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

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Date:	Mon, 27 Oct 2008 12:27:50 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Jiri Kosina <jkosina@...e.cz>
Cc:	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.


* Jiri Kosina <jkosina@...e.cz> wrote:

> Ok, so another important datapoint:
> 
> with c1e4fe711a4 (just before CFS has been merged for 2.6.23), the dbench 
> throughput measures
> 
> 	187.7 MB/s
> 
> in our testing conditions (default config).
> 
> With c31f2e8a42c4 (just after CFS has been merged for 2.6.23), the 
> throughput measured by dbench is
> 
> 	82.3 MB/s
> 
> This is the huge drop we have been looking for. After this, the 
> performance was still going down gradually, up to ~45 MS/ we are 
> measuring for 2.6.27. But the biggest drop (more than 50%) points 
> directly to CFS merge.

that is a well-known property of dbench: it rewards unfairness in IO, 
memory management and scheduling.

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

the moment the clients are allowed to overlap, the moment their requests 
are executed more fairly, the dbench numbers drop.

	Ingo
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