Re: MAKE.PENTIUM.FAST (was Re: Bogomips)
kwrohrer@enteract.com
Sun, 17 Aug 1997 00:45:07 -0500 (CDT)
And lo, ganesh@kailash.cse.iitb.ernet.in saith unto me:
> Bogomips are calculated by timing an empty loop. Because of some
> branch prediction weirdness the Pentium performs rather poorly while
> executing an empty loop, incurring a pipeline flush at every jump.
> The following patch (2.0.30) boosts my P100 from a lowly 39.94 to 99.74
> Of course it's purely cosmetic - don't expect your pentium to run thrice
> as fast :)
Congratulations, you've just optimized an idle loop. Or rather, you've
just forgotten to optimize an idle loop: the one(s) that depend on bogomips
running the same code they do in order to do precisely timed busy waits.
I'm sure there's still at least one of those in the kernel, since not
every kernel can depend on having the pentium instruction counter doodad
in its CPU...
Keith