Re: [Gems-users] executed instructions

[Dan Gibson <degibson@xxxxxxxx>]

Braulio,

In multiprocessor systems, the execution path (and the number of 
instructions) will vary from execution to execution, depending on (among 
other things) minor variations in the observed memory latencies. Even 
small deltas can cause locks to be acquired in different orders, causing 
different executions.  These locks can reside in the OS, remember, so 
even single-threaded code can suffer on multithreaded machines. We 
combat this variation by running multiple simulations of each 
configuration and computing 95% confidence intervals.
As far as comparing the number of instructions executed, you might want 
to ensure that simulation really has stopped in the same "place" -- eg. 
the insturction pointers of each processor are identical for both of 
your protocols when execution finishes. When using magic instructions in 
more than one location, problems can arise when they are reached in 
different orders.
Regards,
Dan

Braulio Pino wrote:

> Hello,
> I have run two different implementations of the MOSI protocol and I want to 
> check whether one of them improves the other in terms of  execution cycles. 
> To this end, I have created a checkpoint and I use the magic instructions in 
> order to the two simulations run the same part of the application that I 
> want to simulate. After running them, I have seen that the number of 
> executed instructions is different in spite of simulating the same part of 
> the application.  Therefore, I do not know if it is possible that I have 
> done something wrong or, on the other hand, those results could be right.  I 
> think that if I simulate the same part of the code, both simulations should 
> run the same number of instructions, shouldn't they? In case the simulations 
> are correct, do I have to take into account the number of executed 
> instructions to compare the execution time or I can compare the ruby cycles 
> directly?
> Thanks you.
>
> Regards.
>
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