Re: hugepage compaction causes performance drop

[Vlastimil Babka <vbabka@suse.cz>]

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 tell us if the large 
difference between "spanned" and "present"/"managed" is due to a large 
hole, or many smaller holes...

compact_migrate_scanned 1982396
compact_free_scanned 40576943
compact_isolated 2096602
compact_stall 9070
compact_fail 6025
compact_success 3045

So it's struggling to find free pages, no wonder about that. I'm working 
on a series that should hopefully help here, and Joonsoo as well.


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