compiler optimizations [was: i686 repository]

[Michal Zeravik]

Hi Florin,

I tried to recompile quanta 3.2 (very slow app)
and It seems to be about 10% faster with cflags set for athlon-xp.
Now I'm trying to rebuild kde32 and going to rebuild X.
What are chances in optimizing multimedia application
for runtime speed and stability?

michal


Florin Andrei wrote:

>On Tue, 2004-03-09 at 05:25, Phil Hannent wrote:
>
>  
>
>>Does anyone know of a repository that has i686 binaries compiled for fedora?
>>While the argument is that there isn't much benefit from having
>>optimised binaries, the fact that Gentoo linux exists shows there is a
>>demand.
>>    
>>
>
>For normal packages you shouldn't need a thorough optimization, because
>the gains are so small it's difficult to even measure them.
>And by the way, optimizing for i686 is a "suboptimal optimization" :-)
>nowadays.
>
>I just use the default packages most of the time. There are cases when
>an aggressive optimization is desireable and yields measurable results
>(multimedia apps such as transcoders, music apps, etc.), but then you
>have to perform a full optimization on them.
>
>As a rule, if an application does not fall under at least one of these
>categories, it's not worth pursuing aggressive optimizations:
>- it runs at 100% CPU usage for a long time (e.g. when transcoding from
>DVD to DivX)
>- it has to deliver very low latency (e.g. when recording audio with the
>computer)
>If at least one criterion is satisfied, you may try to optimize (but
>don't expect miracles every single time).
>
>Myself, i compile (rebuild RPMs) those apps with "-march=athlon-xp
>-mcpu=athlon-xp" (because my system is an AthlonXP/1800) and some other
>minor tweaks. I keep that in my ~/.rpmrc file (sorry if my mailer splits
>the line):
>
>optflags: athlon -O3 -march=athlon-xp -mcpu=athlon-xp -mfpmath=sse -pipe
>-fomit-frame-pointer
>
>But remember, i'm for the most part focused on multimedia type of
>things.
>
>Often, the difference between -O2 and -O3 is bigger than what
>-march=athlon-xp provides (and no, -O3 is not always faster). Also, all
>those -fomit-frame-pointer type of arcane options can deliver unexpected
>results.
>
>There is no such thing as "this is The Magic Gcc Option that will make
>all your apps run 10x faster". Optimizing apps is DIFFICULT. What are
>you optimizing for? Speed? Latency? Memory usage? Are you running other
>apps at the same time? How big is your CPU's cache? Are you sure the
>app's code can take advantage of all those gcc bells and whistles? And
>last but not least, do you know how to properly benchmark different
>application versions?
>
>Believe me, optimizing an app is like cracking open a clockwork puppet:
>all of a sudden all kinds of tiny little springs and gear pop out of
>nowhere, leaving you scratching your head and asking yourself how the
>hell are you going to put all that back inside.
>
>If you really want to squeeze all speed out of an app, you'll spend an
>incredible amount of time tweaking gcc options and doing benchmarks for
>days and weeks. And then a minor detail changes with the conditions
>(like, you start using another app at the same time) and that changes
>everything. :-(
>It sucks. Just use some generic optimizations, you'll do yourself a
>service. Unless your needs are very special, such as if you build some
>kind of appliance or something (but then you probably wouldn't use a
>generic distribution such as Fedora).
>
>  
>

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