Re: [PATCH] CFS: Fix missing digit off in wmult table

[James Bruce]

Ingo Molnar wrote:
> * Ingo Molnar <mingo@xxxxxxx> wrote:
> > * James Bruce <bruce@xxxxxxxxxxxxxx> wrote:
> > > While we're at it, isn't the comment above the wmult table incorrect? 
> > > The multiplier is 1.25, meaning a 25% change per nice level, not 10%.
> > yes, the weight multiplier 1.25, but the actual difference in CPU 
> > utilization, when running two CPU intense tasks, is ~10%:
> >
> >   PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
> >  8246 mingo     20   0  1576  244  196 R   55  0.0   0:11.96 loop
> >  8247 mingo     21   1  1576  244  196 R   45  0.0   0:10.52 loop
> >
> > so the first task 'wins' +10% CPU utilization (relative to the 50% it 
> > had before), the second task 'loses' -10% CPU utilization (relative to 
> > the 50% it had before).
> >
> > so what the comment says is true:
> >
> >  * The "10% effect" is relative and cumulative: from _any_ nice level,
> >  * if you go up 1 level, it's -10% CPU usage, if you go down 1 level
> >  * it's +10% CPU usage.
> >
> > for there to be a ~+10% change in CPU utilization for a task that 
> > races against another CPU-intense task there needs to be a ~25% change 
> > in the weight.
>
> in any case more documentation is justified, so i've added some 
> clarification to the comments - see the patch below.

Ah ok so it's 10% of the original CPU usage, not relative to a tasks
share from before.  While I guess I still think in terms of relative CPU
share, your comments now make sense to me.  Thanks for the
clarification.

 - Jim

> ------------------------>
> Subject: sched: improve weight-array comments
> From: Ingo Molnar <mingo@xxxxxxx>
>
> improve the comments around the wmult array (which controls the weight
> of niced tasks). Clarify that to achieve a 10% difference in CPU
> utilization, a weight multiplier of 1.25 has to be used.
>
> Signed-off-by: Ingo Molnar <mingo@xxxxxxx>
> ---
>  kernel/sched.c |    4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>
> Index: linux/kernel/sched.c
> ===================================================================
> --- linux.orig/kernel/sched.c
> +++ linux/kernel/sched.c
> @@ -736,7 +736,9 @@ static void update_curr_load(struct rq *
>   *
>   * The "10% effect" is relative and cumulative: from _any_ nice level,
>   * if you go up 1 level, it's -10% CPU usage, if you go down 1 level
> - * it's +10% CPU usage.
> + * it's +10% CPU usage. (to achieve that we use a multiplier of 1.25.
> + * If a task goes up by ~10% and another task goes down by ~10% then
> + * the relative distance between them is ~25%.)
>   */
>  static const int prio_to_weight[40] = {
>  /* -20 */ 88818, 71054, 56843, 45475, 36380, 29104, 23283, 18626, 14901, 11921,

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