Reduce number of mmap calls from __libc_memalign in ld.so
Commit Message
__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.
Tested on x86-64. OK for master?
H.J.
---
* elf/dl-minimal.c (__libc_memalign): Mmap one extra page.
---
elf/dl-minimal.c | 1 +
1 file changed, 1 insertion(+)
Comments
On 02 Apr 2016 08:34, H.J. Lu wrote:
> __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.
>
> --- a/elf/dl-minimal.c
> +++ b/elf/dl-minimal.c
> @@ -75,6 +75,7 @@ __libc_memalign (size_t align, size_t n)
> return NULL;
> nup = GLRO(dl_pagesize);
> }
> + nup += GLRO(dl_pagesize);
should this be in the else case ?
also the comment above this code needs updating
-mike
@@ -75,6 +75,7 @@ __libc_memalign (size_t align, size_t n)
return NULL;
nup = GLRO(dl_pagesize);
}
+ nup += GLRO(dl_pagesize);
page = __mmap (0, nup, PROT_READ|PROT_WRITE,
MAP_ANON|MAP_PRIVATE, -1, 0);
if (page == MAP_FAILED)