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content-class:	urn:content-classes:message
Subject:	RE: pthread_cond_wait does not relock mutex on release
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Date:	Thu, 18 Apr 2002 16:57:13 +1000
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From:	"Robert Collins" <robert DOT collins AT itdomain DOT com DOT au>
To:	"Michael Labhard" <ince AT pacifier DOT com>, <cygwin AT cygwin DOT com>
X-MIME-Autoconverted:	from quoted-printable to 8bit by delorie.com id g3I6vYE16857

> -----Original Message-----
> From: Michael Labhard [mailto:ince AT pacifier DOT com] 
> Sent: Wednesday, April 17, 2002 10:51 PM
> To: cygwin AT cygwin DOT com
> Subject: Re: pthread_cond_wait does not relock mutex on release
> 
> 
> Robert and Gerald:
> 
>          Both quite right.  Although adding the SAFE_PRINTF made no 
> difference in the 
> output, checking a condition value in a loop made all the 
> difference.  
> Putting a printf in the loop revealed to my surprise that 
> "spurious wakeups" 
> were occurring thousands of times per second.  I had naively 
> assumed that a 
> condition would stay set until signalled.  Now I'm led to wonder if a 
> condition variable is any performance improvement over a 
> simple loop and 
> short sleep.  Any thoughts?

Without knowing what you are trying to achieve, I can't comment. However: in multi-threaded programs, a condition variable will almost invariably be more efficient than a loop + sleep.

As for conditions staying set, that isn't the purpose of condition variables: they are designed to allow signalling between threads, to allow efficient use of cpu resources - and to avoid that loop+sleep approach.

Rob

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