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[fluid-dev] Multi core support not so great

From: josh
Subject: [fluid-dev] Multi core support not so great
Date: Mon, 28 Sep 2009 00:29:00 -0700
User-agent: Internet Messaging Program (IMP) H3 (4.1.6)

I finished implementing a first pass at multi-core support. While it was a fun task, it didn't really yield the kind of performance I was hoping for. For those interested here is a description of the current logic:

Added a synth.cpu-cores setting.
Additional core threads are created in new_fluid_synth() (synth.cpu-cores - 1).
Primary synthesis thread signals secondary core threads when there is work.
Primary and secondary synthesis threads process individual voices in parallel.
Primary thread mixes all voices to left/right, reverb and chorus buffers.

Having multiple cores really just gives you the ability to have more voices in the case of live performance (before maxing the CPU) or *should* make your -F (fast MIDI render to file) operations go faster. The reason I say *should* is because it really depends on how complex the MIDI file is. If there aren't a lot of voices, it may in fact be slightly worse performance. Best case I have seen so far was about a 20% increase in speed (for the -F render case), which is something. Interestingly the 2 cores were still not quite maxed.

One issue that I have stumbled upon, is in regards to thread priorities. We want the secondary core threads to be running at the same priority as the primary synthesis thread, for round robin sort of response (though it may not matter that much if they are on separate CPUs). In the case of -F fast rendering you definitely don't want your processes running high priority (especially on Linux). In the live case though, the audio driver will be running high priority, so you want the secondary core threads also running high priority. The issue is, that currently the secondary core threads are created in new_fluid_synth(), while the synthesis "thread" is created by audio drivers or via other means. There needs to be some way to ensure that the secondary threads end up having the same priority. Any ideas? Perhaps a one time creation of the secondary threads within the fluid_synth_one_block routine and an attempt to make them identical in priority, would make sense.

In summary:
I realized through all this, that optimization is probably more important than multi-core support. Enabling multi-core support introduces additional overhead, so unless you are trying to get more voices in the realtime case or render MIDI files slightly faster, you're better off not enabling it.

So now that I learned my lesson. Should I commit the code? ;) Does it seem worth it? At the moment there may be some very minimal additional overhead in the single core case (compared to before), but that is probably so minimal as to be lost in the noise.


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