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Re: Use same variable in different threads [message #30138 is a reply to message #30137]

Sat, 11 December 2010 01:37

gprentice is currently offline

gprentice
Messages: 260
Registered: November 2005
Location: New Zealand

Experienced Member

Didier wrote on Sat, 11 December 2010 06:50

Hi all,

Quote:

Actually, on second thoughts I think the code as written is fine without volatile because the mutex forces the shared data to be updated in memory. Possibly an atomic variable that is read or written outside of a mutex region is more likely to need volatile.

Yes this is the case, and is what I explained in my earlier post.

Just to be clear, apparently there are situations when volatile is useful
http://www.drdobbs.com/high-performance-computing/212701484

but in general, you need a memory barrier when accessing shared data.

After googling for a while, I haven't found any clear explanation of how a mutex solves the caching and memory visibility issues. I'm guessing that both acquire mutex and release mutex flush the entire memory cache for all CPUs/caches and prevent hardware re-ordering across the memory barrier - which deals with the hardware problem. For dealing with compiler/linker optimisation, I'm guessing that the call to the "external" acquire/release mutex function forces the compiler not to cache values across that call because it can't tell what memory that function call is going to access. Explanations of how a mutex works don't seem to mention these issues.

That AtomicVar class looks like a good idea. Should the copy constructor be
AtomicVar( const AtomicVar& p) { AtomicWrite(val, AtomicRead(p.val)); }

Graeme

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[Message index]

		Use same variable in different threads By: koldo on Thu, 09 December 2010 18:26
		Re: Use same variable in different threads By: dolik.rce on Thu, 09 December 2010 18:57
		Re: Use same variable in different threads By: koldo on Thu, 09 December 2010 22:04
		Re: Use same variable in different threads By: Didier on Fri, 10 December 2010 00:26
		Re: Use same variable in different threads By: dolik.rce on Fri, 10 December 2010 06:49
		Re: Use same variable in different threads By: koldo on Fri, 10 December 2010 10:01
		Re: Use same variable in different threads By: dolik.rce on Fri, 10 December 2010 11:34
		Re: Use same variable in different threads By: koldo on Fri, 10 December 2010 13:19
		Re: Use same variable in different threads By: gprentice on Fri, 10 December 2010 14:08
		Re: Use same variable in different threads By: dolik.rce on Fri, 10 December 2010 14:16
		Re: Use same variable in different threads By: gprentice on Fri, 10 December 2010 14:34
		Re: Use same variable in different threads By: Didier on Fri, 10 December 2010 18:50
		Re: Use same variable in different threads By: gprentice on Sat, 11 December 2010 01:37
		Re: Use same variable in different threads By: Didier on Sat, 11 December 2010 23:13
		Re: Use same variable in different threads By: gprentice on Sun, 12 December 2010 01:57
		Re: Use same variable in different threads By: mirek on Sun, 12 December 2010 09:43
		Re: Use same variable in different threads By: koldo on Sun, 12 December 2010 23:16
		Re: Use same variable in different threads By: tojocky on Mon, 13 December 2010 11:35
		Re: Use same variable in different threads By: gprentice on Mon, 13 December 2010 11:46
		Re: Use same variable in different threads By: Didier on Mon, 13 December 2010 22:48
		Re: Use same variable in different threads By: koldo on Tue, 14 December 2010 09:02
		Re: Use same variable in different threads By: mirek on Sat, 25 December 2010 20:45
		Re: Use same variable in different threads By: tojocky on Sun, 02 January 2011 15:35
		Re: Use same variable in different threads By: mirek on Sun, 02 January 2011 18:38
		Re: Use same variable in different threads By: koldo on Sun, 02 January 2011 19:06
		Re: Use same variable in different threads By: mirek on Sun, 02 January 2011 22:56
		Re: Use same variable in different threads By: koldo on Mon, 03 January 2011 09:17
		Re: Use same variable in different threads By: koldo on Sun, 02 January 2011 01:12
		Re: Use same variable in different threads By: gprentice on Sun, 02 January 2011 03:08
		Re: Use same variable in different threads By: Didier on Sun, 02 January 2011 12:18
		Re: Use same variable in different threads By: gprentice on Sun, 02 January 2011 13:11

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