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SubjectRe: [CFS Bandwidth Control v4 0/7] Introduction
On Fri, Feb 25, 2011 at 5:06 AM, jacob pan
<jacob.jun.pan@linux.intel.com> wrote:
> On Fri, 25 Feb 2011 02:03:54 -0800
> Paul Turner <pjt@google.com> wrote:
>
>> On Thu, Feb 24, 2011 at 4:11 PM, jacob pan
>> <jacob.jun.pan@linux.intel.com> wrote:
>> > On Tue, 15 Feb 2011 19:18:31 -0800
>> > Paul Turner <pjt@google.com> wrote:
>> >
>> >> Hi all,
>> >>
>> >> Please find attached v4 of CFS bandwidth control; while this rebase
>> >> against some of the latest SCHED_NORMAL code is new, the features
>> >> and methodology are fairly mature at this point and have proved
>> >> both effective and stable for several workloads.
>> >>
>> >> As always, all comments/feedback welcome.
>> >>
>> >
>> > Hi Paul,
>> >
>> > Your patches provide a very useful but slightly different feature
>> > for what we need to manage idle time in order to save power. What we
>> > need is kind of a quota/period in terms of idle time. I have been
>> > playing with your patches and noticed that when the cgroup cpu usage
>> > exceeds the quota the effect of throttling is similar to what I have
>> > been trying to do with freezer subsystem. i.e. freeze and thaw at
>> > given period and percentage runtime.
>> > https://lkml.org/lkml/2011/2/15/314
>> >
>> > Have you thought about adding such feature (please see detailed
>> > description in the link above) to your patches?
>> >
>>
>> So reading the description it seems like rooting everything in a
>> 'freezer' container and then setting up a quota of
>>
>> (1 - frozen_percentage)  * nr_cpus * frozen_period * sec_to_usec
>>
> I guess you meant frozen_percentage is less than 1, i.e. 90 is .90. my
> code treat 90 as 90. just a clarification.
>> on a period of
>>
>> frozen_period * sec_to_usec
>>
>> Would provide the same functionality.  Is there other unduplicated
>> functionality beyond this?

Sorry -- I was out last week; comments inline.

> Do you mean the same functionality as your patch? Not really, since my
> approach will stop the tasks based on hard time slices
>. But seems your
> patch will allow them to run if they don't exceed the quota. Am i
> missing something?

Right, this is what was discussed above.

> That is the only functionality difference i know.
>
> Like the reviewer of freezer patch pointed out, it is a more logical
> fit to implement such feature in scheduler/yours in stead of freezer. So
> i am wondering if your patch can be expended to include limiting quota
> on real time.

The following two configurations should effectively exactly mirror the
freezer behavior without modification.

A) background while(1) thread on each cpu within the cgroup
This will result in synchronous consumption / exhaustion of quota in a
manor that duplicates the periodic freezing.

Given the goal is power-saving, this is obviously non-ideal. However:

B) A userspace daemon toggles quota at the desired interval

Supposing you wanted a freezer period of 100ms per second, then having
a daemon wake up at 900ms into the interval and then setting a quota
amount that is effectively zero will then "freeze" the group. Said
daemon can then release things by returning the group to an infinite
quota in 100ms, and then sleeping for another 900ms.

Is there particular advantage of doing this in-kernel?


>
> I did a comparison study between CFS BW and freezer patch on skype with
> identical quota setting as you pointed out earlier. Both use 2 sec
> period and .2 sec quota (10%). Skype typically uses 5% of the CPU on my
> system when placing a call(below cfs quota) and it wakes up every 100ms
> to do some quick checks. Then I run skype in cpu then freezer cgroup
> (with all its children). Here is my result based on timechart and
> powertop.
>
> patch name      wakeups         skype call?
> ------------------------------------------------------------------
> CFS BW          10/sec          yes
> freezer         1/sec           no
>

Is this a true saving? While the actual task wake-up has been hidden,
the cpu is still coming out of a halt/idle state and processing the
interrupt/etc.

Have you had the chance to measure the actual comparative power-usage
in this case?

> Skype might not be the best example to illustrate the real usage of the
> feature, but we are targeting mobile device where they are mostly off or
> often have only one application allowed in foreground. So we want to
> reduce wakeups coming from the tasks that are not in the foreground.
>

If reducing wake-ups (at the userspace level) is proven to deliver
performance improvements, then it might be more productive to approach
that directly by considering strategies such as batching wakeups and
processing them periodically.

This would not have the negative performance impact of the current
approach, as well as being more deterministic.

>> One thing that does seem undesirable about your approach is (as it
>> seems to be described) threads will not be able to take advantage of
>> naturally occurring idle cycles and will incur a potential performance
>> penalty even at use << frozen_percentage.
>>
>> e.g. From your post
>>
>>        |  |<-- 90% frozen -     ->|  |
>> |  | ____|  |________________x_|  |__________________|  |_____
>>
>>         |<---- 5 seconds     ---->|
>>
>>
>> Suppose no threads active until the wake up at x, suppose there is an
>> accompanying 1 second of work for that thread to do.  That execution
>> time will be dilated to ~1.5 seconds (as it will span the 0.5 seconds
>> the freezer will stall for).  But the true usage for this period is
>> ~20% <<< 90%
> I agree my approach does not consider the natural cycle. But I am not
> sure if a thread can wake up at x when FROZEN.
>

While the ascii is a little mailer-mangled, in the diagram above x was
intended to precede the "frozen" time segment, but at a point where
the work it wants to do exceeds the time-before-freeze resulting in
dilation of execution and a performance regression.
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