Re: [PATCH 00/11] cgroup: separate rstat trees

JP Kobryn
2025-02-27T23:44:16Z

On 2/20/25 7:51 AM, Tejun Heo wrote:
> Hello,
>
> On Mon, Feb 17, 2025 at 07:14:37PM -0800, JP Kobryn wrote:
> ...
>> The first experiment consisted of a parent cgroup with memory.swap.max=0
>> and memory.max=1G. On a 52-cpu machine, 26 child cgroups were created and
>> within each child cgroup a process was spawned to encourage the updating of
>> memory cgroup stats by creating and then reading a file of size 1T
>> (encouraging reclaim). These 26 tasks were run in parallel. While this was
>> going on, a custom program was used to open cpu.stat file of the parent
>> cgroup, read the entire file 1M times, then close it. The perf report for
>> the task performing the reading showed that most of the cycles (42%) were
>> spent on the function mem_cgroup_css_rstat_flush() of the control side. It
>> also showed a smaller but significant number of cycles spent in
>> __blkcg_rstat_flush. The perf report for patched kernel differed in that no
>> cycles were spent in these functions. Instead most cycles were spent on
>> cgroup_base_stat_flush(). Aside from the perf reports, the amount of time
>> spent running the program performing the reading of cpu.stats showed a gain
>> when comparing the control to the experimental kernel.The time in kernel
>> mode was reduced.
>>
>> before:
>> real 0m18.449s
>> user 0m0.209s
>> sys 0m18.165s
>>
>> after:
>> real 0m6.080s
>> user 0m0.170s
>> sys 0m5.890s
>>
>> Another experiment on the same host was setup using a parent cgroup with
>> two child cgroups. The same swap and memory max were used as the previous
>> experiment. In the two child cgroups, kernel builds were done in parallel,
>> each using "-j 20". The program from the previous experiment was used to
>> perform 1M reads of the parent cpu.stat file. The perf comparison showed
>> similar results as the previous experiment. For the control side, a
>> majority of cycles (42%) on mem_cgroup_css_rstat_flush() and significant
>> cycles in __blkcg_rstat_flush(). On the experimental side, most cycles were
>> spent on cgroup_base_stat_flush() and no cycles were spent flushing memory
>> or io. As for the time taken by the program reading cpu.stat, measurements
>> are shown below.
>>
>> before:
>> real 0m17.223s
>> user 0m0.259s
>> sys 0m16.871s
>>
>> after:
>> real 0m6.498s
>> user 0m0.237s
>> sys 0m6.220s
>>
>> For the final experiment, perf events were recorded during a kernel build
>> with the same host and cgroup setup. The builds took place in the child
>> node. Control and experimental sides both showed similar in cycles spent
>> on cgroup_rstat_updated() and appeard insignificant compared among the
>> events recorded with the workload.
>
> One of the reasons why the original design used one rstat tree is because
> readers, in addition to writers, can often be correlated too - e.g. You'd
> often have periodic monitoring tools which poll all the major stat files
> periodically. Splitting the trees will likely make those at least a bit
> worse. Can you test how much worse that'd be? ie. Repeat the above tests but
> read all the major stat files - cgroup.stat, cpu.stat, memory.stat and
> io.stat.

Sure. I changed the experiment to read all of these files. It still
showed an improvement in performance. You can see the details in
v2 [0] which I sent out earlier today.

[0]
https://lore.kernel.org/all/20250227215543.49928-1-inwardvessel@gmail.com/
>
> Thanks.
>