lkml.org 
[lkml]   [2014]   [Jul]   [30]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH v5 07/14] mm, compaction: khugepaged should not give up due to need_resched()
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...



\
 
 \ /
  Last update: 2014-07-30 11:41    [W:0.933 / U:0.064 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site