linux-kernel - Re: [PATCH 3/5] lockstat: core infrastructure

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Date:	Fri, 01 Jun 2007 09:11:03 -0700
From:	Daniel Walker <dwalker@...sta.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Ingo Molnar <mingo@...e.hu>, Steven Rostedt <rostedt@...dmis.org>,
	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jason Baron <jbaron@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 3/5] lockstat: core infrastructure

On Fri, 2007-06-01 at 17:52 +0200, Peter Zijlstra wrote:
> On Fri, 2007-06-01 at 08:26 -0700, Daniel Walker wrote:
> > On Fri, 2007-06-01 at 15:12 +0200, Ingo Molnar wrote:
> > > * Daniel Walker <dwalker@...sta.com> wrote:
> > > 
> > > > On Wed, 2007-05-30 at 19:16 +0200, Peter Zijlstra wrote:
> > > 
> > > > > I think you are mistaken here; the two are similar but not 
> > > > > identical.
> > > > > 
> > > > > I see sched_clock() as fast first, accurate second. Whereas the 
> > > > > clocksource thing is accurate first, fast second.
> > > > 
> > > > This is true .. However, if there is a speed different it's small.
> > > 
> > > Ugh. Have you ever compared pmtimer (or even hpet) against TSC based 
> > > sched_clock()? What you write is so wrong that it's not even funny. You 
> > > keep repeating this nonsense despite having been told multiple times 
> > > that you are dead wrong.
> > 
> > Yes I have, and your right there is a difference, and a big
> > difference .. Above I was referring only to the TSC clocksource, since
> > that's an apples to apples comparison .. I would never compare the TSC
> > to the acpi_pm, that's no contest ..
> > 
> > The acpi_pm as sched_clock() with hackbench was about %25 slower, the
> > pit was 10x slower approximately. (I did this months ago.)
> 
> The whole issue is that you don't have any control over what clocksource
> you'll end up with. If it so happens that pmtimer gets selected your
> whole box will crawl if its used liberaly, like the patch under
> discussion does.

You can have control over it, which I think the whole point of this
discussion ..

> So, having two interfaces, one fast and one accurate is the right answer
> IMHO.

In the case of lockstat you have two cases fast and functional, and
non-functional .. Right now your patch has no slow and functional state.

The non-functional state is even the majority from my perspective.

> And I agree, that if the arch has a fast clock but doesn't use it for
> sched_clock() that would be a shortcoming of that arch.

As I said before there is no reason why and architectures should be
forced to implement sched_clock() .. Is there some specific reason why
you think it should be mandatory?

Daniel

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