linux-kernel - Re: [PATCH] sched/fair: refine maximum periods for decay

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Date:	Fri, 5 Aug 2016 09:56:46 +0200
From:	Vincent Guittot <vincent.guittot@...aro.org>
To:	Leo Yan <leo.yan@...aro.org>
Cc:	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] sched/fair: refine maximum periods for decay_load

Hi Leo,

On 5 August 2016 at 08:34, Leo Yan <leo.yan@...aro.org> wrote:
>
> In current code, decay_load() will consider to set load value to zero
> after passing 32*64 ms. So this means max_load * (0.5^64) ~= 0.
>
> Kernel can support maximum number of processes and threads to 2^29 and
> set task with highest priority with nice=-20 (weight = 88761). So in
> worst case, one CPU may have maximum load value is:
>
>     max_load = 2^29 * 88761 < 2^46
>
> In theory after pass 46 periods we can ensure load value will be decayed
> to zero. So this patch is to change maximum periods from 64 to 48.
>
> Signed-off-by: Leo Yan <leo.yan@...aro.org>
> ---
>  kernel/sched/fair.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index e342159..55cb134 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -2622,7 +2622,7 @@ static __always_inline u64 decay_load(u64 val, u64 n)
>
>         if (!n)
>                 return val;
> -       else if (unlikely(n > LOAD_AVG_PERIOD * 63))
> +       else if (unlikely(n > LOAD_AVG_PERIOD * 47))

In the equation above, you use 46 then you mentioned : "change maximum
periods from 64 to 48." and finally you use 47.

>                 return 0;
>
>         /* after bounds checking we can collapse to 32-bit */
> --
> 1.9.1
>