Messages in this thread |  | | From | Vincent Guittot <> | Date | Thu, 27 Oct 2022 18:34:47 +0200 | Subject | Re: [PATCH v5 0/7] Add latency priority for CFS class |
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Hi Prateek,
On Tue, 25 Oct 2022 at 08:36, K Prateek Nayak <kprateek.nayak@amd.com> wrote: > > Hello Vincent, > > I've rerun some tests with a different configuration with more > contention for CPU and I can see a linear behavior. Sharing the > results below. > > On 10/13/2022 8:54 PM, Vincent Guittot wrote: > > > > [..snip..] > >> > >> o Hackbench and Cyclictest in NPS1 configuration > >> > >> perf bench sched messaging -p -t -l 100000 -g 16& > >> cyclictest --policy other -D 5 -q -n -H 20000 > >> > >> ----------------------------------------------------------------------------------------------------------------- > >> |Hackbench | Cyclictest LN = 19 | Cyclictest LN = 0 | Cyclictest LN = -20 | > >> |LN |--------------------------------|---------------------------------|-----------------------------| > >> |v | Min | Avg | Max | Min | Avg | Max | Min | Avg | Max | > >> |--------------|--------|---------|-------------|----------|---------|------------|----------|---------|--------| > >> |0 | 54.00 | 117.00 | 3021.67 | 53.67 | 65.33 | 133.00 | 53.67 | 65.00 | 201.33 | ^ > >> |19 | 50.00 | 100.67 | 3099.33 | 41.00 | 64.33 | 1014.33 | 54.00 | 63.67 | 213.33 | > >> |-20 | 53.00 | 169.00 | 11661.67 | 53.67 | 217.33 | 14313.67 | 46.00 | 61.33 | 236.00 | ^ > >> ----------------------------------------------------------------------------------------------------------------- > > > > The latency results look good with Cyclictest LN:0 and hackbench LN:0. > > 133us max latency. This suggests that your system is not overloaded > > and cyclictest doesn't really compete with others to run. > > Following is the result of running cyclictest alongside hackbench with 32 groups: > > perf bench sched messaging -p -l 100000 -g 32& > cyclictest --policy other -D 5 -q -n -H 20000 > > ---------------------------------------------------------------------------------------------------------- > | Hackbench | Cyclictest LN = 19 | Cyclictest LN = 0 | Cyclictest LN = -20 | > | LN |------------------------------|-------------------------------|---------------------------| > | | Min | Avg | Max | Min | Avg | Max | Min | Avg | Max | > |-------------|--------|---------|-----------|--------|---------|------------|--------|-------|----------| > | 0 | 54.00 | 165.00 | 6899.00 | 22.00 | 85.00 | 3294.00 | 23.00 | 64.00 | 276.00 | > | 19 | 53.00 | 173.00 | 3275.00 | 40.00 | 60.00 | 2276.00 | 13.00 | 59.00 | 94.00 | > | -20 | 52.00 | 293.00 | 19980.00 | 52.00 | 280.00 | 14305.00 | 53.00 | 95.00 | 5713.00 | > ---------------------------------------------------------------------------------------------------------- > > I see a spike for Max in (0, 0) configuration and the latency decreases > monotonically with lower latency nice value.
Your results looks good
> > > > >> > >> o Hackbench and schbench in NPS1 configuration > >> > >> perf bench sched messaging -p -t -l 1000000 -g 16& > >> schebcnh -m 1 -t 64 -s 30s > >> > >> ------------------------------------------------------------------------------------------------------------ > >> |Hackbench | schbench LN = 19 | schbench LN = 0 | schbench LN = -20 | > >> |LN |----------------------------|--------------------------------|-----------------------------| > >> |v | 90th | 95th | 99th | 90th | 95th | 99th | 90th | 95th | 99th | > >> |--------------|--------|--------|----------|---------|---------|------------|---------|----------|--------| > >> |0 | 4264 | 6744 | 15664 | 17952 | 32672 | 55488 | 15088 | 25312 | 50112 | > >> |19 | 288 | 613 | 2332 | 274 | 1015 | 3628 | 374 | 1394 | 4424 | > >> |-20 | 35904 | 47680 | 79744 | 87168 | 113536 | 176896 | 13008 | 21216 | 42560 | ^ > >> ------------------------------------------------------------------------------------------------------------ > > > > For the schbench, your test is 30 seconds long which is longer than > > the duration of perf bench sched messaging -p -t -l 1000000 -g 16& > > > > The duration of the latter varies depending of latency nice value so > > schbench is disturb more time in some cases > > I've rerun this with hackbench running 128 groups alongside schbench > with 2 messenger and 1 worker each. With larger worker count, I still > see non-monotonic behavior in 99th percentile latency of schbench. > I also see number of latency samples collected by schbench to vary > over the 30 second run for different latency nice values which could > also pay a part in seeing the unexpected behavior. For lower worker > count, I see the number of samples collected is similar. Following > is the configuration and the latency reported by schbench: > > perf bench sched messaging -p -t -l 150000 -g 128& > schbench -m 2 -t 1 -s 30s > > Note: In all cases, hackbench runs longer than schbench. > > ------------------------------------------------------------------------------------------------- > | Hackbench | schbench LN = 19 | schbench LN = 0 | schbench LN = -20 | > | LN |----------------------------|---------------------------|--------------------------| > | | 90th | 95th | 99th | 90th | 95th | 99th | 90th | 95th | 99th | > |-----------|--------|--------|----------|--------|--------|---------|--------|--------|--------| > | 0 | 42 | 92 | 2972 | 26 | 49 | 2356 | 9 | 11 | 20 | > | 19 | 35 | 424 | 4984 | 13 | 390 | 5096 | 8 | 10 | 14 | ^ > | -19 | 144 | 3516 | 110208 | 61 | 807 | 34880 | 25 | 39 | 295 | > ------------------------------------------------------------------------------------------------- > > I see 90th and 95th percentile latency decrease monotonically with > latency nice value of schbench (for a fixed latency nice value of > hackbench) but there are cases where 99th percentile latency > reported by schbench may not strictly decrease with lower latency > nice value (Marked with ^) > > Note: Only a small number of bad samples can affect the 99th > percentile latency for the above configuration. The monotonic > behavior in 90th and 95th percentile latency is a good data point > to show latency nice is indeed working as expected.
Yes, I think you are right that the 99th percentile is not stable enough because it can be impacted by a small number of bad samples
> > If there is any specific workload you would like me to run on the > test system, or any additional data you would like for above > workloads, please do let me know.
Thanks a lot for your tests. I'm about to send v6
> > -- > Thanks and Regards, > Prateek
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