[fluid-dev] Multi core support committed and improved

[josh]

After implementing a mostly lock-less version of the multi-core  
support and optimizing it nicely (locks are only held for thread  
synchronization, which occurs once per fluid_synth_one_block), I went  
ahead and committed my changes.  Basically all the voices are  
synthesized to their own dedicated audio buffers, which turns out to  
be not all that much data (FLUID_BUFSIZE * 256 for polyphony=256 is  
64K for 32 bit float data) and simplified the logic greatly (no need  
to handle buffer starvation).  Once all voices have been synthesized,  
the primary synthesis thread mixes them all to the  
left/right/chorus/reverb buffers.  At this point, I think the overhead  
is pretty minimal.  I ran up to 8 cores on my single core laptop and  
saw fairly small overall CPU increase compared to single core.  So I'm  
much happier with this now.

I tested it on my little brother's core duo laptop and saw -F fast  
rendering increases of anywhere from 11-25%.  A decrease of 25% in  
render time doesn't seem all that bad, if you consider that the  
maximum theoretical reduction of 2 CPUs would be 50%.  I imagine some  
of the other code paths (reverb/chorus, etc) are probably rather  
significant and could perhaps also benefit from multi-cores (perhaps  
do reverb and chorus separately or something).

I also added audio.realtime=yes/no, midi.realtime=yes/no,  
audio.realtime-prio=1-99 and midi.realtime-prio=1-99 as I discussed  
before.  The core threads take advantage of this, to try and be the  
same priority as the audio threads.  The possible priority  
inconsistency is still an issue for programs embedding FluidSynth  
though, but it would still work, just perhaps not optimally.

A couple of bug fixes got committed too, which I caught with valgrind.  
 The -F fast render option is really useful for benchmarking and  
debugging.  Thanks for adding this! :)

I'd like to hear of other user's experiences with the new multi core  
support, on multi-core machines (a quad core or higher would be nice  
to get results on).  In summary, its useful if you want to maximize  
the number of potential FluidSynth voices on your system or -F fast  
render quicker to an audio file.  The largest benefit is seen when  
there are a lot of voices in a MIDI composition or sequence.  To try  
it, just pass -o synth.cpu-cores=N, where N is something like 2, 4, 8  
or 256 ;)

Cheers.

Josh