Re: [PATCH] nptl: Disable THP on thread stack if it incurs in large RSS usage

[Cupertino Miranda <cupertino.miranda@oracle.com>]

Wilco Dijkstra writes:

> Hi Adhemerval,
>
>>> The question is, under what circumstances are huge pages in stacks beneficial and
>>> in which cases are they not? If have a good answer to that, then we can automatically
>>> do the right thing without needing a tuning.
>>>
>>
>> Afaiu the issue is not whether huge page in stacks is beneficial, but rather
>> when kernel will fall back to default pages anyway which will be just waste
>> cycles.  Another option would be tune the default stack and guard size
>> to avoid this issue, but this might require some more heuristics to find a
>> good spot to avoid too much VMA waste.
>
> The kernel won't fallback to standard pages - like I said, it really allocates a huge
> page when it can (based on alignment of the stack) and that is what causes the
> increase of RSS size. But that's not evidence of an issue.
>
> So the real question is when do huge pages make sense for stacks?
The dual question of when huge pages make sense, is when they don't make
sense.
In current proposal of Adhemerval he is really focusing on the dual
question, and identifying a specific case where they do not make sense.

IMO, huge pages make sense for the stack every time the programmer or the
user of the system so thinks and profiles as a benefit. It really
depends on the use case.  If stack allocation is big enough to make use
of huge pages, it is definetely a benefit to use them to reduce TLB
pressure.

Regarding precisely identifying when hugepages are benefitial this is
really an unpredictable case by case problem and for that matter hard to
predict.
I don't think you can easily find an heuristic that will solve the
properly detect that.

Cheers,
Cupertino

>
> Cheers,
> Wilco