Re: [RFC 1/2] mm: add framework for PCP high auto-tuning

[Huang, Ying]

Michal Hocko <mhocko@xxxxxxxx> writes:

> On Wed 12-07-23 15:45:58, Huang, Ying wrote:
>> Michal Hocko <mhocko@xxxxxxxx> writes:
>> 
>> > On Mon 10-07-23 14:53:24, Huang Ying wrote:
>> >> The page allocation performance requirements of different workloads
>> >> are usually different.  So, we often need to tune PCP (per-CPU
>> >> pageset) high to optimize the workload page allocation performance.
>> >> Now, we have a system wide sysctl knob (percpu_pagelist_high_fraction)
>> >> to tune PCP high by hand.  But, it's hard to find out the best value
>> >> by hand.  And one global configuration may not work best for the
>> >> different workloads that run on the same system.  One solution to
>> >> these issues is to tune PCP high of each CPU automatically.
>> >> 
>> >> This patch adds the framework for PCP high auto-tuning.  With it,
>> >> pcp->high will be changed automatically by tuning algorithm at
>> >> runtime.  Its default value (pcp->high_def) is the original PCP high
>> >> value calculated based on low watermark pages or
>> >> percpu_pagelist_high_fraction sysctl knob.  To avoid putting too many
>> >> pages in PCP, the original limit of percpu_pagelist_high_fraction
>> >> sysctl knob, MIN_PERCPU_PAGELIST_HIGH_FRACTION, is used to calculate
>> >> the max PCP high value (pcp->high_max).
>> >
>> > It would have been very helpful to describe the basic entry points to
>> > the auto-tuning. AFAICS the central place of the tuning is tune_pcp_high
>> > which is called from the freeing path. Why?  Is this really a good place
>> > considering this is a hot path? What about the allocation path? Isn't
>> > that a good spot to watch for the allocation demand? 
>> 
>> Yes.  The main entry point to the auto-tuning is tune_pcp_high().  Which
>> is called from the freeing path because pcp->high is only used by page
>> freeing.  It's possible to call it in allocation path instead.  The
>> drawback is that the pcp->high may be updated a little later in some
>> situations.  For example, if there are many page freeing but no page
>> allocation for quite long time.  But I don't think this is a serious
>> problem.
>
> I consider it a serious flaw in the framework as it cannot cope with the
> transition of the allocation pattern (e.g. increasing the allocation
> pressure).

Sorry, my previous words are misleading.  What I really wanted to say is
that the problem may be just theoretical.  Anyway, I will try to avoid
this problem in the future version.

>> > Also this framework seems to be enabled by default. Is this really
>> > desirable? What about workloads tuning the pcp batch size manually?
>> > Shouldn't they override any auto-tuning?
>> 
>> In the current implementation, the pcp->high will be tuned between
>> original pcp high (default or tuned manually) and the max pcp high (via
>> MIN_PERCPU_PAGELIST_HIGH_FRACTION).  So the high value tuned manually is
>> respected at some degree.
>> 
>> So you think that it's better to disable auto-tuning if PCP high is
>> tuned manually?
>
> Yes, I think this is a much safer option. For two reasons 1) it is less
> surprising to setups which know what they are doing by configuring the
> batching and 2) the auto-tuning needs a way to get disabled in case
> there are pathological patterns in behavior.

OK.

Best Regards,
Huang, Ying