linux-kernel - Re: [RFC PATCH v1 0/3] Scaled statistics using APERF/MPERF in x86

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Date:	Tue, 27 May 2008 20:50:33 +0530
From:	Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>
To:	Arjan van de Ven <arjan@...radead.org>
Cc:	balbir@...ux.vnet.ibm.com,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	venkatesh.pallipadi@...el.com, suresh.b.siddha@...el.com,
	Michael Neuling <mikey@...ling.org>,
	"Amit K. Arora" <aarora@...ux.vnet.ibm.com>
Subject: Re: [RFC PATCH v1 0/3] Scaled statistics using APERF/MPERF in x86

* Arjan van de Ven <arjan@...radead.org> [2008-05-27 07:19:00]:

> \> > 
> > > that's a case where it really makes sense; it's the case where the
> > > thing that controls the cpu P-state actually learns about how much
> > > work was done to reevaluate what the cpu frequency should be going
> > > forward. Eg it's a case of comparing actual frequency (APERF/MPERF)
> > > to see what's useful to set next.
> > > IDA makes this all needed due to the dynamic nature of the concept
> > > of "frequency".
> > 
> > Scaled statistics relative to maximum CPU capacity is just a method of
> > exposing the actual CPU utilisation of applications independent of CPU
> > frequency changes.
> > 
> > Reason behind the metric is same as the above fact that you have
> > mentioned.  The CPU frequency governors cannot make decisions only
> > based on idle time ratio.  It needs to know current utilisation (used
> > cycles) relative to maximum capacity so that the frequency can be
> > changed to next higher level.
> > 
> > Higher level management software that wants to control CPU capacity
> > externally will need similar information.
> >
> I entirely understand that desire.

Good :)

> But you're not giving it that information!
> The patch is giving it a really poor approximation, an approximation
> that will get worse and worse in upcoming cpu generations.

I agree that power capping and acceleration makes the metric
approximate.  But was are trying to be as accurate and meaningful as
APERF/MPERF ratio is in the processor hardware.

Can I state the problem like this:  The metric is as accurate and
meaningful as APERF/MEPRF ratio, but the interpretation of the metric
is subject to the knowledge of power constraint or acceleration
currently in effect.

--Vaidy

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