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

[Huang, Ying]

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.

> 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?

Best Regards,
Huang, Ying