linux-kernel - Re: [PATCH v3 17/22] sched: packing small tasks in wake/exec balancing

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Message-ID: <20130116150849.GB30805@e103034-lin>
Date:	Wed, 16 Jan 2013 15:08:49 +0000
From:	Morten Rasmussen <morten.rasmussen@....com>
To:	Alex Shi <alex.shi@...el.com>
Cc:	"mingo@...hat.com" <mingo@...hat.com>,
	"peterz@...radead.org" <peterz@...radead.org>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"arjan@...ux.intel.com" <arjan@...ux.intel.com>,
	"bp@...en8.de" <bp@...en8.de>, "pjt@...gle.com" <pjt@...gle.com>,
	"namhyung@...nel.org" <namhyung@...nel.org>,
	"efault@....de" <efault@....de>,
	"vincent.guittot@...aro.org" <vincent.guittot@...aro.org>,
	"gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
	"preeti@...ux.vnet.ibm.com" <preeti@...ux.vnet.ibm.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 17/22] sched: packing small tasks in wake/exec
 balancing

On Wed, Jan 16, 2013 at 07:32:49AM +0000, Alex Shi wrote:
> On 01/15/2013 01:00 AM, Morten Rasmussen wrote:
> >>> Why multiply rq->util by nr_running?
> >>> > > 
> >>> > > Let's take an example where rq->util = 50, nr_running = 2, and putil =
> >>> > > 10. In this case the value of putil doesn't really matter as vacancy
> >>> > > would be negative anyway since FULL_UTIL - rq->util * nr_running is -1.
> >>> > > However, with rq->util = 50 there should be plenty of spare cpu time to
> >>> > > take another task.
> >> > 
> >> > for this example, the util is not full maybe due to it was just wake up,
> >> > it still is possible like to run full time. So, I try to give it the
> >> > large guess load.
> > I don't see why rq->util should be treated different depending on the
> > number of tasks causing the load. rq->util = 50 means that the cpu is
> > busy about 50% of the time no matter how many tasks contibute to that
> > load.
> > 
> > If nr_running = 1 instead in my example, you would consider the cpu
> > vacant if putil = 6, but if nr_running > 1 you would not. Why should the
> > two scenarios be treated differently?
> > 
> >>> > > 
> >>> > > Also, why multiply putil by 8? rq->util must be very close to 0 for
> >>> > > vacancy to be positive if putil is close to 12 (12.5%).
> >> > 
> >> > just want to pack small util tasks, since packing is possible to hurt
> >> > performance.
> > I agree that packing may affect performance. But why don't you reduce
> > FULL_UTIL instead of multiplying by 8? With current expression you will
> > not pack a 10% task if rq->util = 20 and nr_running = 1, but you would
> > pack a 6% task even if rq->util = 50 and the resulting cpu load is much
> > higher.
> > 
> 
> Yes, the threshold has no strong theory or experiment support. I had
> tried cyclitest which Vicent used, the case's load avg is too small to
> be caught. so just use half of Vicent value as 12.5%. If you has more
> reasonable value, let me know.
> 
> As to nr_running engaged as multiple mode. it's base on 2 reasons.
> 1, load avg/util need 345ms to accumulate as 100%. so, if a tasks is
> cost full cpu time, it still has 345ms with rq->util < 1.

I agree that load avg may not be accurate, especially for new tasks. But
why use it if you don't trust its value anyway?

The load avg (sum/period) of a new task will reach 100% instantly if the
task is consuming all the cpu time it can get. An old task can reach 50%
within 32ms. So you should fairly quickly be able to see if it is a
light task or not. You may under-estimate its load in the beginning, but
only for a very short time.

> 2, if there are more tasks, like 2 tasks running on one cpu, it's
> possible to has capacity to burn 200% cpu time, while the biggest
> rq->util is still 100%.

If you want to have a better metric for how much cpu time the task on
the runqueue could potentially use, I would suggest using
cfs_rq->runnable_load_avg which is the load_avg_contrib sum of all tasks
on the runqueue. It would give you 200% in your example above.

On the other hand, I think rq->util is fine for this purpose. If
rq->util < 100% you know for sure that cpu is not fully utilized no
matter how many tasks you have on the runqueue. So as long as rq->util
is well below 100% (like < 50%) it should be safe to pack more small
tasks on that cpu even if it has multiple tasks running already.

> 
> Consider to figure out precise utils is complicate and cost much. I do
> this simple calculation. It is not very precise, but it is efficient and
> more bias toward performance.

It is indeed very biased towards performance. I would prefer more focus
on saving power in a power scheduling policy :)

Morten

> 
> -- 
> Thanks Alex
> --
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