[LKP] [mm] ac5b2c1891: vm-scalability.throughput -61.3% regression
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  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 .
> 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  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
> 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.
>>  https://firstname.lastname@example.org
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