[concurrency-interest] propper way to use CAS ?

Mike Skells mike.skells at ebizz-consulting.com
Mon Oct 10 09:48:30 EDT 2005


Hi,
Doesnt this presuppose that you are running on a system the is pre-emptive,
So if you are running on a non pre-emptive system then the unsuccessful CAS
retries will just continue wont they?

Mike

> -----Original Message-----
> From: concurrency-interest-bounces at cs.oswego.edu 
> [mailto:concurrency-interest-bounces at cs.oswego.edu] On Behalf 
> Of Brian Goetz
> Sent: 09 October 2005 16:18
> To: Yechiel Feffer
> Cc: concurrency-interest at altair.cs.oswego.edu
> Subject: Re: [concurrency-interest] propper way to use CAS ?
> 
> > in many concurrent classes when compare-and-set (cas) is 
> used- it is 
> > an endless loop of competing threads trying to fight over an object 
> > (,say, a queue's tail in order to connect a new element 
> (reference)) .
> > 
> > Wouldn't it be benefitial to perform a thread.Yield() after N
> > (pre-configured) unsuccessful CAS retries ? wouldnt it give better 
> > overall performance ?
> 
> It depends on the degree of contention you expect to experience.
> 
> In most cases, the standard approach (retry continuously) is 
> best.  Only when you are expecting very heavy contention, 
> where you expect most CAS attempts to fail, is a more 
> sophisticated backoff strategy beneficial.
> 
> When there is no contention for a CAS, it always succeeds.  
> When multiple threads compete for a CAS, one will always win 
> and make progress.  So the question is, what percent of CAS 
> attempts will lose and need to be retried?  Only if this 
> ratio is high will a better contention management strategy be 
> a win.  This is a function of how many threads are involved, 
> and the ratio of CAS to "other work".
> 
> If your contention is so high that most CAS attempts fail, 
> you might be better off with a lock, as locking automatically 
> addresses the problem you raise fairly well, by blocking so 
> as not to create more contention.
> 
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