Re: [PATCH 1/6] numa,sched,mm: remove p->numa_migrate_deferred

From: Mel Gorman
Date: Tue Jan 21 2014 - 06:52:39 EST

On Mon, Jan 20, 2014 at 02:21:02PM -0500, riel@xxxxxxxxxx wrote:
> From: Rik van Riel <riel@xxxxxxxxxx>
> Excessive migration of pages can hurt the performance of workloads
> that span multiple NUMA nodes. However, it turns out that the
> p->numa_migrate_deferred knob is a really big hammer, which does
> reduce migration rates, but does not actually help performance.
> Now that the second stage of the automatic numa balancing code
> has stabilized, it is time to replace the simplistic migration
> deferral code with something smarter.
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Mel Gorman <mgorman@xxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Chegu Vinod <chegu_vinod@xxxxxx>
> Signed-off-by: Rik van Riel <riel@xxxxxxxxxx>

When I added a tracepoint to track deferred migration I was surprised how
often it triggered for some workloads. I agree that we want to do something
better because it was a crutch albeit a necessary one at the time.

Note that the knob was not about performance as such, it was about avoiding
worst-case behaviour. We should keep an eye out for bugs that look like
excessive migration on workloads that are not converging. Reintroducing this
hammer would be a last resort for working around the problem.

Finally, the sysctl is documented in Documentation/sysctl/kernel.txt and
this patch should also remove it.

Functionally, the patch looks fine and it's time to reinvestigate if
it's necessary so assuming the documentation gets removed;

Acked-by: Mel Gorman <mgorman@xxxxxxx>

Mel Gorman
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