linux-kernel - Re: [tbench regression fixes]: digging out smelly deadmen.

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Message-Id: <1225109171.4238.23.camel@marge.simson.net>
Date:	Mon, 27 Oct 2008 13:06:11 +0100
From:	Mike Galbraith <efault@....de>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
Cc:	Ingo Molnar <mingo@...e.hu>, Jiri Kosina <jkosina@...e.cz>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	David Miller <davem@...emloft.net>, rjw@...k.pl,
	s0mbre@...rvice.net.ru, linux-kernel@...r.kernel.org,
	netdev@...r.kernel.org
Subject: Re: [tbench regression fixes]: digging out smelly deadmen.

On Mon, 2008-10-27 at 11:33 +0000, Alan Cox wrote:
> > The way to get the best possible dbench numbers in CPU-bound dbench 
> > runs, you have to throw away the scheduler completely, and do this 
> > instead:
> > 
> >  - first execute all requests of client 1
> >  - then execute all requests of client 2
> >  ....
> >  - execute all requests of client N
> 
> Rubbish. If you do that you'll not get enough I/O in parallel to schedule
> the disk well (not that most of our I/O schedulers are doing the job
> well, and the vm writeback threads then mess it up and the lack of Arjans
> ioprio fixes then totally screw you) </rant>
> 
> > the moment the clients are allowed to overlap, the moment their requests 
> > are executed more fairly, the dbench numbers drop.
> 
> Fairness isn't everything. Dbench is a fairly good tool for studying some
> real world workloads. If your fairness hurts throughput that much maybe
> your scheduler algorithm is just plain *wrong* as it isn't adapting to
> workload at all well.

Doesn't seem to be scheduler/fairness.  2.6.22.19 is O(1), and falls
apart too, I posted the numbers and full dbench output yesterday.

	-Mike

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