linux-kernel - Re: [PATCH] sched: Folding nohz load accounting more accurate

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Date:	Tue, 12 Jun 2012 11:56:10 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Charles Wang <muming.wq@...il.com>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
	Charles Wang <muming.wq@...bao.com>, Tao Ma <tm@....ma>,
	含黛 <handai.szj@...bao.com>,
	Doug Smythies <dsmythies@...us.net>
Subject: Re: [PATCH] sched: Folding nohz load accounting more accurate

Also added Doug to CC, hopefully we now have everybody who pokes at this
stuff.

On Tue, 2012-06-12 at 17:34 +0800, Charles Wang wrote:
> consider following case:
> 
> 5HZ+1
> | cpu0_load     cpu1    cpu2    cpu3    calc_load_tasks
> |    1           1       1       1      
> |  -->calc_load                             1
> |    1           1       1       1      
> |              -->calc_load                 2
> |    0           0       1       0
> |                      -->calc_load         2+1-3=1

Not sure but last time I did the math 2+1-3 ended up being 0.

> |    1           1       0       1      
> |                            -->calc_load   1-1=0
> V
> 5HZ+11     -->calc_global_load              0
>
> actually the load should be around 3, but shows nearly 0.
> 
> 1 tick is much long for some workloads. 

Yes, one tick is long for some stuff, but seeing we sample once every 5
seconds a little fuzz around sampling the nr_running+nr_uninterruptible
thing shouldn't be too bad.

But I think I see what you're getting at.. lemme get more tea and ponder
this a bit.
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