=9381367.000000, AVG=2290.372803
RUNS=4096, TOTAL=9495634.000000, AVG=2318.270020
RUNS=4096, TOTAL=9654729.000000, AVG=2357.111572
RUNS=4096, TOTAL=9202857.000000, AVG=2246.791260
RUNS=4096, TOTAL=9444178.000000, AVG=2305.707520
RUNS=4096, TOTAL=9105736.000000, AVG=2223.080078
With mmap:
RUNS=4096, TOTAL=10211110.000000, AVG=2492.946777
RUNS=4096, TOTAL=10429254.000000, AVG=2546.204590
RUNS=4096, TOTAL=10463869.000000, AVG=2554.655518
RUNS=4096, TOTAL=10599614.000000, AVG=2587.796387
RUNS=4096, TOTAL=10318008.000000, AVG=2519.044922
RUNS=4096, TOTAL=10509473.000000, AVG=2565.789307
RUNS=4096, TOTAL=10325819.000000, AVG=2520.951904
RUNS=4096, TOTAL=10497513.000000, AVG=2562.869385
which gives a roughly 300us advantage per call on average, which is
quite significant. However, since the madvise will still break when
vm.overcommit_memory == 2, I tried a different approach for the patch.
Instead of removing that check, I introduced some Linux-specific code
that checks the contents of /proc/sys/vm.overcommit_memory and if it is
2, uses MAP_FIXED to drop the PTEs. I have attached the patch for
review.
Regards,
Siddhesh
ChangeLog:
* malloc/arena.c: Include malloc-sysdep.h.
(shrink_heap): New static variable may_shrink_heap to decide if
madvise is sufficient to shrink the heap or an unmap is needed.
* sysdeps/generic/Makefile (sysdep_headers): Add
malloc-sysdep.h as a dependency when building malloc.
* sysdeps/generic/malloc-sysdep.h: New file. Define
new function check_may_shrink_heap.
* sysdeps/unix/sysv/linux/Makefile (sysdep_headers): Add
malloc-sysdep.h as dependency when building malloc.
* sysdeps/unix/sysv/linux/malloc-sysdep.h: New file. Define
new function check_may_shrink_heap.
[1] The benchmark code:
#include <pthread.h>
#include <stdlib.h>
#include <string.h>
#include <stdio.h>
#include <malloc.h>
#include <assert.h>
#define NTHR 8
#define BLOCKSIZE 16*1024*1024
#define COUNT 1024
pthread_mutex_t printf_lock = PTHREAD_MUTEX_INITIALIZER;
void *
thr (void *num)
{
double total = 0.0;
int i;
for (i = 0; i < COUNT; i++)
{
/* 4 blocks of 16M each to fill my 64M heap. */
int j = 0;
void *m[4];
for (j = 0; j < 4; j++)
{
m[j] = malloc (BLOCKSIZE);
assert (m[j] != NULL);
memset (m[j], 0, BLOCKSIZE);
}
for (j = 3; j >= 0; j--)
{
struct timespec before, after;
double elapsed;
clock_gettime (CLOCK_MONOTONIC_RAW, &before);
free (m[j]);
clock_gettime (CLOCK_MONOTONIC_RAW, &after);
elapsed = (after.tv_sec - before.tv_sec) * 1000000;
elapsed += (after.tv_nsec - before.tv_nsec) / 1000;
total += elapsed;
pthread_mutex_lock (&printf_lock);
printf ("%d:%d:%d: %lf\n", (long)num, i, j, elapsed);
pthread_mutex_unlock (&printf_lock);
}
}
pthread_mutex_lock (&printf_lock);
for (i = 0; i < 80; i++)
putchar ('=');
putchar ('\n');
printf ("RUNS=%d, TOTAL=%lf, AVG=%lf\n", COUNT * 4,
total, total / (COUNT * 4));
pthread_mutex_unlock (&printf_lock);
}
int
main ()
{
long i;
pthread_t t[NTHR];
mallopt (M_MMAP_THRESHOLD, 31 * 1024 * 1024);
for (i = 0; i < NTHR; i++)
{
pthread_create (&t[i], NULL, thr, (void *)i);
}
for (i = 0; i < NTHR; i++)
{
pthread_join (t[i], NULL);
}
return 0;
}
diff --git a/malloc/arena.c b/malloc/arena.c
index 9e5e332..b7aacc4 100644
--- a/malloc/arena.c
+++ b/malloc/arena.c
@@ -20,6 +20,9 @@
#include <stdbool.h>
+/* Get the implementation for check_may_shrink_heap. */
+#include <malloc-sysdep.h>
+
/* Compile-time constants. */
#define HEAP_MIN_SIZE (32*1024)
@@ -618,13 +621,17 @@ static int
shrink_heap(heap_info *h, long diff)
{
long new_size;
+ static int may_shrink_heap = -1;
+
+ if (__builtin_expect (may_shrink_heap < 0, 0))
+ may_shrink_heap = check_may_shrink_heap ();
new_size = (long)h->size - diff;
if(new_size < (long)sizeof(*h))
return -1;
- /* Try to re-map the extra heap space freshly to save memory, and
- make it inaccessible. */
- if (__builtin_expect (__libc_enable_secure, 0))
+ /* Try to re-map the extra heap space freshly to save memory, and make it
+ inaccessible. See malloc-sysdep.h to know when this is true. */
+ if (__builtin_expect (may_shrink_heap, 0))
{
if((char *)MMAP((char *)h + new_size, diff, PROT_NONE,
MAP_FIXED) == (char *) MAP_FAILED)
diff --git a/sysdeps/generic/Makefile b/sysdeps/generic/Makefile
index f74109d..247cdd3 100644
--- a/sysdeps/generic/Makefile
+++ b/sysdeps/generic/Makefile
@@ -20,6 +20,10 @@ ifeq ($(subdir),string)
CFLAGS-wordcopy.c += -Wno-uninitialized
endif
+ifeq ($(subdir),malloc)
+sysdep_headers += malloc-sysdep.h
+endif
+
ifeq ($(subdir),elf)
ifeq (yes:yes,$(build-shared):$(unwind-find-fde))
# This is needed to support g++ v2 and v3.
diff --git a/sysdeps/generic/malloc-sysdep.h b/sysdeps/generic/malloc-sysdep.h
new file mode 100644
index 0000000..cbb34d6
--- /dev/null
+++ b/sysdeps/generic/malloc-sysdep.h
@@ -0,0 +1,27 @@
+/* Copyright (C) 2012 Free Software Foundation, Inc.
+ This file is part of the GNU C Library.
+
+ The GNU C Library is free software; you can redistribute it and/or
+ modify it under the terms of the GNU Lesser General Public
+ License as published by the Free Software Foundation; either
+ version 2.1 of the License, or (at your option) any later version.
+
+ The GNU C Library is distributed in the hope that it will be useful,
+ but WITHOUT ANY WARRANTY; without even the implied warranty of
+ MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+ Lesser General Public License for more details.
+
+ You should have received a copy of the GNU Lesser General Public
+ License along with the GNU C Library; if not, see
+ <http://www.gnu.org/licenses/>. */
+
+/* Force an unmap on heap shrink for setuid programs. This is so that the
+ kernel immediately drops the PTE for the mapping. */
+
+#include <unistd.h>
+
+static inline int
+check_may_shrink_heap (void)
+{
+ return __libc_enable_secure;
+}
diff --git a/sysdeps/unix/sysv/linux/Makefile b/sysdeps/unix/sysv/linux/Makefile
index ddae686..f129f45 100644
--- a/sysdeps/unix/sysv/linux/Makefile
+++ b/sysdeps/unix/sysv/linux/Makefile
@@ -9,6 +9,7 @@ endif
ifeq ($(subdir),malloc)
CFLAGS-malloc.c += -DMORECORE_CLEARS=2
+sysdep_headers += malloc-sysdep.h
endif
ifeq ($(subdir),socket)
diff --git a/sysdeps/unix/sysv/linux/malloc-sysdep.h b/sysdeps/unix/sysv/linux/malloc-sysdep.h
new file mode 100644
index 0000000..4b66b70
--- /dev/null
+++ b/sysdeps/unix/sysv/linux/malloc-sysdep.h
@@ -0,0 +1,57 @@
+/* Copyright (C) 2012 Free Software Foundation, Inc.
+ This file is part of the GNU C Library.
+
+ The GNU C Library is free software; you can redistribute it and/or
+ modify it under the terms of the GNU Lesser General Public
+ License as published by the Free Software Foundation; either
+ version 2.1 of the License, or (at your option) any later version.
+
+ The GNU C Library is distributed in the hope that it will be useful,
+ but WITHOUT ANY WARRANTY; without even the implied warranty of
+ MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+ Lesser General Public License for more details.
+
+ You should have received a copy of the GNU Lesser General Public
+ License along with the GNU C Library; if not, see
+ <http://www.gnu.org/licenses/>. */
+
+/* The Linux kernel overcommits address space by default and if there is not
+ enough memory available, it uses various parameters to decide the process to
+ kill. It is however possible to disable or curb this overcommit behaviour
+ by setting the proc sysctl vm.overcommit_memory to the value '2' and with
+ that, a process is only allowed to use the maximum of a pre-determined
+ fraction of the total address space. In such a case, we want to make sure
+ that we are judicious with our heap usage as well, and explicitly give away
+ the freed top of the heap to reduce our commit charge. See the proc(5) man
+ page to know more about overcommit behaviour.
+
+ Other than that, we also force an unmap for setuid programs. */
+
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <fcntl.h>
+#include <unistd.h>
+
+static inline int
+check_may_shrink_heap (void)
+{
+ int fd;
+
+ if (__libc_enable_secure)
+ return 1;
+
+ fd = open ("/proc/sys/vm/overcommit_memory", O_RDONLY);
+
+ if (fd != -1)
+ {
+ char val;
+ int n = read (fd, &val, 1);
+ if (n > 0)
+ val -= '0';
+
+ if (val == 2)
+ return 1;
+ }
+
+ return 0;
+}