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

* Colin Fowler <elethiomel@gmail.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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