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Re: [adam@flounder.net: Re: testing and no release schedule]

On Tue, 30 Mar 2004 05:37, Thiemo Seufer <ica2_ts@csv.ica.uni-stuttgart.de> 
> 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.

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