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Reduce number of mmap calls from __libc_memalign in ld.so

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__libc_memalign in ld.so allocates one page at a time and tries to
optimize consecutive __libc_memalign calls by hoping that the next
mmap is after the current memory allocation.

However, the kernel hands out mmap addresses in top-down order, so
this optimization in practice never happens, with the result that we
have more mmap calls and waste a bunch of space for each __libc_memalign.

This change makes __libc_memalign to mmap one page extra.  Worst case,
the kernel never puts a backing page behind it, but best case it allows
__libc_memalign to operate much much better.  For elf/tst-align --direct,
it reduces number of mmap calls from 12 to 9.

	* elf/dl-minimal.c (__libc_memalign): Mmap one extra page.
This commit is contained in:
H.J. Lu
2016-04-23 06:05:01 -07:00
parent 00277a3f81
commit 2bc983b78c
2 changed files with 9 additions and 7 deletions
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12
elf/dl-minimal.c
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@ -66,15 +66,13 @@ __libc_memalign (size_t align, size_t n)
if (alloc_ptr + n >= alloc_end || n >= -(uintptr_t) alloc_ptr)
{
/* Insufficient space left; allocate another page. */
/* Insufficient space left; allocate another page plus one extra
page to reduce number of mmap calls. */
caddr_t page;
size_t nup = (n + GLRO(dl_pagesize) - 1) & ~(GLRO(dl_pagesize) - 1);
if (__glibc_unlikely (nup == 0))
{
if (n)
return NULL;
nup = GLRO(dl_pagesize);
}
if (__glibc_unlikely (nup == 0 && n != 0))
return NULL;
nup += GLRO(dl_pagesize);
page = __mmap (0, nup, PROT_READ|PROT_WRITE,
MAP_ANON|MAP_PRIVATE, -1, 0);
if (page == MAP_FAILED)