Re: [RFC PATCH 00/19] Cleanup and optimise the page allocator V2

Zhang, Yanmin
2009-03-09T01:50:50Z

On Fri, 2009-03-06 at 13:03 +0000, Mel Gorman wrote:
> On Fri, Mar 06, 2009 at 10:39:18AM +0100, Ingo Molnar wrote:
> >
> > * Lin Ming <ming.m.lin@intel.com> wrote:
> >
> > > Thanks, I have used "perfstat -s" to collect cache misses
> > > data.
> > >
> > > 2.6.29-rc7-tip: tip/perfcounters/core (b5e8acf)
> > > 2.6.29-rc7-tip-mg2: v2 patches applied to tip/perfcounters/core
> > >
> > > I collected 5 times netperf UDP-U-4k data with and without
> > > mg-v2 patches applied to tip/perfcounters/core on a 4p
> > > quad-core tigerton machine, as below "value" means UDP-U-4k
> > > test result.
> > >
> > > 2.6.29-rc7-tip
> > > ---------------
> > > value cache misses CPU migrations cachemisses/migrations
> > > 5329.71 391094656 1710 228710
> > > 5641.59 239552767 2138 112045
> > > 5580.87 132474745 2172 60992
> > > 5547.19 86911457 2099 41406
> > > 5626.38 196751217 2050 95976
> > >
> > > 2.6.29-rc7-tip-mg2
> > > -------------------
> > > value cache misses CPU migrations cachemisses/migrations
> > > 4749.80 649929463 1132 574142
> > > 4327.06 484100170 1252 386661
> > > 4649.51 374201508 1489 251310
> > > 5655.82 405511551 1848 219432
> > > 5571.58 90222256 2159 41788
> > >
> > > Lin Ming
> >
> > Hm, these numbers look really interesting and give us insight
> > into this workload. The workload is fluctuating but by measuring
> > 3 metrics at once instead of just one we see the following
> > patterns:
> >
> > - Less CPU migrations means more cache misses and less
> > performance.
> >
>
> I also happen to know that V2 was cache unfriendly in a number of
> respects. I've been trying to address it in V3 but still the netperf
> performance in general is being very tricky even though profiles tell me
> the page allocator is lighter and incurring fewer cache misses.
>
> (aside, thanks for saying how you were running netperf. It allowed me to
> take shortcuts writing the automation as I knew what parameters to use)
The script chooses to bind client/server to cores of different physical cpu.
You could also try:
1) no-binding;
2) Start CPU_NUM clients;

>
> Here is the results from one x86-64 machine running an unreleased version
> of the patchset
>
> Netperf UDP_STREAM Comparison
> ----------------------------
> clean opt-palloc diff
> UDP_STREAM-64 68.63 73.15 6.18%
> UDP_STREAM-128 149.77 144.33 -3.77%
> UDP_STREAM-256 264.06 280.18 5.75%
> UDP_STREAM-1024 1037.81 1058.61 1.96%
> UDP_STREAM-2048 1790.33 1906.53 6.09%
> UDP_STREAM-3312 2671.34 2744.38 2.66%
> UDP_STREAM-4096 2722.92 2910.65 6.45%
> UDP_STREAM-8192 4280.14 4314.00 0.78%
> UDP_STREAM-16384 5384.13 5606.83 3.97%
> Netperf TCP_STREAM Comparison
> ----------------------------
> clean opt-palloc diff
> TCP_STREAM-64 180.09 204.59 11.98%
> TCP_STREAM-128 297.45 812.22 63.38%
> TCP_STREAM-256 1315.20 1432.74 8.20%
> TCP_STREAM-1024 2544.73 3043.22 16.38%
> TCP_STREAM-2048 4157.76 4351.28 4.45%
> TCP_STREAM-3312 4254.53 4790.56 11.19%
> TCP_STREAM-4096 4773.22 4932.61 3.23%
> TCP_STREAM-8192 4937.03 5453.58 9.47%
> TCP_STREAM-16384 6003.46 6183.74 2.92%
>
> WOooo, more or less awesome. Then here are the results of a second x86-64
> machine
>
> Netperf UDP_STREAM Comparison
> ----------------------------
> clean opt-palloc diff
> UDP_STREAM-64 106.50 106.98 0.45%
> UDP_STREAM-128 216.39 212.48 -1.84%
> UDP_STREAM-256 425.29 419.12 -1.47%
> UDP_STREAM-1024 1433.21 1449.20 1.10%
> UDP_STREAM-2048 2569.67 2503.73 -2.63%
> UDP_STREAM-3312 3685.30 3603.15 -2.28%
> UDP_STREAM-4096 4019.05 4252.53 5.49%
> UDP_STREAM-8192 6278.44 6315.58 0.59%
> UDP_STREAM-16384 7389.78 7162.91 -3.17%
> Netperf TCP_STREAM Comparison
> ----------------------------
> clean opt-palloc diff
> TCP_STREAM-64 694.90 674.47 -3.03%
> TCP_STREAM-128 1160.13 1159.26 -0.08%
> TCP_STREAM-256 2016.35 2018.03 0.08%
> TCP_STREAM-1024 4619.41 4562.86 -1.24%
> TCP_STREAM-2048 5001.08 5096.51 1.87%
> TCP_STREAM-3312 5235.22 5276.18 0.78%
> TCP_STREAM-4096 5832.15 5844.42 0.21%
> TCP_STREAM-8192 6247.71 6287.93 0.64%
> TCP_STREAM-16384 7987.68 7896.17 -1.16%
>
> Much less awesome and the cause of much frowny face and contemplation as to
> whether I'd be much better off hitting the bar for a tasty beverage or 10.
>
> I'm trying to pin down why there are such large differences between machines
> but it's something with the machine themselves as the results between runs
> is fairly consistent. Annoyingly, the second machine showed good results
> for kernbench (allocator heavy), sysbench (not allocator heavy), was more
> or less the same for hackbench but regressed tbench and netperf even though
> the page allocator overhead was less. I'm doing something screwy with cache
> but don't know what it is yet.
>
> netperf is being run on different CPUs and is possibly maximising the amount
> of cache bounces incurred by the page allocator as it splits and merges
> buddies. I'm experimenting with the idea of delaying bulk PCP frees but it's
> also possible the network layer is having trouble with cache line bounces when
> the workload is run over localhost and my modifications are changing timings.
Ingo's analysis is on the right track. Both netperf and tbench have dependency on
process scheduler. Perhaps V2 has some impact on scheduler?

>
> > The lowest-score runs had the lowest CPU migrations count,
> > coupled with a high amount of cachemisses.
> >
> > This _probably_ means that in this workload migrations are
> > desired: the sooner two related tasks migrate to the same CPU
> > the better. If they stay separate (migration count is low) then
> > they interact with each other from different CPUs, creating a
> > lot of cachemisses and reducing performance.
> >
> > You can reduce the migration barrier of the system by enabling
> > CONFIG_SCHED_DEBUG=y and setting sched_migration_cost to zero:
> >
> > echo 0 > /proc/sys/kernel/sched_migration_cost
> >
> > This will hurt other workloads - but if this improves the
> > numbers then it proves that what this particular workload wants
> > is easy migrations.
> >
> > Now the question is, why does the mg2 patchset reduce the number
> > of migrations? It might not be an inherent property of the mg2
> > patches: maybe just unlucky timings push the workload across
> > sched_migration_cost.
> >
> > Setting sched_migration_cost to either zero or to a very high
> > value and repeating the test will eliminate this source of noise
> > and will tell us about other properties of the mg2 patchset.
> >
> > There might be other effects i'm missing. For example what kind
> > of UDP transport is used - localhost networking? That means that
> > sender and receiver really wants to be coupled strongly and what
> > controls this workload is whether such a 'pair' of tasks can
> > properly migrate to the same CPU.