Re: [adam@flounder.net: Re: testing and no release schedule]
Russell Coker <russell@coker.com.au> writes:
> On Tue, 30 Mar 2004 05:37, Thiemo Seufer <ica2_ts@csv.ica.uni-stuttgart.de>
> wrote:
> > Matthias Urlichs wrote:
> > > Hi, Ingo Juergensmann wrote:
> > > > However, I'm quite "confused" about those "This is a buildd, please do
> > > > not use it for manual builds!" motds on some machines. Makes no sense
> > > > to me.
> > >
> > > Context switches eat into those machines' cache, thus two compile jobs in
> > > parallel run N times slower than the same compile jobs in sequence.
> >
> > Context switches don't eat any interesting amount of performance
> > on the usual buildd hardware. And quite on the contrary, parallel
> > compiler runs tend to be faster even on a UP machine with enough
> > ram, because the disk I/O is scheduled better.
>
> That depends on the hardware. When using a machine with a moderately fast CPU
> and two disks in a RAID-1 configuration I have not noticed any significant
> benefit in building with -J2 or in doing two builds at the same time.
>
> Having two compiles running at the same time means that when one process
> blocks on IO the other can use CPU time. The trade-off of this is that two
> processes means that there is less memory available for cache, and that cache
> memory is divided between the two processes. If you have a machine with 256M
> of RAM that is compiling a kernel then the cache will be very effective
> (everything you use in the source tree will fit). If you have two kernel
> builds on the same machine then as soon as you un-tar the second source tree
> you start kicking the first one out of cache.
>
> I agree that context switching is not going to cause any significant harm on
> most machines. But disk cache use is a more serious issue. On a modern
> machine with a couple of Gig of RAM it probably won't be a problem to have a
> dozen compiles running at the same time on account of disk cache. On an
> older and smaller machine things will be quite different. I wouldn't run two
> compiles at the same time on one of the original Pentium machines either.
Just for comparison: openh323 on the avg m68k buildd gives a timeout
(1200m without output) on the buildd when it uses -pipe. Thats
excessive swapping. Without -pipe 300m timeout or even 150m timeout is
enough.
MfG
Goswin
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