linux-kernel - Re: [4.17 regression] Performance drop on kernel-4.17 visible on Stream, Linpack and NAS parallel benchmarks

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Date:   Fri, 8 Jun 2018 10:24:51 +0100
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Jirka Hladky <jhladky@...hat.com>
Cc:     Jakub Racek <jracek@...hat.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Len Brown <lenb@...nel.org>, linux-acpi@...r.kernel.org
Subject: Re: [4.17 regression] Performance drop on kernel-4.17 visible on
 Stream, Linpack and NAS parallel benchmarks

On Fri, Jun 08, 2018 at 10:49:03AM +0200, Jirka Hladky wrote:
> Hi Mel,
> 
> automatic NUMA balancing doesn't run long enough to migrate all the
> > memory. That would definitely be the case for STREAM.
> 
> This could explain the behavior we observe. stream is running ~20 seconds
> at the moment. I can easily change the runtime by changing the number of
> iterations. What is the time period when you expect the memory to be fully
> migrated?
> 

Unknown and unknowable. It depends entirely on the reference pattern of
the different threads. If they are fully parallelised with private buffers
that are page-aligned then I expect it to be quick (to pass the 2-reference
filter). If threads are sharing data on a 4K (base page case) or 2M boundary
(THP enabled) then it may take longer as two or more threads will disagree
on what the appropriate placement for a page is.

> I have now checked numastat logs and after 15 seconds I see roughly 80MiB
> out of 200MiB of the allocated memory migrated for each of 10 processes
> which have changed the NUMA CPU node after started. This is on 2
> socket Gold 6126 CPU @ 2.60GHz server with DDR4 2666 MHz. That's 800 MiB of
> memory migrated in 15 seconds which is results in the average migration
> rate of 50MiB/s - is this an expected value?
> 

I expect that to be far short of the capabilities of the machine.
Again, migrations can be delayed indefinitely if threads have buffers
that are not page-aligned (4K or 2M depending).

-- 
Mel Gorman
SUSE Labs