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Ingo Molnar wrote:
> * Ingo Molnar <mingo@elte.hu> wrote:
>> * James Bruce <bruce@andrew.cmu.edu> 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@elte.hu>
>
> 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@elte.hu>
> ---
> 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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