linux-kernel - Re: Meaningless load?

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Date:	Thu, 11 Oct 2012 10:13:27 +0200
From:	Mike Galbraith <efault@....de>
To:	Simon Klinkert <simon.klinkert@...il.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>, mingo@...hat.com,
	Trond.Myklebust@...app.com
Subject: Re: Meaningless load?

On Thu, 2012-10-11 at 09:19 +0200, Simon Klinkert wrote:
> On 10.10.2012, at 18:22, Peter Zijlstra wrote:
> 
> > On Wed, 2012-10-10 at 17:44 +0200, Simon Klinkert wrote:
> >> I'm just wondering if the 'load' is really meaningful in this
> >> scenario. The machine is the whole time fully responsive and looks
> >> fine to me but maybe I didn't understand correctly what the load
> >> should mean. Is there any sensible interpretation of the load?
> > 
> > I'll leave meaningful aside, but uninterruptible (D state) is part
> of
> > how the load thing is defined, so your 500 result is correct.
> 
> Yes, the calculation of the load is correct but I still don't know how
> I should interpret the load…
> 
> On 11.10.2012, at 06:02, Mike Galbraith wrote:
> 
> > Makes perfect sense to me. Work _is_ stack this high. We don't and
> > can't know whether the mountain is made of popcorn balls or
> boulders.
> 
> That's the point. Afaik the D state never represents 'work'. These
> processes are waiting for something.

Yeah, the whole pile is waiting, but they're not idle.  There are N
tasks pointed at CPUs.
> 
> Let's say we have 10,000 processes in the D state (and thus a load of
> ~10,000) doing nothing. What should the load tell me? The machine is
> under fire? There is nothing to do? There might be something to do but
> the machine doesn't know?

They are doing something, just not at the particular instant you see
them in D state.  D state pushing load through the roof tells you that
you have a bottleneck.  Whether the bottleneck is a bit of spinning rust
or insufficient NR_CPUS doesn't matter much, both are bottlenecks.

-Mike

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