linux-kernel - Re: hackbench [pthread mode] regression with 2.6.29-rc3

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Message-ID: <20090201100410.GA22903@elte.hu>
Date:	Sun, 1 Feb 2009 11:04:10 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Lin Ming <ming.m.lin@...el.com>,
	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: hackbench [pthread mode] regression with 2.6.29-rc3


* Peter Zijlstra <peterz@...radead.org> wrote:

> On Sun, 2009-02-01 at 10:17 +0100, Peter Zijlstra wrote:
> > On Sun, 2009-02-01 at 16:29 +0800, Lin Ming wrote:
> > > > Bisect located below patch.
> > > > commit 490dea45d00f01847ebebd007685d564aaf2cd98
> > > > Author: Peter Zijlstra <peterz@...radead.org>
> > > > Date:   Mon Nov 24 17:06:57 2008 +0100
> > > > 
> > > >    itimers: remove the per-cpu-ish-ness
> > > > 
> > > >    Either we bounce once cacheline per cpu per tick, yielding n^2 bounces
> > > >    or we just bounce a single..
> > > > 
> > > >    Also, using per-cpu allocations for the thread-groups complicates the
> > > >    per-cpu allocator in that its currently aimed to be a fixed sized
> > > >    allocator and the only possible extention to that would be vmap based,
> > > >    which is seriously constrained on 32 bit archs.
> > > > 
> > > > 
> > > > After above patch is reverted, hackbench result is restored.
> > > 
> > > oltp has ~3% regression with 2.6.29-rc3 on 4core*2p stokley machine.
> > > After above patch reverted, the regression disappeared.
> > 
> > *sigh*, did they gain anything with introduction of the per-cpu crap?
> 
> No it wouldn't have, I just missed something obvious,.. :-(
> 
> I wish we never merged that crap...

oh, it certainly had its use: it highlighted that we have crappy 
threading+timers code (on hackbench_pth) since the beginning of the Linux 
SMP times.

	Ingo
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