On Wed, 22 Jun 2005, Colm Buckley wrote:
> On 22 Jun 2005, at 16:18, Vishal Vatsa wrote:
>> > Also most people don't want to spend like a couple of days of setting up the
> > system for 10-20% boost. Its just not worth the compile time if don't have
> > the latest and greatest CPU. And if you do then do you really care about 10%
> > boost?
>> 10-20% is *extremely* unlikely. It might be plausible if you were running
> i386 code on a P4, but not when using funroll-loops etc against stock -O2 on
10-20% sounds too high.
> Also note that most of the recommended Gentoo optimisations such as
> funroll-all-loops are optimising CPU usage against disk and RAM usage. On
> most modern systems, where the CPU is very fast and RAM and disk slower, this
> will lead to *decreased* system performance.
funroll-all-loops can be counter-productive. In the event the loops is a
large loop, it can fill the icache. When this happens, performance will
decrease - fast.
It's the same with inline functions. They *are* faster to call because
they get inlines (hence, they are not really called at all). But if they
are called a lot and are sizable functions, the binary size increases a
lot, trashes the cache and performance goes down the tubes.
One mans performance gain is another mans regression
Part-time Phd Student Java Applications Developer
University of Limerick IBM Dublin Software Lab
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