Re: mmap() scalability in the presence of the MAP_POPULATE flag

[Michel Lespinasse]

Hi Roman,

On Fri, Jan 4, 2013 at 11:43 PM, Michel Lespinasse <walken@xxxxxxxxxx> wrote:
> On Fri, Jan 4, 2013 at 10:40 PM, Roman Dubtsov <dubtsov@xxxxxxxxx> wrote:
>> - POPULATE_VIA_LOOP -- makes the test populate memory using a loop
>> - POPULATE_VIA_MMAP -- makes the test populate memory via MAP_POPULATE
>>
> Heh, very interesting. As it turns out, the problem gets MUCH worse as
> the number of threads increase.
>
> In the POPULATE_VIA_MMAP, follow_page() is called at 4KB increment
> addresses, and it takes the mm->page_table_lock 511 times out of 512
> (that is, every time it falls within a huge page that's just been
> populated). So all OMP_NUM_THREADS threads are constantly bouncing
> over the mm->page_table_lock, and getting terrible performance as a
> result.

FYI, the patchset I just sent out ("fixes for large mm_populate() and
munlock() operations") takes care of this and brings POPULATE_VIA_MMAP
performance up where it should be.

Thanks a lot for the report, as I hadn't noticed this issue before :)

-- 
Michel "Walken" Lespinasse
A program is never fully debugged until the last user dies.
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