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Subject: Re: [patch] sched: schedstat needs a diet
From: Peter Zijlstra <peterz@infradead.org>
To: Ingo Molnar <mingo@elte.hu>
Cc: Ken Chen <kenchen@google.com>,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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	 <20071017072323.GB18044@elte.hu>
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Date: Wed, 17 Oct 2007 11:29:47 +0200
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On Wed, 2007-10-17 at 09:23 +0200, Ingo Molnar wrote:
> * Ken Chen <kenchen@google.com> wrote:
> 
> > schedstat is useful in investigating CPU scheduler behavior.  Ideally, 
> > I think it is beneficial to have it on all the time.  However, the 
> > cost of turning it on in production system is quite high, largely due 
> > to number of events it collects and also due to its large memory 
> > footprint.
> > 
> > Most of the fields probably don't need to be full 64-bit on 64-bit 
> > arch.  Rolling over 4 billion events will most like take a long time 
> > and user space tool can be made to accommodate that.  I'm proposing 
> > kernel to cut back most of variable width on 64-bit system.  (note, 
> > the following patch doesn't affect 32-bit system).
> 
> thanks, applied.
> 
> note that current -git has a whole bunch of new schedstats fields in 
> /proc/<PID>/sched which can be used to track the exact balancing 
> behavior of tasks. It can be cleared via echoing 0 to the file - so 
> overflow is not an issue. Most of those new fields should probably be 
> unsigned int too. (they are u64 right now.)
> 

FWIW I can't see how this patch saves a _lot_ of space. The stats are
per domain or per rq, neither are things that have a lot of instances.

That said, I have no actual objection to the patch, just not getting it.


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