Re: [PATCH 18/19] mm: sched: numa: Implement constant, per task Working Set Sampling (WSS) rate

[Rik van Riel <riel@redhat.com>]

On 11/06/2012 04:14 AM, Mel Gorman wrote:
> From: Peter Zijlstra <a.p.zijlstra@chello.nl>
>
> Note: The scan period is much larger than it was in the original patch.
> 	The reason was because the system CPU usage went through the roof
> 	with a sample period of 100ms but it was unsuitable to have a
> 	situation where a large process could stall for excessively long
> 	updating pte_numa. This may need to be tuned again if a placement
> 	policy converges too slowly.
>
> Previously, to probe the working set of a task, we'd use
> a very simple and crude method: mark all of its address
> space PROT_NONE.
>
> That method has various (obvious) disadvantages:
>
>   - it samples the working set at dissimilar rates,
>     giving some tasks a sampling quality advantage
>     over others.
>
>   - creates performance problems for tasks with very
>     large working sets
>
>   - over-samples processes with large address spaces but
>     which only very rarely execute
>
> Improve that method by keeping a rotating offset into the
> address space that marks the current position of the scan,
> and advance it by a constant rate (in a CPU cycles execution
> proportional manner). If the offset reaches the last mapped
> address of the mm then it then it starts over at the first
> address.
>
> The per-task nature of the working set sampling functionality in this tree
> allows such constant rate, per task, execution-weight proportional sampling
> of the working set, with an adaptive sampling interval/frequency that
> goes from once per 2 seconds up to just once per 32 seconds.  The current
> sampling volume is 256 MB per interval.
>
> As tasks mature and converge their working set, so does the
> sampling rate slow down to just a trickle, 256 MB per 8
> seconds of CPU time executed.
>
> This, beyond being adaptive, also rate-limits rarely
> executing systems and does not over-sample on overloaded
> systems.
>
> [ In AutoNUMA speak, this patch deals with the effective sampling
>    rate of the 'hinting page fault'. AutoNUMA's scanning is
>    currently rate-limited, but it is also fundamentally
>    single-threaded, executing in the knuma_scand kernel thread,
>    so the limit in AutoNUMA is global and does not scale up with
>    the number of CPUs, nor does it scan tasks in an execution
>    proportional manner.
>
>    So the idea of rate-limiting the scanning was first implemented
>    in the AutoNUMA tree via a global rate limit. This patch goes
>    beyond that by implementing an execution rate proportional
>    working set sampling rate that is not implemented via a single
>    global scanning daemon. ]
>
> [ Dan Carpenter pointed out a possible NULL pointer dereference in the
>    first version of this patch. ]
>
> Based-on-idea-by: Andrea Arcangeli <aarcange@redhat.com>
> Bug-Found-By: Dan Carpenter <dan.carpenter@oracle.com>
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
> Cc: Linus Torvalds <torvalds@linux-foundation.org>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
> Cc: Andrea Arcangeli <aarcange@redhat.com>
> Cc: Rik van Riel <riel@redhat.com>
> [ Wrote changelog and fixed bug. ]
> Signed-off-by: Ingo Molnar <mingo@kernel.org>
> Signed-off-by: Mel Gorman <mgorman@suse.de>

Reviewed-by: Rik van Riel <riel@redhat.com>

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