On Jan 15, 2007, at 11:52 AM, Daniel Tourde wrote:
The machine (Inspiron 9400) is fast but I saw at certain moments
something
like "NO-MMX, NO-SSE" (some flags or variables) during the compilation
process. I thought then "How come? What a pity not to use these
instructions".
Can someone tell me what it was and if it is really supposed to be
like this?
You'll find the compiler being passed "-mno-mmx -mno-3dnow -mno-sse -
mno-sse2" throughout the compilation of the kernel and bootstrap
loader, and yes, it is really supposed to be like that.
My roots are in Gentoo Linux where it is possible to get the
maximum out of a
processor when building a system from scratch by using properly
certain C and
C++ flags.
Yes, people writing Linux have spent all kinds of effort writing
various fancy block copies which use MMX or SSE or whatnot, and
having the system perform benchmarks at boot to pick the fastest, but
they are tuning this for microbenchmarks without really considering
the effects of prefetching lots of data when it may already be cached
or by potentially flushing away other useful data in the caches
(especially if you're doing DMA and want the disk drive or NIC to end
up with the data and not your CPU's caches).
There's tons of information and benchmarks here in this thread:
http://lists.freebsd.org/pipermail/freebsd-performance/2006-
December/002375.html
--
-Chuck
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