Subject: Re: [tbench regression fixes]: digging out smelly deadmen.
From: Mike Galbraith <efault@gmx.de>
To: David Miller <davem@davemloft.net>
Cc: rjw@sisk.pl, mingo@elte.hu, s0mbre@tservice.net.ru, a.p.zijlstra@chello.nl,
       linux-kernel@vger.kernel.org, netdev@vger.kernel.org
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Date: Sat, 25 Oct 2008 09:52:02 +0200
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On Sat, 2008-10-25 at 00:24 -0700, David Miller wrote:
> From: Mike Galbraith <efault@gmx.de>
> Date: Sat, 25 Oct 2008 08:53:43 +0200
> 
> > On Sat, 2008-10-25 at 07:58 +0200, Mike Galbraith wrote:
> > 2.6.24.7-up
> > ring-test   - 1.100 us/cycle  = 909 KHz  (gcc-4.1)
> > ring-test   - 1.068 us/cycle  = 936 KHz  (gcc-4.3)
> > netperf     - 122300.66 rr/s  = 244 KHz  sb 280 KHz / 140039.03 rr/s
> > tbench      - 341.523 MB/sec
> > 
> > 2.6.25.17-up
> > ring-test   - 1.163 us/cycle  = 859 KHz  (gcc-4.1)
> > ring-test   - 1.129 us/cycle  = 885 KHz  (gcc-4.3)
> > netperf     - 132102.70 rr/s  = 264 KHz  sb 275 KHz / 137627.30 rr/s
> > tbench      - 361.71 MB/sec
> > 
> > ..in 25, something happened that dropped my max context switch rate from
> > ~930 KHz to ~885 KHz.  Maybe I'll have better luck trying to find that.
> > Added to to-do list.  Benchmark mysteries I'm going to have to leave
> > alone, they've kicked my little butt quite thoroughly ;-)
> 
> But note that tbench performance improved a bit in 2.6.25.

Yeah, netperf too.

> In my tests I noticed a similar effect, but from 2.6.23 to 2.6.24,
> weird.

23->24 I can understand.  In my testing, 23 CFS was not a wonderful
experience for rapid switchers.  24 is cfs-24.1.
 
> Just for the public record here are the numbers I got in my testing.
> Each entry was run purely on the latest 2.6.X-stable tree for each
> release.  First is the tbench score and then there are 40 numbers
> which are sparc64 cpu cycle counts of default_wake_function().

Your numbers seem to ~agree with mine.  And yeah, that hrtick is damned
expensive.  I didn't realize _how_ expensive until I trimmed my config
way way down from distro.  Just having highres timers enabled makes a
very large difference here, even without hrtick enabled, and with the
overhead of a disabled hrtick removed.

	-Mike

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