人页面在这里承诺:https://github.com/mkerrisk/man-pages/blob/master/man3/malloc_trim.3,并按照我的理解,它是由人的页面项目的维护者从头写起,kerrisk 2012年:https://github.com/mkerrisk/man-pages/commit/a15b0e60b297e29c825b7417582a33e6ca26bf65
,我可以grep the glibc's git, there are no man pages in the glibc,并没有承诺malloc_trim手册页记录这个补丁。的glibc的malloc最好的和唯一的文档是它的源代码:https://sourceware.org/git/?p=glibc.git;a=blob;f=malloc/malloc.c 有来自malloc/malloc.c
malloc_trim
评论:
Additional functions:
malloc_trim(size_t pad);
609 /*
610 malloc_trim(size_t pad);
611
612 If possible, gives memory back to the system (via negative
613 arguments to sbrk) if there is unused memory at the `high' end of
614 the malloc pool. You can call this after freeing large blocks of
615 memory to potentially reduce the system-level memory requirements
616 of a program. However, it cannot guarantee to reduce memory. Under
617 some allocation patterns, some large free blocks of memory will be
618 locked between two used chunks, so they cannot be given back to
619 the system.
620
621 The `pad' argument to malloc_trim represents the amount of free
622 trailing space to leave untrimmed. If this argument is zero,
623 only the minimum amount of memory to maintain internal data
624 structures will be left (one page or less). Non-zero arguments
625 can be supplied to maintain enough trailing space to service
626 future expected allocations without having to re-obtain memory
627 from the system.
628
629 Malloc_trim returns 1 if it actually released any memory, else 0.
630 On systems that do not support "negative sbrks", it will always
631 return 0.
632 */
633 int __malloc_trim(size_t);
634
从块中间释放没有记录中的malloc文/ malloc.c并没有记录在man-pages项目中。 2012年的手册页可能是该函数的第一个手册页,不是由glibc的作者撰写的。 glibc的信息页只提到128 KB的M_TRIM_THRESHOLD: https://www.gnu.org/software/libc/manual/html_node/Malloc-Tunable-Parameters.html#Malloc-Tunable-Parameters并且不列出malloc_trim函数https://www.gnu.org/software/libc/manual/html_node/Summary-of-Malloc.html#Summary-of-Malloc(并且它也不记录memusage/memusagestat/libmemusage.so)。
2007年12月,由Ulrich Drepper提交了https://sourceware.org/git/?p=glibc.git;a=commit;f=malloc/malloc.c;h=68631c8eb92ff38d9da1ae34f6aa048539b199cc(它是glibc 2的一部分)。9,并且改变mtrim
实施新)(不过那是glibc无人页)它并没有改变任何文件或手册页:
- 的malloc/malloc.c(public_mTRIm):遍历所有竞技场和电话
mTRIm所有这些。 (mTRIm):此外迭代所有空闲块并使用madvise为所有包含至少一个内存页面的块释放内存。
块的未使用的部分(任何地方,包括在中间块),页面大小对齐,并具有尺寸大于页可被标记为MADV_DONTNEED
https://sourceware.org/git/?p=glibc.git;a=blobdiff;f=malloc/malloc.c;h=c54c203cbf1f024e72493546221305b4fd5729b7;hp=1e716089a2b976d120c304ad75dd95c63737ad75;hb=68631c8eb92ff38d9da1ae34f6aa048539b199cc;hpb=52386be756e113f20502f181d780aecc38cbb66a
INTERNAL_SIZE_T size = chunksize (p);
if (size > psm1 + sizeof (struct malloc_chunk))
{
/* See whether the chunk contains at least one unused page. */
char *paligned_mem = (char *) (((uintptr_t) p
+ sizeof (struct malloc_chunk)
+ psm1) & ~psm1);
assert ((char *) chunk2mem (p) + 4 * SIZE_SZ <= paligned_mem);
assert ((char *) p + size > paligned_mem);
/* This is the size we could potentially free. */
size -= paligned_mem - (char *) p;
if (size > psm1)
madvise (paligned_mem, size & ~psm1, MADV_DONTNEED);
}
这是总两种用法之一的madvise
与glibc的MADV_DONTNEED
现在,一个用于(shrink_heap
)的堆顶部部分和其他被标记的任何块(mtrim
)的:http://code.metager.de/source/search?q=MADV_DONTNEED&path=%2Fgnu%2Fglibc%2Fmalloc%2F&project=gnu
H A D arena.c 643 __madvise ((char *) h + new_size, diff, MADV_DONTNEED);
H A D malloc.c 4535 __madvise (paligned_mem, size & ~psm1, MADV_DONTNEED);
我们可以用这个简单的C程序(test_malloc_trim.c
)和strace
/ltrace
测试malloc_trim
:
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <malloc.h>
int main()
{
int *m1,*m2,*m3,*m4;
printf("%s\n","Test started");
m1=(int*)malloc(20000);
m2=(int*)malloc(40000);
m3=(int*)malloc(80000);
m4=(int*)malloc(10000);
// check that all arrays are allocated on the heap and not with mmap
printf("1:%p 2:%p 3:%p 4:%p\n", m1, m2, m3, m4);
// free 40000 bytes in the middle
free(m2);
// call trim (same result with 2000 or 2000000 argument)
malloc_trim(0);
// call some syscall to find this point in the strace output
sleep(1);
free(m1);
free(m3);
free(m4);
// malloc_stats(); malloc_info(0, stdout);
return 0;
}
gcc test_malloc_trim.c -o test_malloc_trim
,strace ./test_malloc_trim
write(1, "Test started\n", 13Test started
) = 13
brk(0) = 0xcca000
brk(0xcef000) = 0xcef000
write(1, "1:0xcca010 2:0xccee40 3:0xcd8a90"..., 441:0xcca010 2:0xccee40 3:0xcd8a90 4:0xcec320
) = 44
madvise(0xccf000, 36864, MADV_DONTNEED) = 0
...
nanosleep({1, 0}, 0x7ffffafbfff0) = 0
brk(0xceb000) = 0xceb000
所以,有madvise
与MADV_DONTNEED
为malloc_trim(0)
后9页调用时,在堆的中间有40008个字节的空洞。
现在有一种将堆中间的孔返回到操作系统的方法:MADV_DONTNEED(有时是MADV_FREE):http://code.metager.de/source/xref/gnu/glibc/malloc/malloc.c #4535'mtrim(mstate av,size_t pad)... __madvise(paligned_mem,size&〜psm1,MADV_DONTNEED);'。带有这种标志的“madvise”标志着应用程序不需要页面,OS可能会销毁它们的数据并取消映射物理空间;下一次访问页面可能会生成页面错误以将虚拟页面重新映射到物理空间。 – osgx 2017-02-16 11:48:45
代码添加在https://sourceware.org/git/?p=glibc.git;a=commit;f=malloc/malloc.c;h=68631c8eb92ff38d9da1ae34f6aa048539b199cc 68631c8eb92ff38d9da1ae34f6aa048539b199cc“(mTRIm):此外迭代所有空闲块和使用madvise为所有包含至少一个内存页的块释放内存。“ - \t Ulrich Drepper 2007年12月16日(glibc 2.9) – osgx 2017-02-16 11:53:19