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

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Date:	Tue, 27 May 2008 23:56:04 +0530
From:	Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>
To:	Arjan van de Ven <arjan@...radead.org>
Cc:	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

* Arjan van de Ven <arjan@...radead.org> [2008-05-27 09:40:35]:

> On Tue, 27 May 2008 19:34:40 +0530
> Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com> wrote:
> > 
> > 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%.
> 
> When you use the word "capacity" I cringe ;(
> 
> > 
> > The problem in the above two cases is that we had assumed that the
> > maximum CPU capacity is 100% at normal capacity (without IDA).
> > 
> > If the CPU is at half the maximum frequency, then scaled stats should
> > show 50%.  
> 
> see frequency != capacity.
> It's about more than frequency. It's about how much cache you have
> available too. If  you run single threaded on a dual core cpu, you have
> 100% of the cache, but the cpu is 50% idle. But that doesn't mean that
> when you double the load, you actually get 2x the performance. So
> you're not at 50% of capacity!
 
You are right.... I kind of interchangeably used frequency and
capacity assuming capacity is linearly proportional to frequency!

In reality, I agree that capacity is not linearly proportional to
frequency and it is dependent on cache usage etc.

The differences apart, I am sure I have conveyed the relationship
between scaled stats and CPU frequency. I used the term 'capacity' to
generalise CPU performance, but I guess it may lead to a different
discussion.  I will stick to frequency. :)

--Vaidy
 
> > The point I am trying to make is whether scaling should be done
> > relative to CPUs designed maximum capacity or maximum capacity under
> > current constraints is to be discussed.
> 
> now you're back at "capacity".. we were at frequency before ;(
> 
> 
> > 
> > Case A:
> > ------
> > 
> > Scaled stats is stats relative to maximum designed capacity including
> > IDA
> 
> you don't know what that is though.
> 
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