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Re: Objective-C in a Time Constraint Thread

Hi Sekhar,

I would be surprised if making a method call from a CoreAudio ioproc would cause an audio glitch even on slow computers. I had various levels of processing loads in CoreAudio ioprocs when I first started with CoreAudio and have gradually changed some things to lessen the load, but back I never experienced any audio glitches (even on a 500mhz G3).

It's best to plan most efficiently, but I don't think a handful of method calls is going to cause any problems.

-- John


I'm trying to get a better understanding of issues that may arise when using Objective-C inside of a CoreAudio IOProc.

The CoreAudio Hardware Abstraction Layer calls clients to produce audio on a time-constraint thread. Thus, audio generating functions are discouraged from invoking functions that may block the thread or take "a long time" to return (e.g., locking mutexes, allocating memory, reading from disk) [1].

On the CoreAudio list, it was pointed out that "... ObjC message sends are treated as an opportunity for the ObjC runtime to re- evaluate its world, and thus can not only involve locks, but also memory allocations" [2].

I looked at the source for objc_msgSend <http:// darwinsource.opendarwin.org/10.3/objc4-235/runtime/ Messengers.subproj/objc-msg-ppc.s> and, sure enough, there is a comment before the macro LockCache:

; The collecting cache mechanism precludes the need for a cache lock
; in objc_msgSend. The cost of the collecting cache is small: a few
; K of memory for uncollected caches, and less than 1 ms per collection.
; A large app will only run collection a few times.

I'm assuming this is what was being referred to in the ca-api list post.

Is this comment still accurate? If so, calling objc_msgSend in an IOProc is certainly risking audio glitches. Are there ways to decrease the probability of having the cache collected or is there any way to avoid it altogether? For example, will caching the IMP and invoking that directly help?

Thanks.

- sekhar 
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 >Objective-C in a Time Constraint Thread (From: Chandrasekhar Ramakrishnan <email@hidden>)



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