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

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Date:	Sat, 31 May 2008 23:17:50 +0200
From:	Pavel Machek <pavel@...e.cz>
To:	Arjan van de Ven <arjan@...radead.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	venkatesh.pallipadi@...el.com, suresh.b.siddha@...el.com,
	Michael Neuling <mikey@...ling.org>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	"Amit K. Arora" <aarora@...ux.vnet.ibm.com>
Subject: Re: [RFC PATCH v1 0/3] Scaled statistics using APERF/MPERF in x86

Hi!

> > > The following RFC patch tries to implement scaled CPU utilisation
> > > statistics using APERF and MPERF MSR registers in an x86 platform.
> > > 
> > > The CPU capacity is significantly changed when the CPU's frequency is
> > > reduced for the purpose of power savings.  The applications that run
> > > at such lower CPU frequencies are also accounted for real CPU time by
> > > default.  If the applications have been run at full CPU frequency,
> > > they would have finished the work faster and not get charged for
> > > excessive CPU time.
> > > 
> > > One of the solution to this problem it so scale the utime and stime
> > > entitlement for the process as per the current CPU frequency.  This
> > > technique is used in powerpc architecture with the help of hardware
> > > registers that accurately capture the entitlement.
> > > 
> > 
> > there are some issues with this unfortunately, and these make it
> > a very complex thing to do. 
> > Just to mention a few:
> > 1) What if the BIOS no longer allows us to go to the max frequency for
> > a period (for example as a result of overheating); with the approach
> > above, the admin would THINK he can go faster, but he cannot in reality,
> > so there's misleading information (the system looks half busy, while in
> > reality it's actually the opposite, it's overloaded). Management tools
> > will take the wrong decisions (such as moving MORE work to the box, not
> > less)
> > 2) On systems with Intel Dynamic Acceleration technology, you can get
> > over 100% of cycles this way. (For those who don't know what IDA is;
> > IDA is basically a case where if your Penryn based dual core laptop is
> > only using 1 core, the other core can go faster than 100% as long as
> > thermals etc allow it). How do you want to deal with this?
> 
> Hi Arjan,
> 
> Thanks you for the inputs.  The above issues are very valid and our
> solution should be able to react appropriately to the above situation.
> 
> What we are proposing is a scaled time value that is scaled to the
> current CPU capacity.  If the scaled utilisation is 50% when the CPU
> is at 100% capacity, it is expected to remain at 50% even if the CPU's
> capacity is dropped to 50%, while the traditional utilisation value
> will be 100%.

time one-second-busy-loop should return close to one second. That's
current behaviour. You don't like it, but it is useful.

If you change it, that's called 'regression'.
							Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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