Subj : Re: What is the real costs of LOCK on x86 multiprocesor machine?
To   : comp.programming.threads
From : Mayan Moudgill
Date : Thu Aug 04 2005 08:17 am

Joe Seigh wrote:

>>
> 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.
> 

I'm not quite sure I get what you mean. In the MESI protocol, a shared 
line will get invalidated when some other processor wants to write to 
the line (E or S->I). This means that if a processor is writing to an I 
line, then it has to fetch the new value. That is unavoidable (other 
than the case of false sharing). [Caveat: if the line in the other core 
is E but not M, then the data does not have to be fetched, but you still 
have to wait for the ack]

Now, where do you see an opportunity to optimize the transaction? 
(again, excluding false sharing)

One optimization which has been talked about (though I don't know 
whether any processor actually implements it) is to allow loads (and 
dependent ops) to execute speculatively against an I line. If it turns 
out that the line was E then the speculative ops are committed, if it 
turns out it was M, then they are restarted.

.