Re: [linux-next:master] [mm] 1111d46b5c: stress-ng.pthread.ops_per_sec -84.3% regression

Yin, Fengwei
2023-12-22T01:06:34Z

On 12/22/2023 2:14 AM, Matthew Wilcox wrote:
> On Thu, Dec 21, 2023 at 10:07:09AM -0800, Yang Shi wrote:
>> On Wed, Dec 20, 2023 at 8:49 PM Matthew Wilcox <willy@infradead.org> wrote:
>>>
>>> On Thu, Dec 21, 2023 at 08:58:42AM +0800, Yin Fengwei wrote:
>>>> Yes. MAP_STACK is also mentioned in manpage of mmap. I did test to
>>>> filter out of the MAP_STACK mapping based on this patch. The regression
>>>> in stress-ng.pthread was gone. I suppose this is kind of safe because
>>>> the madvise call is only applied to glibc allocated stack.
>>>>
>>>>
>>>> But what I am not sure was whether it's worthy to do such kind of change
>>>> as the regression only is seen obviously in micro-benchmark. No evidence
>>>> showed the other regressionsin this report is related with madvise. At
>>>> least from the perf statstics. Need to check more on stream/ramspeed.
>>>
>>> FWIW, we had a customer report a significant performance problem when
>>> inadvertently using 2MB pages for stacks. They were able to avoid it by
>>> using 2044KiB sized stacks ...
>>
>> Thanks for the report. This provided more justification regarding
>> honoring MAP_STACK on Linux. Some applications, for example, pthread,
>> just allocate a fixed size area for stack. This confuses kernel
>> because kernel tell stack by VM_GROWSDOWN | VM_GROWSUP.
>>
>> But I'm still a little confused by why THP for stack could result in
>> significant performance problems. Unless the applications resize the
>> stack quite often.
>
> We didn't delve into what was causing the problem, only that it was
> happening. The application had many threads, so it could have been as
> simple as consuming all the available THP and leaving fewer available
> for other uses. Or it could have been a memory consumption problem;
> maybe the app would only have been using 16-32kB per thread but was
> now using 2MB per thread and if there were, say, 100 threads, that's an
> extra 199MB of memory in use.
One thing I know is related with the memory zeroing. This is from
the perf data in this report:

0.00 +16.7 16.69 ± 7%
perf-profile.calltrace.cycles-pp.clear_page_erms.clear_huge_page.__do_huge_pmd_anonymous_page.__handle_mm_fault.handle_mm_fault

Zeroing 2M memory costs much more CPU than zeroing 16-32KB memory if
there are many threads.

Regards
Yin, Fengwei