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

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Date:	Mon, 27 Oct 2008 11:42:50 +0100 (CET)
From:	Jiri Kosina <jkosina@...e.cz>
To:	Andrew Morton <akpm@...ux-foundation.org>
cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Mike Galbraith <efault@....de>,
	David Miller <davem@...emloft.net>, rjw@...k.pl,
	Ingo Molnar <mingo@...e.hu>, s0mbre@...rvice.net.ru,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [tbench regression fixes]: digging out smelly deadmen.

On Sun, 26 Oct 2008, Jiri Kosina wrote:

> > > > > 208.4 MB/sec  -- vanilla 2.6.16.60
> > > > > 201.6 MB/sec  -- vanilla 2.6.20.1
> > > > > 172.9 MB/sec  -- vanilla 2.6.22.19
> > > > > 74.2 MB/sec   -- vanilla 2.6.23
> > > > >  46.1 MB/sec  -- vanilla 2.6.24.2
> > > > >  30.6 MB/sec  -- vanilla 2.6.26.1
> > > > > I.e. huge drop for 2.6.23 (this was with default configs for each 
> > > > > respective kernel).
> > Was this when we decreased the default value of
> > /proc/sys/vm/dirty_ratio, perhaps?  dbench is sensitive to that.
> 2.6.28 gives 41.8 MB/s with /proc/sys/vm/dirty_ratio == 50. So small 
> improvement, but still far far away from the throughput of pre-2.6.23 
> kernels.

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.

-- 
Jiri Kosina
SUSE Labs
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