linux-kernel - Re: Stolen and degraded time and schedulers

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Date:	Tue, 13 Mar 2007 23:52:12 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Daniel Walker <dwalker@...sta.com>
CC:	john stultz <johnstul@...ibm.com>, Andi Kleen <ak@...e.de>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Con Kolivas <kernel@...ivas.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Zachary Amsden <zach@...are.com>,
	James Morris <jmorris@...ei.org>,
	Chris Wright <chrisw@...s-sol.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	cpufreq@...ts.linux.org.uk,
	Virtualization Mailing List <virtualization@...ts.osdl.org>
Subject: Re: Stolen and degraded time and schedulers

Daniel Walker wrote:
> The adjustments that I spoke of above are working regardless of ntp ..
> The stability of the TSC directly effects the clock mult adjustments in
> timekeeping, as does interrupt latency since the clock is essentially
> validated against the timer interrupt.
>   

Yep.  But the tsc is just an example of a clocksource, and doesn't have
any real bearing on what I'm saying.

> like I said there are other factors so that's not going to exactly model
> cpu speed changes. You could come up with another method, but that would
> likely require another known constant clock.
>   

Well, it doesn't need to be a constant clock if its modelling a changing
rate.  And it doesn't need to be an exact model; it just needs to be
better than the current situation.

> sched_clock doesn't measure amounts of cpu work either, it's all about
> timing. 
>   

Specifically, how much cpu time a process has used.  But if the CPU is
running at half speed (or 50% duty cycle), then claiming that the
process got the full amount of time is just an error.

>> Well, lots of cpus have dynamic frequencies.  Any scheduler which
>> maintains history will suffer the same problem, even on UP.  If
>> processes A and B are supposed to have the same priority and they both
>> execute for 1ms of real time, did they make the same amount of
>> progress?  Not if the cpu changed speed in between.
>>     
>
> That's true, but given a constant clock (like what sched_clock should
> have) then the accounting is similarly inaccurate. Any connection
> between the scheduler and the TSC frequency changes aren't part of the
> design AFAIK ..
>   

Well, my whole argument is that sched_clock /should not/ be a constant
clock.  And I'm not quite sure why you keep bringing up the tsc, because
it has no relevance.

    J
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