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SubjectRe: [PATCH 07/10] mm, page_alloc: Delete the zonelist_cache
On Mon, Sep 21, 2015 at 11:52:39AM +0100, Mel Gorman wrote:
> The zonelist cache (zlc) was introduced to skip over zones that were
> recently known to be full. This avoided expensive operations such as the
> cpuset checks, watermark calculations and zone_reclaim. The situation
> today is different and the complexity of zlc is harder to justify.
>
> 1) The cpuset checks are no-ops unless a cpuset is active and in general
> are a lot cheaper.
>
> 2) zone_reclaim is now disabled by default and I suspect that was a large
> source of the cost that zlc wanted to avoid. When it is enabled, it's
> known to be a major source of stalling when nodes fill up and it's
> unwise to hit every other user with the overhead.
>
> 3) Watermark checks are expensive to calculate for high-order
> allocation requests. Later patches in this series will reduce the cost
> of the watermark checking.
>
> 4) The most important issue is that in the current implementation it
> is possible for a failed THP allocation to mark a zone full for order-0
> allocations and cause a fallback to remote nodes.
>
> The last issue could be addressed with additional complexity but as the
> benefit of zlc is questionable, it is better to remove it. If stalls
> due to zone_reclaim are ever reported then an alternative would be to
> introduce deferring logic based on a timeout inside zone_reclaim itself
> and leave the page allocator fast paths alone.
>
> The impact on page-allocator microbenchmarks is negligible as they don't
> hit the paths where the zlc comes into play. Most page-reclaim related
> workloads showed no noticeable difference as a result of the removal.
>
> The impact was noticeable in a workload called "stutter". One part uses a
> lot of anonymous memory, a second measures mmap latency and a third copies
> a large file. In an ideal world the latency application would not notice
> the mmap latency. On a 2-node machine the results of this patch are
>
> stutter
> 4.3.0-rc1 4.3.0-rc1
> baseline nozlc-v4
> Min mmap 20.9243 ( 0.00%) 20.7716 ( 0.73%)
> 1st-qrtle mmap 22.0612 ( 0.00%) 22.0680 ( -0.03%)
> 2nd-qrtle mmap 22.3291 ( 0.00%) 22.3809 ( -0.23%)
> 3rd-qrtle mmap 25.2244 ( 0.00%) 25.2396 ( -0.06%)
> Max-90% mmap 48.0995 ( 0.00%) 28.3713 ( 41.02%)
> Max-93% mmap 52.5557 ( 0.00%) 36.0170 ( 31.47%)
> Max-95% mmap 55.8173 ( 0.00%) 47.3163 ( 15.23%)
> Max-99% mmap 67.3781 ( 0.00%) 70.1140 ( -4.06%)
> Max mmap 24447.6375 ( 0.00%) 12915.1356 ( 47.17%)
> Mean mmap 33.7883 ( 0.00%) 27.7944 ( 17.74%)
> Best99%Mean mmap 27.7825 ( 0.00%) 25.2767 ( 9.02%)
> Best95%Mean mmap 26.3912 ( 0.00%) 23.7994 ( 9.82%)
> Best90%Mean mmap 24.9886 ( 0.00%) 23.2251 ( 7.06%)
> Best50%Mean mmap 22.0157 ( 0.00%) 22.0261 ( -0.05%)
> Best10%Mean mmap 21.6705 ( 0.00%) 21.6083 ( 0.29%)
> Best5%Mean mmap 21.5581 ( 0.00%) 21.4611 ( 0.45%)
> Best1%Mean mmap 21.3079 ( 0.00%) 21.1631 ( 0.68%)
>
> Note that the maximum stall latency went from 24 seconds to 12 which is still
> bad but an improvement. The milage varies considerably 2-node machine on an
> earlier test went from 494 seconds to 47 seconds and a 4-node machine that
> tested an earlier version of this patch went from a worst case stall time of
> 6 seconds to 67ms. The nature of the benchmark is inherently unpredictable
> as it is hammering the system and the milage will vary between machines.
>
> There is a secondary impact with potentially more direct reclaim because
> zones are now being considered instead of being skipped by zlc. In this
> particular test run it did not occur so will not be described. However,
> in at least one test the following was observed
>
> 1. Direct reclaim rates were higher. This was likely due to direct reclaim
> being entered instead of the zlc disabling a zone and busy looping.
> Busy looping may have the effect of allowing kswapd to make more
> progress and in some cases may be better overall. If this is found then
> the correct action is to put direct reclaimers to sleep on a waitqueue
> and allow kswapd make forward progress. Busy looping on the zlc is even
> worse than when the allocator used to blindly call congestion_wait().
>
> 2. There was higher swap activity as direct reclaim was active.
>
> 3. Direct reclaim efficiency was lower. This is related to 1 as more
> scanning activity also encountered more pages that could not be
> immediately reclaimed
>
> In that case, the direct page scan and reclaim rates are noticeable but
> it is not considered a problem for a few reasons
>
> 1. The test is primarily concerned with latency. The mmap attempts are also
> faulted which means there are THP allocation requests. The ZLC could
> cause zones to be disabled causing the process to busy loop instead
> of reclaiming. This looks like elevated direct reclaim activity but
> it's the correct action to take based on what processes requested.
>
> 2. The test hammers reclaim and compaction heavily. The number of successful
> THP faults is highly variable but affects the reclaim stats. It's not a
> realistic or reasonable measure of page reclaim activity.
>
> 3. No other page-reclaim intensive workload that was tested showed a problem.
>
> 4. If a workload is identified that benefitted from the busy looping then it
> should be fixed by having direct reclaimers sleep on a wait queue until
> woken by kswapd instead of busy looping. We had this class of problem before
> when congestion_waits() with a fixed timeout was a brain damaged decision
> but happened to benefit some workloads.
>
> If a workload is identified that relied on the zlc to busy loop then it
> should be fixed correctly and have a direct reclaimer sleep on a waitqueue
> until woken by kswapd.
>
> Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
> Acked-by: David Rientjes <rientjes@google.com>
> Acked-by: Christoph Lameter <cl@linux.com>
> Acked-by: Vlastimil Babka <vbabka@suse.cz>
> Acked-by: Michal Hocko <mhocko@suse.com>
> ---
> include/linux/mmzone.h | 74 -----------------
> mm/page_alloc.c | 212 -------------------------------------------------
> 2 files changed, 286 deletions(-)

This patch and its results look great!

And I agree, should this affect the balance between kswapd and direct
reclaim, it should be fixed explicitely and not rely on something as
unrelated as the zonelist cache.

Acked-by: Johannes Weiner <hannes@cmpxchg.org>


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