Re: [RFC 2/2] mm: alloc/free depth based PCP high auto-tuning

[Huang, Ying]

Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> writes:

> On Fri, Jul 21, 2023 at 03:28:43PM +0800, Huang, Ying wrote:
>> Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> writes:
>> 
>> > On Wed, Jul 19, 2023 at 01:59:00PM +0800, Huang, Ying wrote:
>> >> > The big remaaining corner case to watch out for is where the sum
>> >> > of the boosted pcp->high exceeds the low watermark.  If that should ever
>> >> > happen then potentially a premature OOM happens because the watermarks
>> >> > are fine so no reclaim is active but no pages are available. It may even
>> >> > be the case that the sum of pcp->high should not exceed *min* as that
>> >> > corner case means that processes may prematurely enter direct reclaim
>> >> > (not as bad as OOM but still bad).
>> >> 
>> >> Sorry, I don't understand this.  When pages are moved from buddy to PCP,
>> >> zone NR_FREE_PAGES will be decreased in rmqueue_bulk().  That is, pages
>> >> in PCP will be counted as used instead of free.  And, in
>> >> zone_watermark_ok*() and zone_watermark_fast(), zone NR_FREE_PAGES is
>> >> used to check watermark.  So, if my understanding were correct, if the
>> >> number of pages in PCP is larger than low/min watermark, we can still
>> >> trigger reclaim.  Whether is my understanding correct?
>> >> 
>> >
>> > You're right, I didn't check the timing of the accounting and all that
>> > occurred to me was "the timing of when watermarks trigger kswapd or
>> > direct reclaim may change as a result of PCP adaptive resizing". Even
>> > though I got the timing wrong, the shape of the problem just changes.
>> > I suspect that excessively large PCP high relative to the watermarks may
>> > mean that reclaim happens prematurely if too many pages are pinned by PCP
>> > pages as the zone free pages approaches the watermark.
>> 
>> Yes.  I think so too.  In addition to reclaim, falling back to remote
>> NUMA node may happen prematurely too.
>> 
>
> Yes, with the added bonus that this is relatively easy to detect from
> the NUMA miss stats. I say "relative" because in a lot of cases, it'll be
> difficult to distinguish from the noise. Hence, it's better to be explicit in
> the change log that the potential problem is known and has been considered.
> That way, if bisect points the finger at adaptive resizing, there will be
> some notes on how to investigate the bug.

Sure.  Will do that.

>> > While disabling the adaptive resizing during reclaim will limit the
>> > worst of the problem, it may still be the case that kswapd is woken
>> > early simply because there are enough CPUs pinning pages in PCP
>> > lists. Similarly, depending on the size of pcp->high and the gap
>> > between the watermarks, it's possible for direct reclaim to happen
>> > prematurely. I could still be wrong because I'm not thinking the
>> > problem through fully, examining the code or thinking about the
>> > implementation. It's simply worth keeping in mind the impact elevated
>> > PCP high values has on the timing of watermarks failing. If it's
>> > complex enough, it may be necessary to have a separate patch dealing
>> > with the impact of elevated pcp->high on watermarks.
>> 
>> Sure.  I will keep this in mind.  We may need to check zone watermark
>> when tuning pcp->high and free some pages from PCP before falling back
>> to other node or reclaiming.
>> 
>
> That would certainly be one option, a cap on adaptive resizing as memory
> gets lower. It's not perfect but ideally the worst-case behaviour would be
> that PCP adaptive sizing returns to existing behaviour when memory usage
> is persistently high and near watermarks within a zone.

--
Best Regards,
Huang, Ying