linux-kernel - Re: Meaningless load?

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Date:	Thu, 11 Oct 2012 15:17:59 +0200
From:	Simon Klinkert <simon.klinkert@...il.com>
To:	Mike Galbraith <efault@....de>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>, mingo@...hat.com,
	Trond.Myklebust@...app.com
Subject: Re: Meaningless load?


On 11.10.2012, at 10:13, Mike Galbraith wrote:

>> 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.

Your explanation sounds correct to me but I think in my case, there are only 2-3 process 
waiting for spinning rust (or rather nfs) and the other processes are all in a heavy lock 
contention in the VFS layer. So a load of 10,000 is helpful to indicate that there is a (software)
bottleneck but if I want to see the 'real (work)load' on this machine it isn't really helpful to show 
a load of 10,000 instead of three or whatever. It's a question of interpretation.

Simon


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