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Re: Realtime Programing on MacOSX

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On May 19, 2004, at 10:13, Godfrey van der Linden wrote:
I agree with Scott, I recommend seting up a real time thread with then blocking it using nanosleep.

From reading the usleep source it looks like I should be able to use it also as it just calls nanosleep() under the covers.

MacOSX's real-time scheduling is very, very good. But you should be aware of a few things. We implement 'real-time' by using a very high priority thread band. The real-time threads are even higher priority than the I/O threads. This can cause some problems especially if you do TOO MUCH at real time. Careful design is paramount.

I my case it will be sending RTP data over the network. I should be able to limit the amount of time needed.

The 20ms scheduling will be trivial so I shouldn't worry about that. The only thing that can hold of a real-time thread scheduling is another real time thread or blocking your thread in the kernel behind a funnel. The biggest users of real-time threads in our system is CoreAudio and its clients. Be warned there are Apps out there that can use > 75% of the available CPU at real-time priority, Garage Band & Logic spring to mind.

This will be a server based app so I should be ok as there will not be any other CoreAudio type apps running with me.

What sort of work are planning on doing at that high a priority?

Voice over IP systems. I need to be able to pump audio around over the network using RTP and VoIP gets very picky if it does not get sent on time.


As an FYI this approach (usleep + realtime thread) is seeming to be working for the most part currently. There still are some weird delays (30-200ms) at times but I need to do some digging to make sure that something inside my code and network libs is not blocking and causing the problems.

I am currently using the thread_policy_set() call to set my self as real time priority. Is there any other functions you recommend calling to let the system know that it is a critcal thread?

Thanks for the help.

RJ
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References: 
 >Realtime Programing on MacOSX (From: RJ Auburn <email@hidden>)
 >Re: Realtime Programing on MacOSX (From: Scott Lamb <email@hidden>)
 >Re: Realtime Programing on MacOSX (From: Godfrey van der Linden <email@hidden>)



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