Re: [PATCH 16/19] mm: numa: Add pte updates, hinting and migration stats

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

On 11/06/2012 04:14 AM, Mel Gorman wrote:
> It is tricky to quantify the basic cost of automatic NUMA placement in a
> meaningful manner. This patch adds some vmstats that can be used as part
> of a basic costing model.
>
> u    = basic unit = sizeof(void *)
> Ca   = cost of struct page access = sizeof(struct page) / u
> Cpte = Cost PTE access = Ca
> Cupdate = Cost PTE update = (2 * Cpte) + (2 * Wlock)
> 	where Cpte is incurred twice for a read and a write and Wlock
> 	is a constant representing the cost of taking or releasing a
> 	lock
> Cnumahint = Cost of a minor page fault = some high constant e.g. 1000
> Cpagerw = Cost to read or write a full page = Ca + PAGE_SIZE/u
> Ci = Cost of page isolation = Ca + Wi
> 	where Wi is a constant that should reflect the approximate cost
> 	of the locking operation
> Cpagecopy = Cpagerw + (Cpagerw * Wnuma) + Ci + (Ci * Wnuma)
> 	where Wnuma is the approximate NUMA factor. 1 is local. 1.2
> 	would imply that remote accesses are 20% more expensive
>
> Balancing cost = Cpte * numa_pte_updates +
> 		Cnumahint * numa_hint_faults +
> 		Ci * numa_pages_migrated +
> 		Cpagecopy * numa_pages_migrated
>
> Note that numa_pages_migrated is used as a measure of how many pages
> were isolated even though it would miss pages that failed to migrate. A
> vmstat counter could have been added for it but the isolation cost is
> pretty marginal in comparison to the overall cost so it seemed overkill.
>
> The ideal way to measure automatic placement benefit would be to count
> the number of remote accesses versus local accesses and do something like
>
> 	benefit = (remote_accesses_before - remove_access_after) * Wnuma
>
> but the information is not readily available. As a workload converges, the
> expection would be that the number of remote numa hints would reduce to 0.
>
> 	convergence = numa_hint_faults_local / numa_hint_faults
> 		where this is measured for the last N number of
> 		numa hints recorded. When the workload is fully
> 		converged the value is 1.
>
> This can measure if the placement policy is converging and how fast it is
> doing it.
>
> Signed-off-by: Mel Gorman <mgorman@suse.de>

I'm skipping the ACKing of the policy patches, which
appear to be meant to be placeholders for a "real"
policy.  However, you have a few more mechanism patches
left in the series, which would be required regardless
of what policy gets merged, so ...

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

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