Re: [PATCH v3 3/3] mm/page_alloc: Introduce a new sysctl knob vm.pcp_batch_scale_max

["Huang, Ying" <ying.huang@intel.com>]

Yafang Shao <laoar.shao@gmail.com> writes:

> On Mon, Aug 5, 2024 at 12:36 PM Huang, Ying <ying.huang@intel.com> wrote:
>>
>> Yafang Shao <laoar.shao@gmail.com> writes:
>>
>> > On Mon, Aug 5, 2024 at 11:05 AM Huang, Ying <ying.huang@intel.com> wrote:
>> >>
>> >> Yafang Shao <laoar.shao@gmail.com> writes:
>> >>
>> >> > On Mon, Aug 5, 2024 at 9:41 AM Huang, Ying <ying.huang@intel.com> wrote:
>> >> >>
>> >> >> Yafang Shao <laoar.shao@gmail.com> writes:
>> >> >>
>> >> >> [snip]
>> >> >>
>> >> >> >
>> >> >> > Why introduce a systl knob?
>> >> >> > ===========================
>> >> >> >
>> >> >> > From the above data, it's clear that different CPU types have varying
>> >> >> > allocation latencies concerning zone->lock contention. Typically, people
>> >> >> > don't release individual kernel packages for each type of x86_64 CPU.
>> >> >> >
>> >> >> > Furthermore, for latency-insensitive applications, we can keep the default
>> >> >> > setting for better throughput.
>> >> >>
>> >> >> Do you have any data to prove that the default setting is better for
>> >> >> throughput?  If so, that will be a strong support for your patch.
>> >> >
>> >> > No, I don't. The primary reason we can't change the default value from
>> >> > 5 to 0 across our fleet of servers is that you initially set it to 5.
>> >> > The sysadmins believe you had a strong reason for setting it to 5 by
>> >> > default; otherwise, it would be considered careless for the upstream
>> >> > kernel. I also believe you must have had a solid justification for
>> >> > setting the default value to 5; otherwise, why would you have
>> >> > submitted your patches?
>> >>
>> >> In commit 52166607ecc9 ("mm: restrict the pcp batch scale factor to
>> >> avoid too long latency"), I tried my best to run test on the machines
>> >> available with a micro-benchmark (will-it-scale/page_fault1) which
>> >> exercises kernel page allocator heavily.  From the data in commit,
>> >> larger CONFIG_PCP_BATCH_SCALE_MAX helps throughput a little, but not
>> >> much.  The 99% alloc/free latency can be kept within about 100us with
>> >> CONFIG_PCP_BATCH_SCALE_MAX == 5.  So, we chose 5 as default value.
>> >>
>> >> But, we can always improve the default value with more data, on more
>> >> types of machines and with more types of benchmarks, etc.
>> >>
>> >> Your data suggest smaller default value because you have data to show
>> >> that larger default value has the latency spike issue (as large as tens
>> >> ms) for some practical workloads.  Which weren't tested previously.  In
>> >> contrast, we don't have strong data to show the throughput advantages of
>> >> larger CONFIG_PCP_BATCH_SCALE_MAX value.
>> >>
>> >> So, I suggest to use a smaller default value for
>> >> CONFIG_PCP_BATCH_SCALE_MAX.  But, we may need more test to check the
>> >> data for 1, 2, 3, and 4, in addtion to 0 and 5 to determine the best
>> >> choice.
>> >
>> > Which smaller default value would be better?
>>
>> This depends on further test results.
>
> I believe you agree with me that you can't test all workloads.
>
>>
>> > How can we ensure that other workloads, which we haven't tested, will
>> > work well with this new default value?
>>
>> We cannot.  We can only depends on the data available.  If there are
>> new data available in the future, we can make the change accordingly.
>
> So, your solution is to change the hardcoded value for untested
> workloads and then release the kernel package again?
>
>>
>> > If you have a better default value in mind, would you consider sending
>> > a patch for it? I would be happy to test it with my test case.
>>
>> If you can test the value 1, 2, 3, and 4 with your workload, that will
>> be very helpful!  Both allocation latency and total free time (if
>> possible) are valuable.
>
> You know I can't verify it with all workloads, right?
> You have so much data to verify, which indicates uncertainty about any
> default value. Why not make it tunable and let the user choose the
> value they prefer?

We only make decision based on data available.  In theory, we cannot
test all workloads, because there will be new workloads in the future.
If we have data to show that smaller value will cause performance
regressions for some reasonable workloads, we can make it user tunable.

--
Best Regards,
Huang, Ying