Re: Benchmarking on CentOS 5

[Andrea Arcangeli <andrea@qumranet.com>]

On Fri, May 30, 2008 at 07:30:41PM +0200, Farkas Levente wrote:
> without crash) and the performance is similar (ie 80-90%) to the 
> development version. unfortunately it seems currently there is no such 

I hope you didn't reach this conclusion about different performance
between enterprise kernel host and mainline kernel host because I
didn't answer promptly to your previous email, sorry!

With regard to your performance question, preempt notifier emulation
really shouldn't be noticeable in real life. It worth to run the
fastest code only if you're doing pure benchmarking, in which case you
want to avoid any unnecessary exception and heavyweight exit (that
should reduces the time to schedule of a couple thousand cycles I
guess, but re-schedule events aren't so frequent and this is a
per-task per-cpu breakpoint with my latest emulation logic taking
advantage of the __switch_to internals).

I can hardly measure any difference here between SLES10 SP2 host and
mainline host with a simple dd from pagecache to /dev/null. The change
between enterprise kernel and current mainline kernel may be more
significant for the final performance than the impact of the preempt
notifier emulation. In any case the difference should be much less
than 5% as far as preempt notifier emulation is concerned. It should
be couple usec lost every couple msec or so.