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Alternatives to CPU intensive while loop in Linux C

Posted on 2010-09-21
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Last Modified: 2012-05-10
Follow up from this question about atomic i++:

http://www.experts-exchange.com/Programming/Languages/C/Q_26488392.html?cid=239#a33725922

I would like to avoid doing a:

while (ring_buffer.count > 0);

If I was using mutex, I could use a conditional mutex which would effectively make the waiting thread sleep until it was alerted some data was ready. I'm trying to avoid mutex in this case so I would love an alternative.

Code snippet requested :-)
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Question by:PMembrey
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7 Comments
 
LVL 32

Expert Comment

by:phoffric
ID: 33726286
For the reader to avoid doing a polling loop in their reading thread, then it needs to wait on a signal (a counting semaphore can be used to implement that - not a mutex). But it sounds like you do not wish to use any kernel calls, so in that case, you have to poll (with at least some sleep).
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Author Comment

by:PMembrey
ID: 33726326
That's what I was afraid of :-)
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Expert Comment

by:phoffric
ID: 33726342
Using a signal (as opposed to mutex) should not result in performance degradation.
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LVL 40

Expert Comment

by:evilrix
ID: 33726823
There is little you can do to avoid a "busy" wait as long as the thread is getting a quantum. It will continue to do this unless the kernel is told to make it sleep. It will do this either if the thread is waiting on a kernel primitive (such as an event or mutex) or if you have called sleep() to tell it to yield it's quantum. The latter will still result in a busy wait but at least the thread will yield preventing thread starvation for other threads.

Any reason you don't want to use a proper threading primitive? More info would allow to give a refined answer.
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phoffric earned 2000 total points
ID: 33726890
If the reader is waiting on a semaphore signal from the writer, then it doesn't have to be that the signal is sent for each byte (or even each item). Instead, the writer could send a signal when there is a batch of items for the reader to process. The right balance for maximum throughput and reducing kernel overhead is application dependent.
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Author Closing Comment

by:PMembrey
ID: 33731618
Can't win them all :)
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LVL 32

Expert Comment

by:phoffric
ID: 33731678
If you want to discuss the details of your real-time requirement(s), we may be able to help soundboard your architecture for better performance.
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