linux-kernel - Re: Performance loss 2.6.22->22.6.23->2.6.24-rc7 on CPU intensive benchmark on 8 Core Xeon

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Message-ID: <20080114185520.GA26540@elte.hu>
Date:	Mon, 14 Jan 2008 19:55:20 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Colin Fowler <elethiomel@...il.com>
Cc:	linux-kernel@...r.kernel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: Performance loss 2.6.22->22.6.23->2.6.24-rc7 on CPU intensive
	benchmark on 8 Core Xeon


* Colin Fowler <elethiomel@...il.com> wrote:

> Benchmark : A ray-trace is performed on 500 times on 17 separate 
> scenes. Workload is distributed by tiling the framebuffer into N 32x32 
> pixel tiles. Each CPU grabs one of N tiles out of the queue and 
> repeats until no jobs are left. Rendering is to a shared framebuffer 
> (obviously this causes problems with caching). Locking and 
> synchronization is done using pthreads.
> 
> Other details: The system is cleanly booted for each run. No I/O is 
> performed during the timed portions of the test. The benchmark does 
> however read a model file from the drive and build a data structure 
> from it before each timed portion.
> 
> On the 2.6.22 series of kernels results are pretty much the same. On
> 2.6.23 series kernels I see a loss in speed of ~2% across the board.
> On 2.6.24-rc7 that loss in speed is perhaps very slightly worse (~3%).
> 2.6.22 Kernels tested: 22.9(Ubuntu Stock Kernel), 22.14, 22.15
> 2.6.23 Kernels tested: 23.1, 23.3,  23.13
> 2.6.24 Kernels tested: 24-rc7
> 
> I have my kernel compiled to use the SLAB allocator. All other 
> tweaking options are set as defaults. My config files are available at 
> http://vangogh.cs.tcd.ie/fowler/configs . Perhaps I'm configuring 
> something wrong for the type of work I do?

Could you try CONFIG_SCHED_DEBUG=y and CONFIG_SCHEDSTATS=y and double 
the value of /proc/sys/kernel/sched_latency_ns - does that make any 
difference? Please also run the following script while the ray-trace app 
is running:

  http://people.redhat.com/mingo/cfs-scheduler/tools/cfs-debug-info.sh

and send me the output of it, so that we can have an idea about what's 
going on in your system during this workload.

	Ingo
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