Re: [Gems-users] Relation between SIMICS cycle & GEMS cycle

["Min Xu (Hsu)" <xu@xxxxxxxxxxx>]

Simics cycle number includes both dynamic
instructions and exceptions/interrupts.

When running with opal, each dynamic instruction
increment simics cycle number by 1, but the
same instruction may have much longer latency
in terms of the "GEMS cycle". The reason that
simics cycle is larger than the number of dynamic
instructions in your experiment is that simics
cycle number also counts the exceptions and
interrupts. I am curious what workload you are
using, because the number of exceptions and
interrupts seems high.

We are aware that because simics cycle
number don't really reflect the instruction
latency, interrupts are perhaps delivered
more frequent than it needs to be.

On Wed, 29 Mar 2006 Weihang Jiang wrote :
> I used both.
> 
> On 3/29/06, Min Xu (Hsu) <xu@xxxxxxxxxxx> wrote:
> > Did you use Opal or just Ruby only? It matters.
> >
> > On Wed, 29 Mar 2006 Weihang Jiang wrote :
> > > Hi,
> > >   Can anybody help me understand the relation between SIMICS
> > > cycle and GEMS cycle.
> > >
> > >   SIMICS cycle means the number of cycles SIMICS thinks the
> > > simulation has lasted, which can be calculated through
> > > calling SIM_cycle_count() at the beginning and the end of
> > > one simulation.
> > >   GEMS cycle is referring to the number of cycles opal/ruby
> > > has advanced.
> > >
> > >   Understanding the relation between SIMICS cycle and GEMS
> > > cycle is important for utilizing system level profiling
> > > tools. For example, vmstat reports both number of events
> > > (e.g. bi, bo) and relative execution time (e.g. us,sy,id).
> > > Other profiling tools report absolute execution time.
> > >
> > >   I did a small experiment to testify the relation. I ran
> > > SIMICS+GEMS with two different cache configurations for
> > > 10,000,000 GEMS cycles.
> > >   In experiment 1, using fast cache, 407,808 instructions
> > > have been executed and the SIMICS cycle number is 1,711,569.
> > > In experiment 2, using slow cache, 401,245 have been
> > > executed. However, the SIMICS cycle number is 1,664,671
> > > (different from exp1).
> > >
> > >    Based on this experiment, my understanding is that,
> > > SIMICS cycle is not an accurate measurement and those system
> > > level profiling tools can not be used without care.
> > >
> > >    So, I have 2 questions:
> > >
> > > 1. Is my understanding correct?
> > > 2. How much effort does it require to let SIMICS cycle have
> > > more sense(i.e. let SIMICS cycle == GEMS cycle)?
> > >
> > >
> > > --
> > > Weihang Jiang,  UIUC
> > >
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> 
> 
> --
> Weihang Jiang
>