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

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Message-ID: <20081027113306.5b1d5898@lxorguk.ukuu.org.uk>
Date:	Mon, 27 Oct 2008 11:33:06 +0000
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Jiri Kosina <jkosina@...e.cz>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Mike Galbraith <efault@....de>,
	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.

> 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.

Alan
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