* Ingo Molnar <[EMAIL PROTECTED]> wrote: > * James Bruce <[EMAIL PROTECTED]> 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. Ingo ------------------------> Subject: sched: improve weight-array comments From: Ingo Molnar <[EMAIL PROTECTED]> 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 <[EMAIL PROTECTED]> --- 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, - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/