Re: [PATCH 1/2] mm, numa: Remove rate-limiting of automatic numa balancing migration

[Srikar Dronamraju]

* Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> [2018-10-01 11:05:24]:

> Rate limiting of page migrations due to automatic NUMA balancing was
> introduced to mitigate the worst-case scenario of migrating at high
> frequency due to false sharing or slowly ping-ponging between nodes.
> Since then, a lot of effort was spent on correctly identifying these
> pages and avoiding unnecessary migrations and the safety net may no longer
> be required.
> 
> Jirka Hladky reported a regression in 4.17 due to a scheduler patch that
> avoids spreading STREAM tasks wide prematurely. However, once the task
> was properly placed, it delayed migrating the memory due to rate limiting.
> Increasing the limit fixed the problem for him.
> 
> Currently, the limit is hard-coded and does not account for the real
> capabilities of the hardware. Even if an estimate was attempted, it would
> not properly account for the number of memory controllers and it could
> not account for the amount of bandwidth used for normal accesses. Rather
> than fudging, this patch simply eliminates the rate limiting.
> 
> However, Jirka reports that a STREAM configuration using multiple
> processes achieved similar performance to 4.16. In local tests, this patch
> improved performance of STREAM relative to the baseline but it is somewhat
> machine-dependent. Most workloads show little or not performance difference
> implying that there is not a heavily reliance on the throttling mechanism
> and it is safe to remove.
> 
> STREAM on 2-socket machine
>                          4.19.0-rc5             4.19.0-rc5
>                          numab-v1r1       noratelimit-v1r1
> MB/sec copy     43298.52 (   0.00%)    44673.38 (   3.18%)
> MB/sec scale    30115.06 (   0.00%)    31293.06 (   3.91%)
> MB/sec add      32825.12 (   0.00%)    34883.62 (   6.27%)
> MB/sec triad    32549.52 (   0.00%)    34906.60 (   7.24%
> 
> Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
> ---
Reviewed-by: Srikar Dronamraju <srikar@xxxxxxxxxxxxxxxxxx>

-- 
Thanks and Regards
Srikar Dronamraju