Ingo Molnar wrote:
* 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.
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 <[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,
-
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