diff mbox

Reduce number of mmap calls from __libc_memalign in ld.so

Message ID 20160402153421.GA28788@intel.com
State New
Headers show

Commit Message

H.J. Lu April 2, 2016, 3:34 p.m. UTC 
__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

Mike Frysinger April 2, 2016, 5:33 p.m. UTC | #1 
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
diff mbox

Patch

diff --git a/elf/dl-minimal.c b/elf/dl-minimal.c
index 762e65b..d6f87f1 100644
--- 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);
       page = __mmap (0, nup, PROT_READ|PROT_WRITE,
 		     MAP_ANON|MAP_PRIVATE, -1, 0);
       if (page == MAP_FAILED)