On 26/01/2012 23:37, Evan Laforge wrote:
I'm slightly surprised by this - in my experience parallel builds beat --make as long as the parallelism is a factor of 2 or more. Is your dependency graph very narrow, or do you have lots of very small modules?
I get full parallelism, 4 threads at once on a 2 core machine * 2 hyperthread/whatever core i5, and SSD. Maybe I should try with just 2 threads. I only ever get 200% CPU at most, so it seems like the hyperthreads are not really much like a whole core.
The modules are usually around 150-250 lines. Here are the timings for an older run:
from scratch (191 modules): runghc Shake/Shakefile.hs build/debug/seq 128.43s user 20.04s system 178% cpu 1:23.01 total no link: runghc Shake/Shakefile.hs build/debug/seq 118.92s user 19.21s sys tem 249% cpu 55.383 total make -j3 build/seq 68.81s user 9.98s system 98% cpu 1:19.60 total
This looks a bit suspicious. The Shake build is doing nearly twice as much work as the --make build, in terms of CPU time, but because it is getting nearly 2x parallelism it comes in a close second. How many processes is the Shake build using? I'd investigate this further. Are you sure there's no swapping going on? How many processes is the Shake build creating - perhaps too many? Cheers, Simon