[LKP] [mm] ac5b2c1891: vm-scalability.throughput -61.3% regression

Vlastimil Babka vbabka at suse.cz
Fri Dec 14 13:33:42 PST 2018

On 12/14/18 10:04 PM, David Rientjes wrote:
> On Wed, 12 Dec 2018, Vlastimil Babka wrote:


> Reclaim likely could be deterministically useful if we consider a redesign 
> of how migration sources and targets are determined in compaction.
> Has anybody tried a migration scanner that isn't linearly based, rather 
> finding the highest-order free page of the same migratetype, iterating the 
> pages of its pageblock, and using this to determine whether the actual 
> migration will be worthwhile or not?

Not exactly that AFAIK, but a year ago in my series [1] patch 6 made
migration scanner 'prescan' the block of requested order before actually
trying to isolate anything for migration.

> I could imagine pageblock_skip being 
> repurposed for this as the heuristic.
> Finding migration targets would be more tricky, but if we iterate the 
> pages of the pageblock for low-order free pages and find them to be mostly 
> used, that seems more appropriate than just pushing all memory to the end 
> of the zone?

Agree. That was patch 8/8 of the same series [1].

> It would be interesting to know if anybody has tried using the per-zone 
> free_area's to determine migration targets and set a bit if it should be 
> considered a migration source or a migration target.  If all pages for a 
> pageblock are not on free_areas, they are fully used.

Repurposing/adding a new pageblock bit was in my mind to help multiple
compactors not undo each other's work in the scheme where there's no
free page scanner, but I didn't implement it yet.

>>> otherwise we fail and defer because it wasn't able 
>>> to make a hugepage available.
>> Note that THP fault compaction doesn't actually defer itself, which I
>> think is a weakness of the current implementation and hope that patch 3
>> in my series from yesterday [1] can address that. Because defering is
>> the general feedback mechanism that we have for suppressing compaction
>> (and thus associated reclaim) in cases it fails for any reason, not just
>> the one you mention. Instead of inspecting failure conditions in detail,
>> which would be costly, it's a simple statistical approach. And when
>> compaction is improved to fail less, defering automatically also happens
>> less.
> I couldn't get the link to work, unfortunately, I don't think the patch 
> series made it to LKML :/  I do see it archived for linux-mm, though, so 
> I'll take a look, thanks!

Yeah I forgot to Cc: LKML, but you were also in direct To: so you should
have received them directly. Also the abovementioned series, but that's
year ago. My fault for not returning to it after being done with the
Meltdown fun. I hope to do that soon.

[1] https://marc.info/?l=linux-mm&m=151315560308753

>> [1] https://lkml.kernel.org/r/20181211142941.20500-1-vbabka@suse.cz

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