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

[Michal Hocko]

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).

> > 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.
-- 
Michal Hocko
SUSE Labs