Re: [PATCH 00/10] mm: PCP high auto-tuning

[Mel Gorman]

On Wed, Sep 20, 2023 at 09:41:18AM -0700, Andrew Morton wrote:
> On Wed, 20 Sep 2023 14:18:46 +0800 Huang Ying <ying.huang@xxxxxxxxx> wrote:
> 
> > The page allocation performance requirements of different workloads
> > are often different.  So, we need to tune the PCP (Per-CPU Pageset)
> > high on each CPU automatically to optimize the page allocation
> > performance.
> 
> Some of the performance changes here are downright scary.
> 
> I've never been very sure that percpu pages was very beneficial (and
> hey, I invented the thing back in the Mesozoic era).  But these numbers
> make me think it's very important and we should have been paying more
> attention.
> 

FWIW, it is because not only does it avoid lock contention issues, it
avoids excessive splitting/merging of buddies as well as the slower
paths of the allocator. It is not very satisfactory and frankly, the
whole page allocator needs a revisit to account for very large zones but
it is far from a trivial project. PCP just masks the worst of the issues
and replacing it is far harder than tweaking it.

> > The list of patches in series is as follows,
> > 
> >  1 mm, pcp: avoid to drain PCP when process exit
> >  2 cacheinfo: calculate per-CPU data cache size
> >  3 mm, pcp: reduce lock contention for draining high-order pages
> >  4 mm: restrict the pcp batch scale factor to avoid too long latency
> >  5 mm, page_alloc: scale the number of pages that are batch allocated
> >  6 mm: add framework for PCP high auto-tuning
> >  7 mm: tune PCP high automatically
> >  8 mm, pcp: decrease PCP high if free pages < high watermark
> >  9 mm, pcp: avoid to reduce PCP high unnecessarily
> > 10 mm, pcp: reduce detecting time of consecutive high order page freeing
> > 
> > Patch 1/2/3 optimize the PCP draining for consecutive high-order pages
> > freeing.
> > 
> > Patch 4/5 optimize batch freeing and allocating.
> > 
> > Patch 6/7/8/9 implement and optimize a PCP high auto-tuning method.
> > 
> > Patch 10 optimize the PCP draining for consecutive high order page
> > freeing based on PCP high auto-tuning.
> > 
> > The test results for patches with performance impact are as follows,
> > 
> > kbuild
> > ======
> > 
> > On a 2-socket Intel server with 224 logical CPU, we tested kbuild on
> > one socket with `make -j 112`.
> > 
> > 	build time	zone lock%	free_high	alloc_zone
> > 	----------	----------	---------	----------
> > base	     100.0	      43.6          100.0            100.0
> > patch1	      96.6	      40.3	     49.2	      95.2
> > patch3	      96.4	      40.5	     11.3	      95.1
> > patch5	      96.1	      37.9	     13.3	      96.8
> > patch7	      86.4	       9.8	      6.2	      22.0
> > patch9	      85.9	       9.4	      4.8	      16.3
> > patch10	      87.7	      12.6	     29.0	      32.3
> 
> You're seriously saying that kbuild got 12% faster?
> 
> I see that [07/10] (autotuning) alone sped up kbuild by 10%?
> 
> Other thoughts:
> 
> - What if any facilities are provided to permit users/developers to
>   monitor the operation of the autotuning algorithm?
> 

Not that I've seen yet but I'm still in part of the series. It could be
monitored with tracepoints but it can also be inferred from lock
contention issue. I think it would only be meaningful to developers to
monitor this closely, at least that's what I think now. Honestly, I'm
more worried about potential changes in behaviour depending on the exact
CPU and cache implementation than I am about being able to actively
monitor it.

> - I'm not seeing any Documentation/ updates.  Surely there are things
>   we can tell users?
> 
> - This:
> 
>   : It's possible that PCP high auto-tuning doesn't work well for some
>   : workloads.  So, when PCP high is tuned by hand via the sysctl knob,
>   : the auto-tuning will be disabled.  The PCP high set by hand will be
>   : used instead.
> 
>   Is it a bit hacky to disable autotuning when the user alters
>   pcp-high?  Would it be cleaner to have a separate on/off knob for
>   autotuning?
> 

It might be but tuning the allocator is very specific and once we
introduce that tunable, we're probably stuck with it. I would prefer to
see it introduced if and only if we have to.

>   And how is the user to determine that "PCP high auto-tuning doesn't work
>   well" for their workload?

Not easily. It may manifest as variable lock contention issues when the
workload is at a steady state but that would increase the pressure to
split the allocator away from being zone-based entirely instead of tweaking
PCP further.

-- 
Mel Gorman
SUSE Labs