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Re: [RFC PATCH 00/27] Move LRU page reclaim from zones to nodes v2

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On Tue, Feb 23, 2016 at 04:12:01PM -0800, Johannes Weiner wrote:
> > > > > If reclaim can't guarantee a balanced zone utilization then the
> > > > > allocator has to keep doing it. :(
> > > > 
> > > > That's the key issue - the main reason balanced zone utilisation is
> > > > necessary is because we reclaim on a per-zone basis and we must avoid
> > > > page aging anomalies. If we balance such that one eligible zone is above
> > > > the watermark then it's less of a concern.
> > > 
> > > Yes, but only if there can't be extended reclaim stretches that prefer
> > > the pages of a single zone. Yet it looks like this is still possible.
> > 
> > And that is a problem if a workload is dominated by allocations
> > requiring the lower zones. If that is the common case then it's a bust
> > and fair zone allocation policy is still required. That removes one
> > motivation from the series as it leaves some fatness in the page
> > allocator paths.
> 
> With your above explanations, I'm now much more confident this series
> is doing the right thing. Thanks.
> 
> The uncertainty over low-zone allocation floods is real, but what is
> also unsettling is that, where the fair zone code used to shield us
> from kswapd changes, we now open ourselves up to subtle aging bugs,
> which are no longer detectable via the zone placement statistics. And
> we have changed kswapd around quite extensively in the recent past.
> 
> A good metric for aging distortion might be able to mitigate both
> these things. Something to keep an eye on when making changes to
> kswapd, or when analyzing performance problems with a workload.
> 
> What I have in mind is per-classzone counters of reclaim work. If we
> had exact numbers on how much zone-restricted reclaim is being done
> relative to unrestricted scans, we could know how severely the aging
> process is being distorted under any given workload. That would allow
> us to validate these changes here, future kswapd and allocator
> changes, and help us identify problematic workloads.
> 

Ok, that makes me think that I should keep the per-zone pgscan figures
even if they are based on node LRU reclaim because we'll know what the
per-zone scan activity is. We already know how many pages get skipped
when reclaiming for lower zones.

> And maybe we can change the now useless pgalloc_ stats from counting
> zone placement to counting allocation requests by classzone.

I can't convince myself about this one way or the other.

> We could
> then again correlate the number of requests to the amount of work
> done. A high amount of restricted reclaim on behalf of mostly Normal
> allocation requests would detect the bug I described above, e.g. And
> we could generally tell how expensive restricted allocations are in
> the new node-LRUs.
> 

I keep thinking the skip statistics gives us similar data -- it does
not tell us how many restricted allocations that resulted in reclaim was
but we do get an idea of the amount of work caused.

I'll think about it some more and see what I come up with.

-- 
Mel Gorman
SUSE Labs

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[RFC PATCH 00/27] Move LRU page reclaim from zones to nodes v2 Mel Gorman <mgorman@techsingularity.net> - 2016-02-23 16:10 +0100
  [PATCH 06/27] mm, vmscan: Begin reclaiming pages on a per-node basis Mel Gorman <mgorman@techsingularity.net> - 2016-02-23 16:10 +0100
    Re: [PATCH 06/27] mm, vmscan: Begin reclaiming pages on a per-node  basis Johannes Weiner <hannes@cmpxchg.org> - 2016-02-23 20:00 +0100
      Re: [PATCH 06/27] mm, vmscan: Begin reclaiming pages on a per-node  basis Johannes Weiner <hannes@cmpxchg.org> - 2016-02-23 20:10 +0100
      Re: [PATCH 06/27] mm, vmscan: Begin reclaiming pages on a per-node  basis Mel Gorman <mgorman@techsingularity.net> - 2016-02-24 11:30 +0100
  [PATCH 15/27] mm, workingset: Make working set detection node-aware Mel Gorman <mgorman@techsingularity.net> - 2016-02-23 16:10 +0100
    Re: [PATCH 15/27] mm, workingset: Make working set detection  node-aware Johannes Weiner <hannes@cmpxchg.org> - 2016-02-28 17:20 +0100
  [PATCH 13/27] mm, vmscan: Make shrink_node decisions more node-centric Mel Gorman <mgorman@techsingularity.net> - 2016-02-23 16:10 +0100
  [PATCH 09/27] mm, vmscan: Simplify the logic deciding whether kswapd sleeps Mel Gorman <mgorman@techsingularity.net> - 2016-02-23 16:10 +0100
    Re: [PATCH 09/27] mm, vmscan: Simplify the logic deciding whether  kswapd sleeps Johannes Weiner <hannes@cmpxchg.org> - 2016-02-28 17:20 +0100
  [PATCH 10/27] mm, vmscan: By default have direct reclaim only shrink once per node Mel Gorman <mgorman@techsingularity.net> - 2016-02-23 16:10 +0100
    Re: [PATCH 10/27] mm, vmscan: By default have direct reclaim only  shrink once per node Johannes Weiner <hannes@cmpxchg.org> - 2016-02-28 17:20 +0100
  [PATCH 12/27] mm: vmscan: Do not reclaim from kswapd if there is any eligible zone Mel Gorman <mgorman@techsingularity.net> - 2016-02-23 16:10 +0100
  [PATCH 07/27] mm, vmscan: Have kswapd only scan based on the highest requested zone Mel Gorman <mgorman@techsingularity.net> - 2016-02-23 16:20 +0100
    Re: [PATCH 07/27] mm, vmscan: Have kswapd only scan based on the  highest requested zone Johannes Weiner <hannes@cmpxchg.org> - 2016-02-25 23:20 +0100
  [PATCH 19/27] mm: Move most file-based accounting to the node Mel Gorman <mgorman@techsingularity.net> - 2016-02-23 16:20 +0100
  [PATCH 18/27] mm: Rename NR_ANON_PAGES to NR_ANON_MAPPED Mel Gorman <mgorman@techsingularity.net> - 2016-02-23 16:20 +0100
  [PATCH 01/27] mm, page_alloc: Use ac->classzone_idx instead of zone_idx(preferred_zone) Mel Gorman <mgorman@techsingularity.net> - 2016-02-23 16:20 +0100
    Re: [PATCH 01/27] mm, page_alloc: Use ac->classzone_idx instead of  zone_idx(preferred_zone) Johannes Weiner <hannes@cmpxchg.org> - 2016-02-23 19:10 +0100
    Re: [PATCH 01/27] mm, page_alloc: Use ac->classzone_idx instead of  zone_idx(preferred_zone) Vlastimil Babka <vbabka@suse.cz> - 2016-03-03 11:40 +0100
  [PATCH 04/27] mm, vmscan: Move lru_lock to the node Mel Gorman <mgorman@techsingularity.net> - 2016-02-23 16:20 +0100
    Re: [PATCH 04/27] mm, vmscan: Move lru_lock to the node Johannes Weiner <hannes@cmpxchg.org> - 2016-02-23 19:50 +0100
  [PATCH 17/27] mm: Move page mapped accounting to the node Mel Gorman <mgorman@techsingularity.net> - 2016-02-23 16:20 +0100
  [PATCH 03/27] mm, vmstat: Add infrastructure for per-node vmstats Mel Gorman <mgorman@techsingularity.net> - 2016-02-23 16:20 +0100
    Re: [PATCH 03/27] mm, vmstat: Add infrastructure for per-node vmstats Johannes Weiner <hannes@cmpxchg.org> - 2016-02-23 19:20 +0100
      Re: [PATCH 03/27] mm, vmstat: Add infrastructure for per-node vmstats Mel Gorman <mgorman@techsingularity.net> - 2016-02-24 10:20 +0100
  [PATCH 21/27] mm, vmscan: Update classzone_idx if  buffer_heads_over_limit Mel Gorman <mgorman@techsingularity.net> - 2016-02-23 16:20 +0100
  [PATCH 11/27] mm, vmscan: Clear congestion, dirty and need for compaction on a per-node basis Mel Gorman <mgorman@techsingularity.net> - 2016-02-23 16:20 +0100
  [PATCH 16/27] mm, page_alloc: Consider dirtyable memory in terms of  nodes Mel Gorman <mgorman@techsingularity.net> - 2016-02-23 16:20 +0100
    Re: [PATCH 16/27] mm, page_alloc: Consider dirtyable memory in terms  of nodes Johannes Weiner <hannes@cmpxchg.org> - 2016-02-28 17:20 +0100
  [PATCH 20/27] mm: Move vmscan writes and file write accounting to  the node Mel Gorman <mgorman@techsingularity.net> - 2016-02-23 16:20 +0100
  [PATCH 02/27] mm, vmscan: Check if cpusets are enabled during direct reclaim Mel Gorman <mgorman@techsingularity.net> - 2016-02-23 16:20 +0100
    Re: [PATCH 02/27] mm, vmscan: Check if cpusets are enabled during  direct reclaim Johannes Weiner <hannes@cmpxchg.org> - 2016-02-23 19:10 +0100
    Re: [PATCH 02/27] mm, vmscan: Check if cpusets are enabled during  direct reclaim Vlastimil Babka <vbabka@suse.cz> - 2016-03-03 12:40 +0100
  [PATCH 27/27] mm: page_alloc: Cache the last node whose dirty limit  is reached Mel Gorman <mgorman@techsingularity.net> - 2016-02-23 16:30 +0100
  [PATCH 25/27] mm, vmscan: Add classzone information to tracepoints Mel Gorman <mgorman@techsingularity.net> - 2016-02-23 16:30 +0100
  [PATCH 23/27] mm, vmscan: Account in vmstat for pages skipped during  reclaim Mel Gorman <mgorman@techsingularity.net> - 2016-02-23 16:30 +0100
  [PATCH 22/27] mm, vmscan: Only wakeup kswapd once per node for the  requested classzone Mel Gorman <mgorman@techsingularity.net> - 2016-02-23 16:30 +0100
  [PATCH 24/27] mm: Convert zone_reclaim to node_reclaim Mel Gorman <mgorman@techsingularity.net> - 2016-02-23 16:30 +0100
  [PATCH 26/27] mm, page_alloc: Remove fair zone allocation policy Mel Gorman <mgorman@techsingularity.net> - 2016-02-23 16:30 +0100
  Re: [RFC PATCH 00/27] Move LRU page reclaim from zones to nodes v2 Christoph Lameter <cl@linux.com> - 2016-02-23 18:20 +0100
  Re: [PATCH 05/27] mm, vmscan: Move LRU lists to node Johannes Weiner <hannes@cmpxchg.org> - 2016-02-23 19:50 +0100
  Re: [RFC PATCH 00/27] Move LRU page reclaim from zones to nodes v2 Johannes Weiner <hannes@cmpxchg.org> - 2016-02-23 21:10 +0100
    Re: [RFC PATCH 00/27] Move LRU page reclaim from zones to nodes v2 Mel Gorman <mgorman@techsingularity.net> - 2016-02-23 21:20 +0100
      Re: [RFC PATCH 00/27] Move LRU page reclaim from zones to nodes v2 Johannes Weiner <hannes@cmpxchg.org> - 2016-02-23 22:00 +0100
        Re: [RFC PATCH 00/27] Move LRU page reclaim from zones to nodes v2 Mel Gorman <mgorman@techsingularity.net> - 2016-02-23 23:00 +0100
          Re: [RFC PATCH 00/27] Move LRU page reclaim from zones to nodes v2 Johannes Weiner <hannes@cmpxchg.org> - 2016-02-24 01:20 +0100
            Re: [RFC PATCH 00/27] Move LRU page reclaim from zones to nodes v2 Mel Gorman <mgorman@techsingularity.net> - 2016-02-24 11:50 +0100

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