Subj : Re: CMPXCHG timing
To   : comp.programming.threads
From : David Schwartz
Date : Fri Apr 01 2005 03:59 pm


"Michael Pryhodko" <mpryhodko@westpac.com.au> wrote in message 
news:1112399107.865711.5270@f14g2000cwb.googlegroups.com...

> Ok. Situation:
> - two processors have empy store buffers
> - two processor were executing similar code for some time (i.e.
> pipeline state should be similar)
> - now they execute:
> mov mem_addr, value
> sfence

> For me it is quite logical that they should spend the same amount of
> clocks  for this operation (suppose that flushing store buffer takes
> the same time  regardless of mem_addr). I understand that this is only
> my assumptions.

    That's an absurd assumption and in general will not be true. This is 
*especially* true if they're running similar code because they're contending 
for resources and each time one CPU will win and one will lose (cachewise).

    There are so many reasons why this won't be true in the real world and 
new ones are appearing all the time. What about interrupts?

    DS

.