Subj : Re: What is the real costs of LOCK on x86 multiprocesor machine?
To   : comp.programming.threads
From : Joe Seigh
Date : Thu Aug 04 2005 08:04 am

chris noonan wrote:
> Joe Seigh wrote:
> 
>>chris noonan wrote:
>>
>>>The way I meant it is already in the literature:
>>>"Highly Efficient Synchronization Based on Active Memory
>>>Operations"
>>>Zhang, Fang, Carter
>>>http://www.cs.utah.edu/~retrac/papers/ipdps04.pdf
>>>
>>
>>It seems to be meant for parallel computation where the
>>cpus are dedicated to the computation and it's ok for a
>>cpu to just wait on the result of the atomic operation.
>>These would be supercomputers with individual computers
>>connected with high speed links and they're interested in
>>reducing network traffic over the links.
>>
>>I suppose this might be applicable to regular multi-threaded
>>programming once you get 50 to 100 core processors widely available
>>and you can dispense with time slicing.
> 
> 
> I suspect it would also be valuable for PC-type systems
> having (say) four processors. These are currently architected
> with a MESI crossbar equalising the processor data caches.
> The purpose of this is presumably to provide *implicit*
> synchronisation between threads running on different
> processors.
> 
> However, because of the Pentium's out-of-order execution
> and write buffering, *explicit* synchronisation (memory
> barriers, bus-locking instructions) is required in any
> case. So what does cache coherency achieve?
> 
> Why not synchronise processors at the physical point
> where they are already connected, at main memory?
> 
MESI is required to maintain coherency so that only one
processor has a cache line exclusive at a time and thus
keeping updates from getting lost.  If you didn't have
false sharing, you wouldn't need this.

The big problem is with the strong part of strongly
coherent.  When you have more cache traffic than you
need due to repeated refreshes of invalidated cache
lines, that has to slow things down.  Hardware designers
don't get it.


-- 
Joe Seigh

When you get lemons, you make lemonade.
When you get hardware, you make software.

.