Re: [PATCH 0/4] reintroduce compaction feedback for OOM decisions

From	Vlastimil Babka <vbabka@suse.cz>
Newsgroups	linux.kernel
Subject	Re: [PATCH 0/4] reintroduce compaction feedback for OOM decisions
Date	2016-09-23 13:00 +0200
Message-ID	<skwmZ-2zw-11@gated-at.bofh.it> (permalink)
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On 09/23/2016 10:26 AM, Michal Hocko wrote:
>>  include/linux/compaction.h |  5 +++--
>>  mm/compaction.c            | 44 +++++++++++++++++++++++---------------------
>>  mm/internal.h              |  1 +
>>  mm/vmscan.c                |  6 ++++--
>>  4 files changed, 31 insertions(+), 25 deletions(-)
> 
> This is much more code churn than I expected. I was thiking about it
> some more and I am really wondering whether it actually make any sense
> to check the fragidx for !costly orders. Wouldn't it be much simpler to
> just put it out of the way for those regardless of the compaction
> priority. In other words does this check makes any measurable difference
> for !costly orders?

I've did some stress tests and sampling
/sys/kernel/debug/extfrag/extfrag_index once per second. The lowest
value I've got for order-2 was 0.705. The default threshold is 0.5, so
this would still result in compaction considered as suitable.

But it's sampling so I might not got to the interesting moments, most of
the time it was -1.000 which means the page should be just available.
Also we would be changing behavior for the user-controlled
vm.extfrag_threshold, so I'm not entirely sure about that.

I could probably reduce the churn so that compaction_suitable() doesn't
need a new parameter. We could just skip compaction_suitable() check
from compact_zone() on the highest priority, and go on even without
sufficient free page gap?

Thread

Re: [PATCH 0/4] reintroduce compaction feedback for OOM decisions Michal Hocko <mhocko@kernel.org> - 2016-09-21 19:20 +0200
  Re: [PATCH 0/4] reintroduce compaction feedback for OOM decisions Vlastimil Babka <vbabka@suse.cz> - 2016-09-22 17:20 +0200
    Re: [PATCH 0/4] reintroduce compaction feedback for OOM decisions Michal Hocko <mhocko@kernel.org> - 2016-09-23 10:30 +0200
      Re: [PATCH 0/4] reintroduce compaction feedback for OOM decisions Vlastimil Babka <vbabka@suse.cz> - 2016-09-23 13:00 +0200
        Re: [PATCH 0/4] reintroduce compaction feedback for OOM decisions Michal Hocko <mhocko@kernel.org> - 2016-09-23 14:20 +0200

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