7+ days of dogfood

David Chisnall David.Chisnall at cl.cam.ac.uk
Mon Feb 11 14:49:28 UTC 2013

On 11 Feb 2013, at 13:56, Fabian Keil <freebsd-listen at fabiankeil.de> wrote:

> real    350m42.363s
> user    253m5.477s
> sys     50m0.024s

These numbers look a bit wrong.  You've got 300 minutes of CPU time, but 350 minutes of real time.  In an ideal world, on your dual-core system you'd see 150 minutes of real time.  Seeing more than 300 implies that you're spending a lot of time waiting for I/O.  The normal recommendation is to use -j x where x is 1.5 times the number of cores, or 1x the number of GBs of RAM, whichever is smaller.  With only 2GB of RAM you might have linking problems with -j3, but it's still worth trying.

One of the more serious problems with our current build system is that it doesn't scale well to large numbers of cores.  On a 32-core system, with -j64, we're very rarely managing to have even 8 things able to run in parallel.  This should be addressed when the bmake import is fully integrated and we can use meta mode for better dependency tracking.  

Ninja has a concept of pools, so you can say 'only run one link job at a time, but you can do two C++ compile jobs or 4 C compile jobs', and it might be interesting to look at adding something similar to bmake, as this can improve scalability a lot.


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