Re: 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>]

Hi Ingo, thanks for the reply.

Modifying /proc/sys/kernel/sched_latency_ns to be double may have in
fact made things slightly worse. I used 24-rc7

Your script was only written to run for 15 seconds, so I ran it so it
multiple times so it covered most of the benchmark.
Other issues with these data may be that for much of the benchmark I
am building data structures utilizing at most 1 to 3 cores. I'm not
concerned with these timings personally as this is considered the
offline part of the render. Once these data structures are built I
proceed to render across 8 cores. This is the section of the benchmark
I get my timings from ( I use RDTSC before and after the render
segment). The majority of the overall time taken for a run is
therefore data structure building. I do not time this.

Colin.


On Jan 14, 2008 6:55 PM, Ingo Molnar <mingo@elte.hu> wrote:
>
> * 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
>