Messages in this thread | | | Date | Wed, 30 Jul 2014 11:08:28 +0200 | From | Vlastimil Babka <> | Subject | Re: [PATCH v5 07/14] mm, compaction: khugepaged should not give up due to need_resched() |
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On 07/30/2014 12:53 AM, David Rientjes wrote: > On Tue, 29 Jul 2014, Vlastimil Babka wrote: > >>> I think there's two ways to go about it: >>> >>> - allow a single thp fault to be expensive and then rely on deferred >>> compaction to avoid subsequent calls in the near future, or >>> >>> - try to make all thp faults be as least expensive as possible so that >>> the cumulative effect of faulting large amounts of memory doesn't end >>> up with lengthy stalls. >>> >>> Both of these are complex because of the potential for concurrent calls to >>> memory compaction when faulting thp on several cpus. >>> >>> I also think the second point from that email still applies, that we >>> should abort isolating pages within a pageblock for migration once it can >>> no longer allow a cc->order allocation to succeed. >> >> That was the RFC patch 15, I hope to reintroduce it soon. > > Which of the points above are you planning on addressing in another patch? > I think the approach would cause the above to be mutually exclusive > options.
Oh I meant the quick abort of a pageblock that's not going to succeed. That was the RFC patch. As for the single expensive fault + defer vs lots of inexpensive faults, I would favor the latter. I'd rather avoid bug reports such as "It works fine for a while and then we get this weird few seconds of stall", which is exactly what you were dealing with IIRC?
>> You could still test >> it meanwhile to see if you see the same extfrag regression as me. In my tests, >> kswapd/khugepaged wasn't doing enough work to defragment the pageblocks that >> the stress-highalloc benchmark (configured to behave like thp page fault) was >> skipping. >> > > The initial regression that I encountered was on a 128GB machine where > async compaction would cause faulting 64MB of transparent hugepages to > excessively stall and I don't see how kswapd can address this if there's > no memory pressure and khugepaged can address it if it has the default > settings which is very slow.
Hm I see. I have been thinking about somehow connecting compaction with the extfrag (page stealing) events. For example, if it's about to allocate UNMOVABLE/RECLAIMABLE page in a MOVABLE pageblock, then try to compact the pageblock first, which will hopefully free enough of it to have it remarked as UNMOVABLE/RECLAIMABLE and satisfy many such allocations without having to steal from another one.
> Another idea I had is to only do async memory compaction for thp on local > zones and avoid defragmenting remotely since, in my experimentation, > remote thp memory causes a performance degradation over regular pages. If > that solution were to involve zone_reclaim_mode and a test of > node_distance() > RECLAIM_DISTANCE, I think that would be acceptable as > well.
Yes, not compacting remote zones on page fault definitely makes sense. Maybe even without zone_reclaim_mode...
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