[CLUE-Tech] Athlon = i686 in kernel 2.4.7 Makefile

[Timothy C. Klein]

* Jed S. Baer (thag at frii.com) wrote:
> As I mentioned before, the test you mention isn't in my makefile. This is
> the 2.4.7-10 kernel. It explicitly sets march=i686 when CONFIG-MK7 is
> defined. The test you mention must be a later kernel release.
> 

Oh yeah, you did say that.  For grins, I downloaded the 2.4.7 source
tree, and looked at it.  That tree does have the line to use the Athlon
arch if it is available.  The relevant lines ar 65-67 of:

linux/arch/i386/Makefile.

So, if it is in that source tree by default, why the heck would RedHat
take it out?  That just baffles me.

> I'll be interested to here how it comes out. I was hoping there was some
> "known" article on why RH (or whomever) didn't use the athlon capability
> in a distribution where the c compiler supports it, as, AFAICT, 2.96 does.
> Or maybe RH didn't tweak the makefile, even though they included an
> athlon-capable compiler.

Well, it finished the compile, for a change, as long as I let the kernel
Makefiles pick the CFLAGS optimizations.  That makes sense, as the
kernel is pretty reliant on certain functions being inlined, certain
functions residing at certain addresses, etc. so it really does need to
be in control of how the kernel is compiled.

So now, I have an Athlon architecture kernel, rather than i686.  I
downloaded lmbench, and ran a quick benchmark on both kernels.  I was
rather surprised:  the Athlon one *is* actually faster at certain tasks.
I would need to do several runs to know for sure, but it does seem to
make a difference, especially for DISK->MEMORY->CPU bandwidth tests.
Looks like about 1% to 10% increase on local communication bandwidths.

So, it might be worth the trouble downloading the latest kernel tarball.
BTW, this kernel is patched for low-latency, and full-preemptability,
but otherwise is just the default 2.4.18 kernel.

HTH

Tim
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== Timothy Klein || teece at silverklein.net   ==
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