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Re: scheduling a block from a render callback
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Re: scheduling a block from a render callback

  • Subject: Re: scheduling a block from a render callback
  • From: Paul Davis <email@hidden>
  • Date: Tue, 07 Jan 2014 21:01:55 -0500



On Tue, Jan 7, 2014 at 8:44 PM, Lucas Ives <email@hidden> wrote:
Hey all,

In a nutshell: from my audio render callback I'd like to trigger work,
on demand, on another thread. What's the preferred way to do this sort
of thing?

Specifically:  after I've rendered a particular audio frame, I'd like to fire
a completion block.  dispatch_async() seems like an obvious way to do
this, but per Apple anything in the BSD layer or anything that may take
a lock -- I believe GCD is a yes on both counts --  is off limits in the
real-time callback.

I could pass data through a lock-free pipe back to another thread and
poll its existence, but my blocks have the property that they both need
to fire as soon as possible and also are used (relatively)
infrequently... so it seems like a waste to be constantly polling for
them using NSTimer or the like.

the darwin kernel (like most (but not all) POSIX style kernels) does not offer any guaranteed realtime safe methods of signalling one thread from another.

you are therefore left to decide which of the "bad" solutions is least bad.

among the options:

    - write a single byte to a POSIX FIFO that is being poll/select-watched by the other thread
    - use a semaphore
    - use a Mach Port

these all have various advantages and disadvantages. they will all work. none are guaranteed RT-safe.
 
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 >scheduling a block from a render callback (From: Lucas Ives <email@hidden>)

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