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SubjectRe: [PATCH] mm, compaction: direct freepage allocation for async direct compaction
On Thu, Nov 23, 2017 at 10:15:17PM +0100, Vlastimil Babka wrote:
> On 11/23/2017 03:08 PM, Mel Gorman wrote:
> >
> > 1. This indirectly uses __rmqueue to allocate a MIGRATE_MOVABLE page but
> > that is allowed to fallback to other pageblocks and potentially even
> > steal them. I think it's very bad that an attempt to defragment can
> > itself indirectly cause more fragmentation events by altering pageblocks.
> > Please consider using __rmqueue_fallback (within alloc_pages_zone of
> > course)
>
> Agree. That should be simpler to do in the new version of the patch and
> its __rmqueue_compact(). It might happen though that we deplete all free
> pages on movable lists. Then the only option is to fallback to others
> (aborting compaction in that case makes little sense IMHO) but perhaps
> without the usual fallback heuristics of trying to steal the largest
> page, whole pageblock etc.
>

I also should have said __rmqueue_smallest. It was __rmqueue_fallback
that needed to be avoided :(

> > 2. One of the reasons a linear scanner was used was because I wanted the
> > possibility that MIGRATE_UNMOVABLE and MIGRATE_RECLAIMABLE pageblocks
> > would also be scanned and we would avoid future fragmentation events.
>
> Hmm are you talking about the free scanner here, or the migration
> scanner? The free scanner generally avoids these pageblocks, by the way
> of suitable_migration_target() (and I think it used to be like this all
> the time). Only recently an override of cc->ignore_block_suitable was added.
>

Migration scanner.

> > This had a lot of overhead and was reduced since but it's still a
> > relevant problem. Granted, this patch is not the correct place to fix
> > that issue and potential solutions have been discussed elsewhere. However,
> > this patch potentially means that never happens. It doesn't necessarily
> > kill the patch but the long-lived behaviour may be that no compaction
> > occurs because all the MIGRATE_MOVABLE pageblocks are full and you'll
> > either need to reclaim to fix it or we'll need kcompactd to migration
> > MIGRATE_MOVABLE pages from UNMOVABLE and RECLAIMABLE pageblocks out
> > of band.
> >
> > For THP, this point doesn't matter but if you need this patch for
> > high-order allocations for network buffers then at some point, you
> > really will have to clean out those pageblocks or it'll degrade.
>
> Hmm this really reads like about the migration scanner. That one is
> unchanged by this patch, there is still a linear scanner. In fact, it
> gets better, because now it can see the whole zone, not just the first
> 1/3 - 1/2 until it meets the free scanner (my past observations). And
> some time ago the async direct compaction was adjusted so that it only
> scans the migratetype matching the allocation (see
> suitable_migration_source()). So to some extent, the cleaning already
> happens.
>

It is true that the migration scanner may see a subset of the zone but
it was important to avoid a previous migration source becoming a
migration target. The problem is completely different when using the
freelist as a hint.

> > 3. Another reason a linear scanner was used was because we wanted to
> > clear entire pageblocks we were migrating from and pack the target
> > pageblocks as much as possible. This was to reduce the amount of
> > migration required overall even though the scanning hurts. This patch
> > takes MIGRATE_MOVABLE pages from anywhere that is "not this pageblock".
> > Those potentially have to be moved again and again trying to randomly
> > fill a MIGRATE_MOVABLE block. Have you considered using the freelists
> > as a hint? i.e. take a page from the freelist, then isolate all free
> > pages in the same pageblock as migration targets? That would preserve
> > the "packing property" of the linear scanner.
> >
> > This would increase the amount of scanning but that *might* be offset by
> > the number of migrations the workload does overall. Note that migrations
> > potentially are minor faults so if we do too many migrations, your
> > workload may suffer.
>
> I have considered the "freelist as a hint", but I'm kinda sceptical
> about it, because with increasing uptime reclaim should be freeing
> rather random pages, so finding some free page in a pageblock doesn't
> mean there would be more free pages there than in the other pageblocks?
>

True, but randomly selecting pageblocks based on the contents of the
freelist is not better. If a pageblock has limited free pages then it'll
be filled quickly and not used as a hint in the future.

> Instead my plan is to make the migration scanner smarter by expanding
> the "skip_on_failure" feature in isolate_migratepages_block(). The
> scanner should not even start isolating if the block ahead contains a
> page that's not free or lru-isolatable/PageMovable. The current
> "look-ahead" is effectively limited by COMPACT_CLUSTER_MAX (32) isolated
> pages followed by a migration, after which the scanner might immediately
> find a non-migratable page, so if it was called for a THP, that work has
> been wasted.
>

That's also not necessarily true because there is a benefit to moving
pages from unmovable blocks to avoid fragmentation later.

> > 5. Consider two processes A and B compacting at the same time with A_s
> > and A_t being the source pageblock and target pageblock that process
> > A is using and B_s/B_t being B's pageblocks. Nothing prevents A_s ==
> > B_t and B_s == A_t. Maybe it rarely happens in practice but it was one
> > problem the linear scanner was meant to avoid.
>
> I hope that ultimately this problem is not worse than the existing
> problem where B would not be compacting, but simply allocating the pages
> that A just created... Maybe if the "look-ahead" idea turns out to have
> high enough success rate of really creating the high-order page where it
> decides to isolate and migrate (which probably depends mostly on the
> migration failure rate?) we could resurrect the old idea of doing a
> pageblock isolation (MIGRATE_ISOLATE) beforehand. That would block all
> interference.
>

Pageblock bits similar to the skip bit could also be used to limit the
problem.

> > I can't shake the feeling I had another concern when I started this
> > email but then forgot it before I got to the end so it can't be that
> > important :(.
>
> Thanks a lot for the feedback. I totally see how the approach of two
> linear scanners makes many things simpler, but seems we are now really
> paying too high a price for the free page scanning. So hopefully there
> is a way out, although not a simple one.


While the linear scanner solved some problems, I do agree that the overhead
is too high today. However, I think it can be fixed by using the freelist
as a hint, possibly combined with a pageblock bit to avoid hitting some
problems the linear scanner avoids. I do think there is a way out even
though I also think that the complexity would not have been justified
when compaction was first introduced -- partially because it was not clear
the time that the overhead was an issue but mostly because compaction was
initially a huge-page-only thing.

--
Mel Gorman
SUSE Labs

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