Re: [Gems-users] Simulated machine execution time

[Dan Gibson <degibson@xxxxxxxx>]

This is a pretty application-specific observation; I am thinking about 
servers in general below:
As far as I am concerned, this is a way to tweak 'realism' vs. 
'processor-centric study'. Basically, at 75 MHz, I/O (lame though some 
of the I/O models are) is fast enough to enable one to study how I/O 
'intensive' workloads, like OLTP workloads, behave /on the processor/.
On the other hand, at 2000MHz, the performance tuning changes completely 
because I/O is a LOT slower relative to the processor speed. You may get 
more clocks per I/O event, but I think they're less likely to correspond 
with useful instructions.
That is, a low /simics processor speed/ is probably a reasonable proxy 
for a well-tuned application. That is, I'd expect to see more processor 
utilization as a consequence of the improved I/O timing.
Regards,
Dan

Greg Byrd wrote:
> Agreed.  Clock-based interrupts will occur less frequently (i.e., more 
> cycles in between).  I/O timing will look different.  Are there other 
> differences that you've encountered that I haven't considered?  (This 
> isn't a rhetorical question -- I'm really interested in what others 
> think about this.)
> ...Greg
>
>
> Mike Marty wrote:
> > > I tend to use the "sarek" configuration.  In 
> > > $SIMICS/home/sarek/sarek-common.simics, there's a python variable "cpu_freq" 
> > > that sets this parameter.  It defaults to 75.
> > > Also, at least for the Ultrasparc processor, it's saved when you do a simics 
> > > checkpoint.  Look for the string "freq_mhz".  You can just change this (for all 
> > > processors) in the checkpoint file directly.
> > > Note that changing this only affects time from the Simics point of view.  It 
> > > doesn't have any effect on Ruby cycles.
> > >   
> > >     
> > Except that it could change the execution path that a program takes
> >
> > --mike
> >
> >
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