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SubjectRe: [PATCH] accelerate newidle balancing in relax_domain

* Hidetoshi Seto <seto.hidetoshi@jp.fujitsu.com> wrote:

> Increasing the value of 'sched_relax_domain_level' in cpuset expands
> the searching range of task balancing on some schedule events. As
> the result it makes the task balancing in the range more aggressive,
> so it will benefit some situation, such as where the latency is
> required even it sacrifices cache hit rate etc. (for such situation,
> it would be ideally best that cpus do not be idle until there are no
> runnable task.)
>
> This patch aimed to accelerate the balancing in the relax_domain.
>
> The newidle balancing is kicked when tasks in a runqueue run out. It
> finds and pulls runnable tasks from other busy cpus, checking load
> imbalance between cpus. Considering above situation, using loads in
> short term is preferred than that in long term because it makes
> balancing more aggressive otherwise it becomes relatively
> conservative. The referenced load is selected by the newidle_idx
> parameter of scheduler domains, so this patch tunes the parameters
> only when domains are in the relax_domain's range. There are no
> effects if you don't use relax_domain.
>
> Following is a result of my short-lightweight-transaction test, showing
> average of requester's latency (ms), 300 couple of threads running 30 sec
> on 8cpu/Itanium:
>
> 1) v2.6.28-rc4
> Average 0.748783 Std Div 1.688022 Throughput 165313
> 2) v2.6.28-rc4 + relax_domain
> Average 0.536867 Std Div 1.115383 Throughput 168492
> 3) v2.6.28-rc4 + relax_domain + patch
> Average 0.385164 Std Div 0.801875 Throughput 170069

that improvement in metric looks good.

> Signed-off-by: Hidetoshi Seto <seto.hidetoshi@jp.fujitsu.com>
> ---
> kernel/sched.c | 2 ++
> 1 files changed, 2 insertions(+), 0 deletions(-)
>
> diff --git a/kernel/sched.c b/kernel/sched.c
> index 57c933f..c970239 100644
> --- a/kernel/sched.c
> +++ b/kernel/sched.c
> @@ -7366,6 +7366,8 @@ static void set_domain_attribute(struct sched_domain *sd,
> } else {
> /* turn on idle balance on this domain */
> sd->flags |= (SD_WAKE_IDLE_FAR|SD_BALANCE_NEWIDLE);
> + /* make newidle balancing more aggressive */
> + sd->newidle_idx = 0;

I agree with making it more sensitive to momentary load fluctuations.
(as long as other metrics do not degrade).

But this solutin basically overrides the newidle_idx tuning in
topology.h.

Is there a strong reason to do this tuning dynamically, or could we
just decrease newidle_idx in the appropriate templates in the
topology.h files?

Ingo


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