Re: [mm, thp] 85b9f46e8e: vm-scalability.throughput -8.7% regression

[Huang, Ying]

David Rientjes <rientjes@xxxxxxxxxx> writes:

> On Tue, 20 Oct 2020, Huang, Ying wrote:
>
>> >> =========================================================================================
>> >> compiler/cpufreq_governor/kconfig/rootfs/runtime/size/tbox_group/test/testcase/ucode:
>> >>   gcc-9/performance/x86_64-rhel-8.3/debian-10.4-x86_64-20200603.cgz/300s/1T/lkp-skl-fpga01/lru-shm/vm-scalability/0x2006906
>> >> 
>> >> commit: 
>> >>   dcdf11ee14 ("mm, shmem: add vmstat for hugepage fallback")
>> >>   85b9f46e8e ("mm, thp: track fallbacks due to failed memcg charges separately")
>> >> 
>> >> dcdf11ee14413332 85b9f46e8ea451633ccd60a7d8c 
>> >> ---------------- --------------------------- 
>> >>        fail:runs  %reproduction    fail:runs
>> >>            |             |             |    
>> >>           1:4           24%           2:4     perf-profile.calltrace.cycles-pp.sync_regs.error_entry.do_access
>> >>           3:4           53%           5:4     perf-profile.calltrace.cycles-pp.error_entry.do_access
>> >>           9:4          -27%           8:4     perf-profile.children.cycles-pp.error_entry
>> >>           4:4          -10%           4:4     perf-profile.self.cycles-pp.error_entry
>> >>          %stddev     %change         %stddev
>> >>              \          |                \  
>> >>     477291            -9.1%     434041        vm-scalability.median
>> >>   49791027            -8.7%   45476799        vm-scalability.throughput
>> >>     223.67            +1.6%     227.36        vm-scalability.time.elapsed_time
>> >>     223.67            +1.6%     227.36        vm-scalability.time.elapsed_time.max
>> >>      50364 ±  6%     +24.1%      62482 ± 10%  vm-scalability.time.involuntary_context_switches
>> >>       2237            +7.8%       2412        vm-scalability.time.percent_of_cpu_this_job_got
>> >>       3084           +18.2%       3646        vm-scalability.time.system_time
>> >>       1921            -4.2%       1839        vm-scalability.time.user_time
>> >>      13.68            +2.2       15.86        mpstat.cpu.all.sys%
>> >>      28535 ± 30%     -47.0%      15114 ± 79%  numa-numastat.node0.other_node
>> >>     142734 ± 11%     -19.4%     115000 ± 17%  numa-meminfo.node0.AnonPages
>> >>      11168 ±  3%      +8.8%      12150 ±  5%  numa-meminfo.node1.PageTables
>> >>      76.00            -1.6%      74.75        vmstat.cpu.id
>> >>       3626            -1.9%       3555        vmstat.system.cs
>> >>    2214928 ±166%     -96.6%      75321 ±  7%  cpuidle.C1.usage
>> >>     200981 ±  7%     -18.0%     164861 ±  7%  cpuidle.POLL.time
>> >>      52675 ±  3%     -16.7%      43866 ± 10%  cpuidle.POLL.usage
>> >>      35659 ± 11%     -19.4%      28754 ± 17%  numa-vmstat.node0.nr_anon_pages
>> >>    1248014 ±  3%     +10.9%    1384236        numa-vmstat.node1.nr_mapped
>> >>       2722 ±  4%     +10.6%       3011 ±  5%  numa-vmstat.node1.nr_page_table_pages
>> >
>> > I'm not sure that I'm reading this correctly, but I suspect that this just 
>> > happens because of NUMA: memory affinity will obviously impact 
>> > vm-scalability.throughput quite substantially, but I don't think the 
>> > bisected commit can be to be blame.  Commit 85b9f46e8ea4 ("mm, thp: track 
>> > fallbacks due to failed memcg charges separately") simply adds new 
>> > count_vm_event() calls in a couple areas to track thp fallback due to 
>> > memcg limits separate from fragmentation.
>> >
>> > It's likely a question about the testing methodology in general: for 
>> > memory intensive benchmarks, I suggest it is configured in a manner that 
>> > we can expect consistent memory access latency at the hardware level when 
>> > running on a NUMA system.
>> 
>> So you think it's better to bind processes to NUMA node or CPU?  But we
>> want to use this test case to capture NUMA/CPU placement/balance issue
>> too.
>> 
>
> No, because binding to a specific socket may cause other performance 
> "improvements" or "degradations" depending on how fragmented local memory 
> is, or whether or not it's under memory pressure.  Is the system rebooted 
> before testing so that we have a consistent state of memory availability 
> and fragmentation across sockets?

Yes.  System is rebooted before testing (0day uses kexec to accelerate
rebooting).

>> 0day solve the problem in another way.  We run the test case
>> multiple-times and calculate the average and standard deviation, then
>> compare.
>> 
>
> Depending on fragmentation or memory availability, any benchmark that 
> assesses performance may be adversely affected if its results can be 
> impacted by hugepage backing.

Best Regards,
Huang, Ying