Re: [PATCH v5] Soft limit rework

[Johannes Weiner <hannes@cmpxchg.org>]

On Tue, Jun 18, 2013 at 02:09:39PM +0200, Michal Hocko wrote:
> My primary test case was a parallel kernel build with 2 groups (make
> is running with -j4 with a distribution .config in a separate cgroup
> without any hard limit) on a 8 CPU machine booted with 1GB memory.  I
> was mostly interested in 2 setups. Default - no soft limit set and - and
> 0 soft limit set to both groups.
> The first one should tell us whether the rework regresses the default
> behavior while the second one should show us improvements in an extreme
> case where both workloads are always over the soft limit.

The most interesting test case would be how it behaves if some groups
are over the soft limits while others are not.  I would expect this to
be the most common situation for when soft limits are used.

On the other hand, setting all soft limits to 0 makes every reclaim
invocation do almost only soft reclaim.  What's the point of that?

> /usr/bin/time -v has been used to collect the statistics and each
> configuration had 3 runs after fresh boot without any other load on the
> system.
> 
> base is mmotm-2013-05-09-15-57
> baserebase is mmotm-2013-06-05-17-24-63 + patches from the current mmots
> without slab shrinkers patchset.
> reworkrebase all patches 8 applied on top of baserebase
> 
> * No-limit
> User
> base: min: 1164.94 max: 1169.75 avg: 1168.31 std: 1.57 runs: 6
> baserebase: min: 1169.46 [100.4%] max: 1176.07 [100.5%] avg: 1172.49 [100.4%] std: 2.38 runs: 6
> reworkrebase: min: 1172.58 [100.7%] max: 1177.43 [100.7%] avg: 1175.53 [100.6%] std: 1.91 runs: 6
> System
> base: min: 242.55 max: 245.36 avg: 243.92 std: 1.17 runs: 6
> baserebase: min: 235.36 [97.0%] max: 238.52 [97.2%] avg: 236.70 [97.0%] std: 1.04 runs: 6
> reworkrebase: min: 236.21 [97.4%] max: 239.46 [97.6%] avg: 237.55 [97.4%] std: 1.05 runs: 6
> Elapsed
> base: min: 596.81 max: 620.04 avg: 605.52 std: 7.56 runs: 6
> baserebase: min: 666.45 [111.7%] max: 710.89 [114.7%] avg: 690.62 [114.1%] std: 13.85 runs: 6
> reworkrebase: min: 664.05 [111.3%] max: 701.06 [113.1%] avg: 689.29 [113.8%] std: 12.36 runs: 6
> 
> Elapsed time regressed by 13% wrt. base but it seems that this came from
> baserebase which regressed by the same amount.

Which mmots does this refer to?  We should probably look into a
regression this size...

> * 0-limit
> User
> base: min: 1188.28 max: 1198.54 avg: 1194.10 std: 3.31 runs: 6
> baserebase: min: 1186.17 [99.8%] max: 1196.46 [99.8%] avg: 1189.75 [99.6%] std: 3.41 runs: 6
> reworkrebase: min: 1169.88 [98.5%] max: 1177.84 [98.3%] avg: 1173.50 [98.3%] std: 2.79 runs: 6
> System
> base: min: 248.40 max: 252.00 avg: 250.19 std: 1.38 runs: 6
> baserebase: min: 240.77 [96.9%] max: 246.74 [97.9%] avg: 243.63 [97.4%] std: 2.23 runs: 6
> reworkrebase: min: 235.19 [94.7%] max: 237.43 [94.2%] avg: 236.35 [94.5%] std: 0.86 runs: 6
> Elapsed
> base: min: 759.28 max: 805.30 avg: 784.87 std: 15.45 runs: 6
> baserebase: min: 881.69 [116.1%] max: 938.14 [116.5%] avg: 911.68 [116.2%] std: 19.58 runs: 6
> reworkrebase: min: 667.54 [87.9%] max: 718.54 [89.2%] avg: 695.61 [88.6%] std: 17.16 runs: 6
> 
> System time is slightly better but I wouldn't consider it relevant.
> 
> Elapsed time is more interesting though. baserebase regresses by 16%
> again which is in par with no-limit configuration.
> 
> With the patchset applied we are 11% better in average wrt. to the
> old base but it is important to realize that this is still 76.3% wrt.
> baserebase so the effect of the series is comparable to the previous
> version. Albeit the whole result is worse.
> 
> Page fault statistics tell us at least part of the story:
> Minor
> base: min: 35941845.00 max: 36029788.00 avg: 35986860.17 std: 28288.66 runs: 6
> baserebase: min: 35852414.00 [99.8%] max: 35899605.00 [99.6%] avg: 35874906.83 [99.7%] std: 18722.59 runs: 6
> reworkrebase: min: 35538346.00 [98.9%] max: 35584907.00 [98.8%] avg: 35562362.17 [98.8%] std: 18921.74 runs: 6
> Major
> base: min: 25390.00 max: 33132.00 avg: 29961.83 std: 2476.58 runs: 6
> baserebase: min: 34224.00 [134.8%] max: 45674.00 [137.9%] avg: 41556.83 [138.7%] std: 3595.39 runs: 6
> reworkrebase: min: 277.00 [1.1%] max: 480.00 [1.4%] avg: 384.67 [1.3%] std: 74.67 runs: 6

The changes are big but the test makes no sense to me.

> While the minor faults are within the noise the major faults are reduced
> considerably. This looks like an aggressive pageout during the reclaim
> and that pageout affects the working set presumably. Please note that
> baserebase has even hight number of major page faults than the older
> mmotm trree.
> 
> While this looks as a nice win it is fair to say that there are some
> workloads that actually benefit from reclaim at 0 priority (from
> background reclaim). E.g. an aggressive streaming IO would like to get
> rid of as many pages as possible and do not block on the pages under
> writeback. This can lead to a higher System time but I generally got
> Elapsed which was comparable.
> 
> The following results are from 2 groups configuration on a 8GB machine
> (A running stream IO with 4*TotalMem with 0 soft limit, B runnning a
> mem_eater which consumes TotalMem-1G without any limit).
> System
> base: min: 124.88 max: 136.97 avg: 130.77 std: 4.94 runs: 3
> baserebase: min: 102.51 [82.1%] max: 108.84 [79.5%] avg: 104.81 [80.1%] std: 2.86 runs: 3
> reworkrebase: min: 108.29 [86.7%] max: 121.70 [88.9%] avg: 114.60 [87.6%] std: 5.50 runs: 3
> Elapsed
> base: min: 398.86 max: 412.81 avg: 407.62 std: 6.23 runs: 3
> baserebase: min: 480.92 [120.6%] max: 497.56 [120.5%] avg: 491.46 [120.6%] std: 7.48 runs: 3
> reworkrebase: min: 397.19 [99.6%] max: 462.57 [112.1%] avg: 436.13 [107.0%] std: 28.12 runs: 3

Do you have individual runtimes for both the streamer and "mem_eater"?
Is mem_eater's memory reclaimable?

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