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>]

Please CC me as I'm not subscribed.

I have (what is to me) a strange and very repeatable slowdown for a
CPU intensive benchmark on my system  on newer kernels.

Hardware : Dell Precision 470.
CPU 2x2.0GHz Quad Core Xeon E5335 CPUs
Memory 4GB ECC RAM.
OS Ubuntu x86_64 7.10 (Gutsy Gibbon)
Compiler : gcc version 4.1.3 20070929 (prerelease) (Ubuntu 4.1.2-16ubuntu2)

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?

regards,
        Colin