Re: [PATCH v3 7/8] mm, compaction: restrict async compaction to pageblocks of same migratetype

[Joonsoo Kim]

On Wed, Mar 29, 2017 at 06:06:41PM +0200, Vlastimil Babka wrote:
> On 03/16/2017 03:14 AM, Joonsoo Kim wrote:
> > On Tue, Mar 07, 2017 at 02:15:44PM +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.
> > 
> > I agree with this idea but have some concerns on this change.
> > 
> > *ASYNC* compaction is designed for reducing latency and this change
> > doesn't fit it. If everything works fine, there is a few movable pages
> > in non-MOVABLE pageblocks as you noted above. Moreover, there is quite
> > less the number of non-MOVABLE pageblock than MOVABLE one so finding
> > non-MOVABLE pageblock takes long time. These two factors will increase
> > the latency of *ASYNC* compaction.
> 
> Right. I lately started to doubt the whole idea of async compaction (for
> non-movable allocations). Seems it's one of the compaction heuristics tuned
> towards the THP usecase. But for non-movable allocations, we just can't have
> both the low latency and long-term fragmentation avoidance. I see now even my
> own skip_on_failure mode in isolate_migratepages_block() as a mistake for
> non-movable allocations.

Why do you think that skip_on_failure mode is a mistake? I think that
it would lead to reduce the latency and it fits the goal of async
compaction.

> 
> Ideally I'd like to make async compaction redundant by kcompactd, and direct
> compaction would mean a serious situation which should warrant sync compaction.
> Meanwhile I see several options to modify this patch
> - async compaction for non-movable allocations will stop doing the
> skip_on_failure mode, and won't restrict the pageblock at all. patch 8/8 will
> make sure that also this kind of compaction finishes the whole pageblock
> - non-movable allocations will skip async compaction completely and go for sync
> compaction immediately

IMO, concept of async compaction is also important for non-movable allocation.
Non-movable allocation is essential for some workload and they hope
the low latency.

Thanks.