Re: More kernel performance tests on FreeBSD 10.0-CURRENT

On Sat, Sep 22, 2012 at 02:20:14PM +0300, Konstantin Belousov wrote:
> On Fri, Sep 21, 2012 at 11:39:40PM +0200, Dimitry Andric wrote:
> > Hi all,
> > 
> > As a followup to my previous post about the performance of FreeBSD 10.0
> > kernels compiled with different compilers (clang and gcc), I did another
> > series of tests, now on a more modern machine (Core i5-based).  I also
> > tested the performance with different compiler optimization settings.
> > 
> > The attached text file[1] contains more information about these tests,
> > performance data, and my conclusions.  Any errors and omissions are also
> > my fault, so if you notice them, please let me know.
> > 
> > The executive summary: GENERIC kernels compiled with clang 3.2 are again
> > a little faster than those compiled with gcc 4.2.1.  For gcc, compiling
> > with -O2 also gives a slightly faster kernel than with -O1, but for
> > clang there is no measurable difference between those flags.
> > 
> > Again, many thanks to Gavin Atkinson for providing the required
> > hardware.
> ...
> 
> > Conclusion:
> > -----------
> > Kernels compiled with clang are a little faster in real time for building world,
> > and in system time the difference is even larger, roughly 10%.  For clang, the
> > difference between -O1 and -O2 is not measurable, but for gcc, -O2 is slightly
> > faster than -O1.
> > 
> 
> Thank you very much for finishing the initial assessment.
> In my opinion, this positively closes the issue of the uncertainicity
> of the performance impact of the proposed clang use by default for the
> base system.

It does not close everything.  The kernel does not use
floating point.  I showed last week that clang may have
problems with floating point.

-- 
Steve