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can load at least Windows 98 (Windows 95 doesn’t require a floating-point coprocessor, which
VSTHost absolutely needs). In case of a multiprocessor machine, however, this lack of interest for its
environment is a bit out of place, since it would mean that VSTHost only uses one processor. Since the
rise of the Core Duo and Athlon64 X2, this isn’t a good idea any more; a steadily increasing number
of machines have multiprocessor cores, and it’s not really fine if VSTHost uses only one half (or less,
in case of a quad core machine) of the available processing power.
Since V1.43, VSTHost can use as many processors as there are in the machine (up to 32, which is a
hard-coded 32-bit Windows limit; the x64 version can use 64 cores). If there’s only one processor, the
overhead is minimal (2 additional “if” statements in the audio processing thread). If there are more,
it’s considerable, but it pays off in the end.
Technical explanation
If you just want to make music, damnit!, you can safely ignore these paragraphs, although they might
help you to get the maximum performance from VSTHost if a certain configuration simply doesn’t
work the way you thought it should.
VSTHost starts as many processing threads as there are processors available. While processing audio,
it determines how many possible thread start points are possible, and then triggers as many threads as
possible and necessary to go to work on them. Each thread finds the same set of thread start points; the
first thread going to work on a start point blocks this path for the other threads, which go on searching
for thread start points to process. This continues until all threads come to the conclusion that there are
no more thread start points to process.
Thread Start Points
What is a “thread start point”? Hmm… well, VSTHost can only parallelize audio processing of
independent PlugIns. If a PlugIn’s input is dependent on another PlugIn’s output, they have to be
processed sequentially. Consider a simple setup:
Audio In PlugIn A
Audio Out
Here, nothing can be run in parallel. Everything has to be done in a strict order. VSTHost doesn’t even
bother to trigger a thread running on a second processor, as there is only one thread start point –
PlugIn A. Now, let’s assume that PlugIn A is a VSTi, and you want to run its output through a delay
effect:
Audio In PlugIn A
PlugIn B Audio Out
Everything still has to be done in a strict order, PlugIn a remains the only thread start point. In such a
configuration, there’s absolutely no performance gain from a multiprocessor machine.
A slightly more complex setup would be this:
Audio In PlugIn A
Audio Out
PlugIn B
Here, VSTHost can run the two PlugIns in parallel; it just to wait with the final audio output until both
PlugIns contributed their outputs. PlugIn A and PlugIn B are thread start points.
Let’s progress to an even more complex example:
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