Re: [PATCH v5 07/14] mm, compaction: khugepaged should not give up due to need_resched()

[David Rientjes]

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.

> 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.

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.
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