Re: [RFC PATCH] sched: Reduce overestimating avg_idle

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

On 07/31/2013 05:37 AM, Jason Low wrote:
> The avg_idle value may sometimes be overestimated, which may cause new idle
> load balance to be attempted more often than it should. Currently, when
> avg_idle gets updated, if the delta exceeds some max value (default 1000000 ns),
> the entire avg gets set to the max value, regardless of what the previous avg
> was. So if a CPU remains idle for 200,000 ns most of the time, and if the CPU
> goes idle for 1,200,000 ns, the average is then pushed up to 1,000,000 ns when
> it should be less.
>
> Additionally, once the avg_idle is at its max, it may take a while to pull the
> avg down to a value that it should be. In the above example, after the avg idle
> is set the max value of 1000000 ns, the CPU's idle durations needs to
> be 200000 ns for the next 8 occurrences before the avg falls below the migration
> cost value.
>
> This patch attempts to avoid these situations by always updating the avg_idle
> value first with the function call to update_avg(). Then, if the avg_idle
> exceeds the max avg value, the avg gets set to the max. Also, this patch lowers
> the max avg_idle value to migration_cost * 1.5 instead of migration_cost * 2 to
> reduce the time it takes to pull the avg idle to a lower value after long idles.
>
> With this change, I got some decent performance boosts in AIM7 workloads on an
> 8 socket machine on the 3.10 kernel. In particular, it boosted the AIM7 fserver
> workload by about 20% when running it with a high # of users.
>
> An avg_idle related question that I have is does migration_cost in idle balance
> need to be the same as the migration_cost in task_hot()? Can we keep
> migration_cost default value used in task_hot() the same, but have a different
> default value or increase migration_cost only when comparing it with avg_idle in
> idle balance?
>
>
> Signed-off-by: Jason Low <jason.low2@hp.com>

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

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