Optimized kernel builds: the straight dope [fp extensions]

[Krzysztof Lichota]

Matt Zimmerman napisał(a):
> On Tue, Aug 15, 2006 at 05:26:03PM +0200, Pavel Rojtberg wrote:
>> Matt Zimmerman wrote:
>>> I'm interested to hear (objective, reasoned) feedback on this data and my
>>> conclusions from the members of this list.
>> it would be interesting to see a i386 vs k7 comparison.  Since the
>> architectural difference is bigger; the k7 architecture has some floating
>> point extensions(3D Now) which usually have a big impact on performance.
>> Therefore we should look if the kernel can handle these well enough
>> dynamically.  Perhaps it would make sense to leave one optimized kernel on
>> i386 for newer CPUs.  This could be SSE optimized then to be more generic
>> (AthlonXP+, Pentium3+)
> 
> I'll add to what Ben has already said that 3DNOW, SSE, MMX and friends are
> not general-purpose extensions which make your system faster.  They are in
> fact very specialized, and specialized for operations that the kernel
> doesn't do.  The kernel very nearly doesn't use floating point at all, much
> less floating point extensions for 3D graphics!

AFAIR gcc uses SIMD instructions for faster memcpy & friends and
probably in some other places. So it could have some effect on
performance (unless this is switched at runtime).
Using prefetch instructions also can speedup some routines (encryption,
compression, etc.).
However, I am not sure it is enabled in kernel compilation.

IMO getting rid of these optimized kernels should be done after much
broader testing than 2 randomly chosen desktops. Think of people using
encrypted filesystems, compressed filesystems, etc.

	Krzysztof Lichota

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