Messages in this thread | | | Date | Fri, 01 Aug 2014 10:51:07 +0200 | From | Vlastimil Babka <> | Subject | Re: [PATCH v5 02/14] mm, compaction: defer each zone individually instead of preferred zone |
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On 07/30/2014 06:22 PM, Vlastimil Babka wrote: > On 07/29/2014 11:12 AM, Vlastimil Babka wrote: >> On 07/29/2014 08:38 AM, Joonsoo Kim wrote: >>> >>> I still don't understand why defer_compaction() is needed here. >>> defer_compaction() is intended for not struggling doing compaction on >>> the zone where we already have tried compaction and found that it >>> isn't suitable for compaction. Allocation failure doesn't tell us >>> that we have tried compaction for all the zone range so we shouldn't >>> make a decision here to defer compaction on this zone carelessly. >> >> OK I can remove that, it should make the code nicer anyway. > > Weird, that removal of this defer_compaction() call seems ho have > quadrupled compact_stall and compact_fail counts. The scanner pages > counters however increased by only 10% so that could indicate the > problem is occuring only in a small zone such as DMA. Could be another > case of mismatch between watermark checking in compaction and > allocation? Perhaps the lack of proper classzone_idx in the compaction > check? Sigh.
Yep so it was the DMA zone returning COMPACT_PARTIAL from the compaction_suitable() check done at the very beginning of compact_zone(). The meaning of that is "the allocation should succeed without compaction", so compaction is not done at all. Yet the COMPACT_PARTIAL return value means it counts as a stall, even with the patch that doesn't count COMPACT_SKIPPED as stalls. The watermark check in try_to_compact_pages() also apparently succeeds as the compaction is not being deferred. With deferral removed from __alloc_pages_direct_compact(), this zone will be attempted uselessly each time, and deferred_compaction is practically never reported back.
So for now I think it would be best to leave the defer_compaction() call in __alloc_pages_direct_compact() as it is. Fixing this in a better way would require more investigation (I guess the lack of classzone_idx in compaction makes the difference for the watermark checks here) and another patch(es), which I'll attempt, but I don't want to further grow this series with new patches right now.
>> I also agree >> with the argument "for all the zone range" and I also realized that it's >> not (both before and after this patch) really the case. I planned to fix >> that in the future, but I can probably do it now. >> The plan is to call defer_compaction() only when compaction returned >> COMPACT_COMPLETE (and not COMPACT_PARTIAL) as it means the whole zone >> was scanned. Otherwise there will be bias towards the beginning of the >> zone in the migration scanner - compaction will be deferred half-way and >> then cached pfn's might be reset when it restarts, and the rest of the >> zone won't be scanned at all. > > Hm despite expectations, this didn't seem to make much difference. But > maybe there will be once I have some idea what happened to those stalls.
Yeah, so it doesn't matter here if I call defer_compaction() with only COMPACT_COMPLETE returned from compact_zone(). The whole thing is only done when watermarks check fails, and it doesn't for the DMA zone.
>>> Thanks. >>> >> >
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