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

[H.J. Lu]

On Sat, Apr 2, 2016 at 10:43 AM, H.J. Lu <hjl.tools@gmail.com> wrote:
> 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.
>

We can just always increment number of pages by one.

-- 
H.J.
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