Optimizationn questions?

Gary Kline kline at tao.thought.org
Thu Mar 15 15:31:50 UTC 2007

On Wed, Mar 14, 2007 at 08:19:49PM -0700, youshi10 at u.washington.edu wrote:
> On Thu, 15 Mar 2007, Danny Pansters wrote:
> >On Thursday 15 March 2007 02:16, Gary Kline wrote:
> >>	Two quick one for kernel and/or compiler wizards:  first, is
> >>	a 400Mz processor considered a 586 (for my KERNELCONF file)?
> >
> >Think its 686 (but really, leaving 486 and 586 in isn't going to slow down
> >booting or anything!) I always say: Use GENERIC unless you have a good 
> >reason
> >not to.
> >
> >>	Second, is it safe to do a buildworld with -O3?  If there are
> >
> >No. It's not supported if things break.
> >
> >>	stability concerns, I'll go with the default when I rebuild my
> >>	6.2 systems.
> >
> >The defaults should be fine. Also, like I said consider just using GENERIC 
> >and
> >load the odd kmod if needed. Generally it's less headache and equal
> >performance.
> >
> >>	thanks in advance,
> >>
> >>	gary
> >
> >Cheers,
> >
> >Dan
> As Dan and Gary said -O3 isn't supported, and in many cases that "level of 
> optimization" gets filtered out while compiling sections of FreeBSD.
> Besides, I've compiled stuff with -O3 and various optimizations in Gentoo 
> Linux before, and let me say that it caused a great deal of headaches... 
> that's why I stick with -O2 now, because it's better to have something in 
> executable shape and a bit slower (arguably because some optimizations slow 
> things down) than it is to have something run fast and break all the time.
> Some food for thought :).

	--Food for thought and a chuckle too!  (not to mention that
	it's waaay early, the chickens are still snoring, and I've
	only had *one* cup of joe)...   I've done some investigation
	with optimizing my own code, usually < 1000 lines, and haven't
	seen much gain between -O2 and -O3. Loop-unrolling may be
	different; one trick that compiler hackers at supercomputer 
	companies use by default in to unroll small loops.  Cray is
	one example.  Soooo, to get any real gain is going to mean
	going thru the most freq used tools (*grep, find, ls) and
	hand-tweak.  Might buy 5 - 7%.  

	have a good one,


> -Garrett
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  Gary Kline  kline at thought.org   www.thought.org  Public Service Unix

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