Re: hugepage compaction causes performance drop

[Joonsoo Kim <iamjoonsoo.kim@lge.com>]

On Fri, Nov 20, 2015 at 11:06:46AM +0100, Vlastimil Babka wrote:
> On 11/20/2015 10:33 AM, Aaron Lu wrote:
> >On 11/20/2015 04:55 PM, Aaron Lu wrote:
> >>On 11/19/2015 09:29 PM, Vlastimil Babka wrote:
> >>>+CC Andrea, David, Joonsoo
> >>>
> >>>On 11/19/2015 10:29 AM, Aaron Lu wrote:
> >>>>The vmstat and perf-profile are also attached, please let me know if you
> >>>>need any more information, thanks.
> >>>
> >>>Output from vmstat (the tool) isn't much useful here, a periodic "cat
> >>>/proc/vmstat" would be much better.
> >>
> >>No problem.
> >>
> >>>The perf profiles are somewhat weirdly sorted by children cost (?), but
> >>>I noticed a very high cost (46%) in pageblock_pfn_to_page(). This could
> >>>be due to a very large but sparsely populated zone. Could you provide
> >>>/proc/zoneinfo?
> >>
> >>Is a one time /proc/zoneinfo enough or also a periodic one?
> >
> >Please see attached, note that this is a new run so the perf profile is
> >a little different.
> >
> >Thanks,
> >Aaron
> 
> Thanks.
> 
> DMA32 is a bit sparse:
> 
> Node 0, zone    DMA32
>   pages free     62829
>         min      327
>         low      408
>         high     490
>         scanned  0
>         spanned  1044480
>         present  495951
>         managed  479559
> 
> Since the other zones are much larger, probably this is not the
> culprit. But tracepoints should tell us more. I have a theory that
> updating free scanner's cached pfn doesn't happen if it aborts due
> to need_resched() during isolate_freepages(), before hitting a valid
> pageblock, if the zone has a large hole in it. But zoneinfo doesn't

Today, I revisit this issue and yes, I think that your theory is
right. isolate_freepages() will not update cached pfn until call
isolate_freepages_block(). So, if there are many holes or many
unmovable pageblocks or !isolation_suitable() pageblocks, cached pfn
will not updated if compaction aborts due to need_resched(). zoneinfo
shows that there is not much holes so I guess that this problem is caused
by latter two cases.

It is better to update cached pfn in these cases. Although I don't see
your solution yet, I guess it will help here.

Thanks.

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