Re: [PATCH v2 00/10] try to reduce fragmenting fallbacks

From: Vlastimil Babka
Date: Wed Feb 15 2017 - 09:30:12 EST

On 02/13/2017 12:07 PM, Mel Gorman wrote:
> On Fri, Feb 10, 2017 at 06:23:33PM +0100, Vlastimil Babka wrote:
> By and large, I like the series, particularly patches 7 and 8. I cannot
> make up my mind about the RFC patches 9 and 10 yet. Conceptually they
> seem sound but they are much more far reaching than the rest of the
> series.
> It would be nice if patches 1-8 could be treated in isolation with data
> on the number of extfrag events triggered, time spent in compaction and
> the success rate. Patches 9 and 10 are tricy enough that they would need
> data per patch where as patches 1-8 should be ok with data gathered for
> the whole series.

I've got the results with mmtests stress-highalloc modified to do
GFP_KERNEL order-4 allocations, on 4.9 with "mm, vmscan: fix zone
balance check in prepare_kswapd_sleep" (without that, kcompactd indeed
wasn't woken up) on UMA machine with 4GB memory. There were 5 repeats of
each run, as the extfrag stats are quite volatile (note the stats below
are sums, not averages, as it was less perl hacking for me).

Success rate are the same, already high due to the low order. THP and
compaction stats also roughly the same. The extfrag stats (a bit
modified/expanded wrt. vanilla mmtests):

(the patches are stacked, and I haven't measured the non-functional-changes
patches separately)
base patch 2 patch 3 patch 4 patch 7 patch 8
Page alloc extfrag event 11734984 11769620 11485185 13029676 13312786 13939417
Extfrag fragmenting 11729231 11763921 11479301 13024101 13307281 13933978
Extfrag fragmenting for unmovable 87848 84906 76328 78613 66025 59261
Extfrag fragmenting unmovable placed with movable 8298 7367 5865 8479 6440 5928
Extfrag fragmenting for reclaimable 11636074 11673657 11397642 12940253 13236444 13869509
Extfrag fragmenting reclaimable placed with movable 389283 362396 330855 374292 390700 415478
Extfrag fragmenting for movable 5309 5358 5331 5235 4812 5208

Going in order, patch 3 might be some improvement wrt polluting
(movable) pageblocks with unmovable, hopefully not noise.

Results for patch 4 ("count movable pages when stealing from pageblock")
are really puzzling me, as it increases the number of fragmenting events
for reclaimable allocations, implicating "reclaimable placed with (i.e.
falling back to) unmovable" (which is not listed separately above, but
follows logically from "reclaimable placed with movable" not changing
that much). I really wonder why is that. The patch effectively only
changes the decision to change migratetype of a pageblock, it doesn't
affect the actual stealing decision (which is always true for
RECLAIMABLE anyway, see can_steal_fallback()). Moreover, since we can't
distinguish UNMOVABLE from RECLAIMABLE when counting, good_pages is 0
and thus even the decision to change pageblock migratetype shouldn't be
changed by the patch for this case. I must recheck the implementation...

Patch 7 could be cautiously labeled as improvement for reduction of
"Fragmenting for unmovable" events, which would be perfect as that was
the intention. For reclaimable it looks worse, but probably just within
noise. Same goes for Patch 8, although the apparent regression for
reclaimable looks even worse there.