Quick question: would that
overhead end up being a factor on something like the iPhone, where the
CPU power is quite limited?
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
Primary and secondary synthesis threads process individual voices in
Primary thread mixes all voices to left/right, reverb and chorus
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.
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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