Re: [PATCH 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 Thu, Jul 11, 2024 at 2:44 PM Huang, Ying <ying.huang@intel.com> wrote:
>>
>> Yafang Shao <laoar.shao@gmail.com> writes:
>>
>> > On Wed, Jul 10, 2024 at 10:51 AM Huang, Ying <ying.huang@intel.com> wrote:
>> >>
>> >> Yafang Shao <laoar.shao@gmail.com> writes:
>> >>
>> >> > The configuration parameter PCP_BATCH_SCALE_MAX poses challenges for
>> >> > quickly experimenting with specific workloads in a production environment,
>> >> > particularly when monitoring latency spikes caused by contention on the
>> >> > zone->lock. To address this, a new sysctl parameter vm.pcp_batch_scale_max
>> >> > is introduced as a more practical alternative.
>> >>
>> >> In general, I'm neutral to the change.  I can understand that kernel
>> >> configuration isn't as flexible as sysctl knob.  But, sysctl knob is ABI
>> >> too.
>> >>
>> >> > To ultimately mitigate the zone->lock contention issue, several suggestions
>> >> > have been proposed. One approach involves dividing large zones into multi
>> >> > smaller zones, as suggested by Matthew[0], while another entails splitting
>> >> > the zone->lock using a mechanism similar to memory arenas and shifting away
>> >> > from relying solely on zone_id to identify the range of free lists a
>> >> > particular page belongs to[1]. However, implementing these solutions is
>> >> > likely to necessitate a more extended development effort.
>> >>
>> >> Per my understanding, the change will hurt instead of improve zone->lock
>> >> contention.  Instead, it will reduce page allocation/freeing latency.
>> >
>> > I'm quite perplexed by your recent comment. You introduced a
>> > configuration that has proven to be difficult to use, and you have
>> > been resistant to suggestions for modifying it to a more user-friendly
>> > and practical tuning approach. May I inquire about the rationale
>> > behind introducing this configuration in the beginning?
>>
>> Sorry, I don't understand your words.  Do you need me to explain what is
>> "neutral"?
>
> No, thanks.
> After consulting with ChatGPT, I received a clear and comprehensive
> explanation of what "neutral" means, providing me with a better
> understanding of the concept.
>
> So, can you explain why you introduced it as a config in the beginning ?

I think that I have explained it in the commit log of commit
52166607ecc9 ("mm: restrict the pcp batch scale factor to avoid too long
latency").  Which introduces the config.

Sysctl knob is ABI, which needs to be maintained forever.  Can you
explain why you need it?  Why cannot you use a fixed value after initial
experiments.

Best Regards,
Huang, Ying