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Re: [PATCH v2 07/10] mm, compaction: restrict async compaction to pageblocks of same migratetype

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  Re: [PATCH v2 07/10] mm, compaction: restrict async compaction to  pageblocks of same migratetype Johannes Weiner <hannes@cmpxchg.org> - 2017-02-14 21:20 +0100
    Re: [PATCH v2 07/10] mm, compaction: restrict async compaction to  pageblocks of same migratetype Vlastimil Babka <vbabka@suse.cz> - 2017-02-17 17:40 +0100
      Re: [PATCH v2 07/10] mm, compaction: restrict async compaction to  pageblocks of same migratetype Johannes Weiner <hannes@cmpxchg.org> - 2017-02-17 19:30 +0100

#1580832 — Re: [PATCH v2 07/10] mm, compaction: restrict async compaction to pageblocks of same migratetype

On Fri, Feb 10, 2017 at 06:23:40PM +0100, Vlastimil Babka wrote:
> The migrate scanner in async compaction is currently limited to MIGRATE_MOVABLE
> pageblocks. This is a heuristic intended to reduce latency, based on the
> assumption that non-MOVABLE pageblocks are unlikely to contain movable pages.
> 
> However, with the exception of THP's, most high-order allocations are not
> movable. Should the async compaction succeed, this increases the chance that
> the non-MOVABLE allocations will fallback to a MOVABLE pageblock, making the
> long-term fragmentation worse.
> 
> This patch attempts to help the situation by changing async direct compaction
> so that the migrate scanner only scans the pageblocks of the requested
> migratetype. If it's a non-MOVABLE type and there are such pageblocks that do
> contain movable pages, chances are that the allocation can succeed within one
> of such pageblocks, removing the need for a fallback. If that fails, the
> subsequent sync attempt will ignore this restriction.
> 
> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>

Yes, IMO we should make the async compaction scanner decontaminate
unmovable blocks. This is because we fall back to other-typed blocks
before we reclaim, so any unmovable blocks that aren't perfectly
occupied will fill with greedy page cache (and order-0 doesn't steal
blocks back to make them compactable again). Subsequent unmovable
higher-order allocations in turn are more likely to fall back and
steal more movable blocks.

As long as we have vastly more movable blocks than unmovable blocks,
continuous page cache turnover will counteract this negative trend -
pages are reclaimed mostly from movable blocks and some unmovable
blocks, while new cache allocations are placed into the freed movable
blocks - slowly moving cache out from unmovable blocks into movable
ones. But that effect is independent of the rate of higher-order
allocations and can be overwhelmed, so I think it makes sense to
involve compaction directly in decontamination.

The thing I'm not entirely certain about is the aggressiveness of this
patch. Instead of restricting the async scanner to blocks of the same
migratetype, wouldn't it be better (in terms of allocation latency) to
simply let it compact *all* block types? Maybe changing it to look at
unmovable blocks is enough to curb cross-contamination. Sure there
will still be some, but now we're matching the decontamination rate to
the rate of !movable higher-order allocations and don't just rely on
the independent cache turnover rate, which during higher-order bursts
might not be high enough to prevent an expansion of unmovable blocks.

Does that make sense?

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#1583588

On 02/14/2017 09:10 PM, Johannes Weiner wrote:
> On Fri, Feb 10, 2017 at 06:23:40PM +0100, Vlastimil Babka wrote:
>> The migrate scanner in async compaction is currently limited to MIGRATE_MOVABLE
>> pageblocks. This is a heuristic intended to reduce latency, based on the
>> assumption that non-MOVABLE pageblocks are unlikely to contain movable pages.
>> 
>> However, with the exception of THP's, most high-order allocations are not
>> movable. Should the async compaction succeed, this increases the chance that
>> the non-MOVABLE allocations will fallback to a MOVABLE pageblock, making the
>> long-term fragmentation worse.
>> 
>> This patch attempts to help the situation by changing async direct compaction
>> so that the migrate scanner only scans the pageblocks of the requested
>> migratetype. If it's a non-MOVABLE type and there are such pageblocks that do
>> contain movable pages, chances are that the allocation can succeed within one
>> of such pageblocks, removing the need for a fallback. If that fails, the
>> subsequent sync attempt will ignore this restriction.
>> 
>> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
> 
> Yes, IMO we should make the async compaction scanner decontaminate
> unmovable blocks. This is because we fall back to other-typed blocks
> before we reclaim,

Which we could change too, patch 9 is a step in that direction.

> so any unmovable blocks that aren't perfectly
> occupied will fill with greedy page cache (and order-0 doesn't steal
> blocks back to make them compactable again).

order-0 allocation can actually steal the block back, the decisions to steal are
based on the order of the free pages in the fallback block, not on the
allocation order. But maybe I'm not sure what exactly you meant here.

> Subsequent unmovable
> higher-order allocations in turn are more likely to fall back and
> steal more movable blocks.

Yes.

> As long as we have vastly more movable blocks than unmovable blocks,
> continuous page cache turnover will counteract this negative trend -
> pages are reclaimed mostly from movable blocks and some unmovable
> blocks, while new cache allocations are placed into the freed movable
> blocks - slowly moving cache out from unmovable blocks into movable
> ones. But that effect is independent of the rate of higher-order
> allocations and can be overwhelmed, so I think it makes sense to
> involve compaction directly in decontamination.

Interesting observation, I agree.

> The thing I'm not entirely certain about is the aggressiveness of this
> patch. Instead of restricting the async scanner to blocks of the same
> migratetype, wouldn't it be better (in terms of allocation latency) to
> simply let it compact *all* block types?

Yes it would help allocation latency, but I'm afraid it will remove most of the
decontamination effect.

> Maybe changing it to look at
> unmovable blocks is enough to curb cross-contamination. Sure there
> will still be some, but now we're matching the decontamination rate to
> the rate of !movable higher-order allocations and don't just rely on
> the independent cache turnover rate, which during higher-order bursts
> might not be high enough to prevent an expansion of unmovable blocks.

The rate of compaction attempts is matched with allocations, but the probability
of compaction scanner being in unmovable block is low when the majority of
blocks are movable. So the decontamination rate is proportional but much smaller.

> Does that make sense?

I guess I can try and look at the stats, but I have doubts.

Thanks for the feedback!

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#1583639

On Fri, Feb 17, 2017 at 05:32:00PM +0100, Vlastimil Babka wrote:
> On 02/14/2017 09:10 PM, Johannes Weiner wrote:
> > On Fri, Feb 10, 2017 at 06:23:40PM +0100, Vlastimil Babka wrote:
> >> The migrate scanner in async compaction is currently limited to MIGRATE_MOVABLE
> >> pageblocks. This is a heuristic intended to reduce latency, based on the
> >> assumption that non-MOVABLE pageblocks are unlikely to contain movable pages.
> >> 
> >> However, with the exception of THP's, most high-order allocations are not
> >> movable. Should the async compaction succeed, this increases the chance that
> >> the non-MOVABLE allocations will fallback to a MOVABLE pageblock, making the
> >> long-term fragmentation worse.
> >> 
> >> This patch attempts to help the situation by changing async direct compaction
> >> so that the migrate scanner only scans the pageblocks of the requested
> >> migratetype. If it's a non-MOVABLE type and there are such pageblocks that do
> >> contain movable pages, chances are that the allocation can succeed within one
> >> of such pageblocks, removing the need for a fallback. If that fails, the
> >> subsequent sync attempt will ignore this restriction.
> >> 
> >> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
> > 
> > Yes, IMO we should make the async compaction scanner decontaminate
> > unmovable blocks. This is because we fall back to other-typed blocks
> > before we reclaim,
> 
> Which we could change too, patch 9 is a step in that direction.

Yep, patch 9 looks good to me too, pending data that confirms it.

> > so any unmovable blocks that aren't perfectly
> > occupied will fill with greedy page cache (and order-0 doesn't steal
> > blocks back to make them compactable again).
> 
> order-0 allocation can actually steal the block back, the decisions to steal are
> based on the order of the free pages in the fallback block, not on the
> allocation order. But maybe I'm not sure what exactly you meant here.

No, that was me misreading the code. Scratch what's in parentheses.

> > The thing I'm not entirely certain about is the aggressiveness of this
> > patch. Instead of restricting the async scanner to blocks of the same
> > migratetype, wouldn't it be better (in terms of allocation latency) to
> > simply let it compact *all* block types?
> 
> Yes it would help allocation latency, but I'm afraid it will remove most of the
> decontamination effect.
> 
> > Maybe changing it to look at
> > unmovable blocks is enough to curb cross-contamination. Sure there
> > will still be some, but now we're matching the decontamination rate to
> > the rate of !movable higher-order allocations and don't just rely on
> > the independent cache turnover rate, which during higher-order bursts
> > might not be high enough to prevent an expansion of unmovable blocks.
> 
> The rate of compaction attempts is matched with allocations, but the probability
> of compaction scanner being in unmovable block is low when the majority of
> blocks are movable. So the decontamination rate is proportional but much smaller.

Yeah, you're right. The unmovable blocks would still expand, we'd just
turn it into a logarithmic curve.

> > Does that make sense?
> 
> I guess I can try and look at the stats, but I have doubts.

I don't insist. Your patch is implementing a good thing, we can just
keep an eye out for a change in allocation latencies before spending
time trying to mitigate a potential non-issue.

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