[PATCH] Reduce number of mmap calls from __libc_memalign in ld.so

[H.J. Lu]

On Sat, Apr 2, 2016 at 10:33 AM, Mike Frysinger <vapier@gentoo.org> wrote:
> 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

You are right.  Here is the updated patch.

-- 
H.J.
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