linux-kernel - Re: [RFC PATCH 0/6] Use local_lock for pcp protection and reduce stat overhead

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Date:   Wed, 31 Mar 2021 10:17:39 +0200
From:   Jesper Dangaard Brouer <brouer@...hat.com>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     Linux-MM <linux-mm@...ck.org>,
        Linux-RT-Users <linux-rt-users@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Chuck Lever <chuck.lever@...cle.com>,
        Matthew Wilcox <willy@...radead.org>, brouer@...hat.com
Subject: Re: [RFC PATCH 0/6] Use local_lock for pcp protection and reduce
 stat overhead

On Wed, 31 Mar 2021 08:38:05 +0100
Mel Gorman <mgorman@...hsingularity.net> wrote:

> On Tue, Mar 30, 2021 at 08:51:54PM +0200, Jesper Dangaard Brouer wrote:
> > On Mon, 29 Mar 2021 13:06:42 +0100
> > Mel Gorman <mgorman@...hsingularity.net> wrote:
> >   
> > > This series requires patches in Andrew's tree so the series is also
> > > available at
> > > 
> > > git://git.kernel.org/pub/scm/linux/kernel/git/mel/linux.git mm-percpu-local_lock-v1r15
> > > 
> > > tldr: Jesper and Chuck, it would be nice to verify if this series helps
> > > 	the allocation rate of the bulk page allocator. RT people, this
> > > 	*partially* addresses some problems PREEMPT_RT has with the page
> > > 	allocator but it needs review.  
> > 
> > I've run a new micro-benchmark[1] which shows:
> > (CPU: Intel(R) Xeon(R) CPU E5-1650 v4 @ 3.60GHz)
> > 
> > <Editting to focus on arrays>
> > BASELINE
> >  single_page alloc+put: 194 cycles(tsc) 54.106 ns
> > 
> > ARRAY variant: time_bulk_page_alloc_free_array: step=bulk size
> > 
> >  Per elem: 195 cycles(tsc) 54.225 ns (step:1)
> >  Per elem: 127 cycles(tsc) 35.492 ns (step:2)
> >  Per elem: 117 cycles(tsc) 32.643 ns (step:3)
> >  Per elem: 111 cycles(tsc) 30.992 ns (step:4)
> >  Per elem: 106 cycles(tsc) 29.606 ns (step:8)
> >  Per elem: 102 cycles(tsc) 28.532 ns (step:16)
> >  Per elem: 99 cycles(tsc) 27.728 ns (step:32)
> >  Per elem: 98 cycles(tsc) 27.252 ns (step:64)
> >  Per elem: 97 cycles(tsc) 27.090 ns (step:128)
> > 
> > This should be seen in comparison with the older micro-benchmark[2]
> > done on branch mm-bulk-rebase-v5r9.
> > 
> > BASELINE
> >  single_page alloc+put: Per elem: 199 cycles(tsc) 55.472 ns
> > 
> > ARRAY variant: time_bulk_page_alloc_free_array: step=bulk size
> > 
> >  Per elem: 202 cycles(tsc) 56.383 ns (step:1)
> >  Per elem: 144 cycles(tsc) 40.047 ns (step:2)
> >  Per elem: 134 cycles(tsc) 37.339 ns (step:3)
> >  Per elem: 128 cycles(tsc) 35.578 ns (step:4)
> >  Per elem: 120 cycles(tsc) 33.592 ns (step:8)
> >  Per elem: 116 cycles(tsc) 32.362 ns (step:16)
> >  Per elem: 113 cycles(tsc) 31.476 ns (step:32)
> >  Per elem: 110 cycles(tsc) 30.633 ns (step:64)
> >  Per elem: 110 cycles(tsc) 30.596 ns (step:128)
> >   
> 
> Ok, so bulk allocation is faster than allocating single pages, no surprise
> there. Putting the array figures for bulk allocation into tabular format
> and comparing we get;
> 
> Array variant (time to allocate a page in nanoseconds, lower is better)
>         Baseline        Patched
> 1       56.383          54.225 (+3.83%)
> 2       40.047          35.492 (+11.38%)
> 3       37.339          32.643 (+12.58%)
> 4       35.578          30.992 (+12.89%)
> 8       33.592          29.606 (+11.87%)
> 16      32.362          28.532 (+11.85%)
> 32      31.476          27.728 (+11.91%)
> 64      30.633          27.252 (+11.04%)
> 128     30.596          27.090 (+11.46%)
> 
> The series is 11-12% faster when allocating multiple pages.  That's a
> fairly positive outcome and I'll include this in the series leader if
> you have no objections.

That is fine by me to add this to the cover letter.  I like your
tabular format as it makes is easier to compare. If you use the
nanosec measurements and not the cycles, you should state that
this was run on a CPU E5-1650 v4 @ 3.60GHz.  You might notice that the
factor between cycles(tsc) and ns is very close to 3.6.

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer