On 07/22/2015 01:07 AM, David Rientjes wrote:
On Tue, 21 Jul 2015, Vlastimil Babka wrote:
Khugepaged benefits from the periodic memory compaction being done
immediately before it attempts to compact memory, and that may be lost
with a de-coupled approach like this.
Meant to say "before it attempts to allocate a hugepage", but it seems you
understood that :)
Right :)
That could be helped with waking up khugepaged after kcompactd is successful
in making a hugepage available.
I don't think the criteria for waking up khugepaged should become any more
complex beyond its current state, which is impacted by two different
tunables, and whether it actually has memory to scan. During this
additional wakeup, you'd also need to pass kcompactd's node and only do
local khugepaged scanning since there's no guarantee khugepaged can
allocate on all nodes when one kcompactd defragments memory.
Keeping track of the nodes where hugepage allocations are expected to
succeed is already done in this series. "local khugepaged scanning" is
unfortunately not possible in general, since the node that will be used
for a given pmd is not known until half of pte's (or more) are scanned.
I think
coupling these two would be too complex and not worth it.
It wouldn't be that complex (see above), and go away if khugepaged
scanning is converted to deferred task work. In that case it's also
possible to assume that it's only worth touching memory local to the
task, so if that node indicates no available hugepages, the scanning can
be skipped.
Also in your rfc you propose the compaction
period to be 15 minutes, while khugepaged wakes up each 10 (or 30) seconds by
default for the scanning and collapsing, so only fraction of the work is
attempted right after the compaction anyway?
The rfc actually proposes the compaction period to be 0, meaning it's
disabled, but suggests in the changelog that we have seen a reproducible
benefit with the period of 15m.
Ah, right.
I'm not concerned about scan_sleep_millisecs here, if khugepaged was able
to successfully allocate in its last scan. I'm only concerned with
alloc_sleep_millisecs which defaults to 60000. I think it would be
unfortunate if kcompactd were to free a pageblock, and then khugepaged
waits for 60s before allocating.
Don't forget that khugepaged has to find a suitable pmd first, which can
take much longer than 60s. It might be rescanning address spaces that
have no candidates, or processes that are sleeping and wouldn't benefit
from THP. Another potential advantage for doing the scanning and
collapses in task context...
Hm reports of even not-so-high-order allocation failures occur from time to
time. Some might be from atomic context, but some are because compaction just
can't help due to the unmovable fragmentation. That's mostly a guess, since
such detailed information isn't there, but I think Joonsoo did some
experiments that confirmed this.
If it's unmovable fragmentation, then any periodic synchronous memory
compaction isn't going to help either.
It can help if it moves away movable pages out of unmovable pageblocks,
so the following unmovable allocations can be served from those
pageblocks and not fallback to pollute another movable pageblock. Even
better if this is done (kcompactd woken up) in response to such
fallback, where unmovable page falls to a partially filled movable
pageblock. Stuffing also this into khugepaged would be really a stretch.
Joonsoo proposed another daemon for that in
https://lkml.org/lkml/2015/4/27/94 but extending kcompactd would be a
very natural way for this.
The page allocator already does
MIGRATE_SYNC_LIGHT compaction on its second pass and that will terminate
when a high-order page is available. If it is currently failing, then I
don't see the benefit of synchronous memory compaction over all memory
that would substantially help this case.
The sync compaction is no longer done for THP page faults, so if there's
no other source of the sync compaction, system can fragment over time
and then it might be too late when the need comes.
Also effects on the fragmentation are evaluated when making changes to
compaction, see e.g. http://marc.info/?l=linux-mm&m=143634369227134&w=2
In the past it has prevented changes that would improve latency of direct
compaction. They might be possible if there was a reliable source of more
thorough periodic compaction to counter the not-so-thorough direct compaction.
Hmm, I don't think we have to select one to the excusion of the other. I
don't think that because khugepaged may do periodic synchronous memory
compaction (to eventually remove direct compaction entirely from the page
fault path, since we have checks in the page allocator that specifically
do that)
That would be nice for the THP page faults, yes. Or maybe just change
the default for thp "defrag" tunable to "madvise".
that we can't do background memory compaction elsewhere. I think
it would be trivial to schedule a workqueue in the page allocator when
MIGRATE_ASYNC compaction fails for a high-order allocation on a node and
to have that local compaction done in the background.
I think pushing compaction in a workqueue would meet a bigger resistance
than new kthreads. It could be too heavyweight for this mechanism and
what if there's suddenly lots of allocations in parallel failing and
scheduling the work items? So if we do it elsewhere, I think it's best
as kcompactd kthreads and then why would we do it also in khugepaged?
I guess a broader input than just us two would help :)
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