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 08:41 am

Mayan Moudgill wrote:
> 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)

There is no reason not to allow stale reads since most memory models
allow it.  A stale read is much faster than a read stalled by a
cache line refresh.  Though since RCU is now a defacto standard you
need new unspecified memory barriers to efficiently support it.

> 
> 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.

Transactional memory?  Sun is probably going to do it.  Somebody leaked
that in the solaris newsgroups.



-- 
Joe Seigh

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

.