Re: [PATCH 09/14] mm, compaction: Ignore the fragmentation avoidance boost for isolation and compaction

Mel Gorman
2018-12-18T14:29:59Z

On Tue, Dec 18, 2018 at 02:58:33PM +0100, Vlastimil Babka wrote:
> On 12/18/18 2:51 PM, Mel Gorman wrote:
> > On Tue, Dec 18, 2018 at 01:36:42PM +0100, Vlastimil Babka wrote:
> >> On 12/15/18 12:03 AM, Mel Gorman wrote:
> >>> When pageblocks get fragmented, watermarks are artifically boosted to pages
> >>> are reclaimed to avoid further fragmentation events. However, compaction
> >>> is often either fragmentation-neutral or moving movable pages away from
> >>> unmovable/reclaimable pages. As the actual watermarks are preserved,
> >>> allow compaction to ignore the boost factor.
> >>
> >> Right, I should have realized that when reviewing the boost patch. I
> >> think it would be useful to do the same change in
> >> __compaction_suitable() as well. Compaction has its own "gap".
> >>
> >
> > That gap is somewhat static though so I'm a bit more wary of it. However,
>
> Well, watermark boost is dynamic, but based on allocations stealing from
> other migratetypes, not reflecting compaction chances of success.
>

True.

> > the check in __isolate_free_page looks too agressive. We isolate in
> > units of COMPACT_CLUSTER_MAX yet the watermark check there is based on
> > the allocation request. That means for THP that we check if 512 pages
> > can be allocated when only somewhere between 1 and 32 is needed for that
> > compaction cycle to complete. Adjusting that might be more appropriate?
>
> AFAIU the code in __isolate_free_page() reflects that if there's less
> than 512 free pages gap, we might form a high-order page for THP but
> won't be able to allocate it afterwards due to watermark.

Yeah but it used to be a lot more important when watermark checking for
high-orders was very different. Now, if the watermark is met for order-0
and a large enough free page is allocated, the allocation succeeds so
it's a lot less relevant than it used to be. kswapd will still run in
the background for order-0 if necessary so a heavy watermark check there
doesn't really help.

--
Mel Gorman
SUSE Labs