Re: [Gems-users] IPC and Ruby cycles

["Lei Yang" <lya755@xxxxxxxxxxxxxxxxxxxx>]

Thanks a lot Dan! That explains it. But what I still can't understand is, 
why would Opal reports such a high IPC for most of processors: P0 1.6, P1 
3.96, P2 3.97, P3 0.62? I thought even with out-of-order pipelined CPU the 
maximum theoretical instruction throughput should be 1 instruction per 
cycle. Also, if I run multiprogrammed benchmarks (mix of SPEC benchmarks) in 
the system, say 4 programs for 4-core CMP, would one processor run one 
program or would there be switching? Thanks again for your comments.
Lei
----- Original Message ----- 
From: "Dan Gibson" <degibson@xxxxxxxx>
To: "Lei Yang" <lya755@xxxxxxxxxxxxxxxxxxxx>
Sent: Wednesday, March 28, 2007 2:22 PM
Subject: Re: [Gems-users] IPC and Ruby cycles


> Opal's sim-step command executes N instructions on processor 0, then stops 
> execution. The other processors are allowed to execute as many 
> instructions as they can in the same duration. I don't think there is a 
> way to force all processors to execute the same number of instructions.
> If you'd like to run for a certain number of *cycles*, you can run Opal 
> for some absurdly large number of instructions and then explicitly break 
> the simulation at some RUBY_CYCLE count. See SIMICS_break_simulation() for 
> details in the Simics Reference Manual.
> Regards,
> Dan
>
> Lei Yang wrote:
> > Thanks Dan! I'm not using Opal now, as I got confused at the way IPC is 
> > calculated. I'm pasting a question I asked before on the list. 
> > Unfortunately it was not answered at that time.
> >  I simulated a four core CMP system with both Opal and Ruby. I found in 
> > results.opal the runtime stats for processor [0],[1],[2],[3].  But as can 
> > be seen from below, how come only processor [0] executed 100 million 
> > instructions specified by "opal0.sim-step 100000000", but processor [1] 
> > and [2] have more total number of instructions than what I've specified ?
> >  Is there a way in Opal to let the simulator run for a certain number of 
> > cycles, instead of instructions (like "cc N" and "c N" in simics)?
> >  [0] *** Runtime statistics:
> > [0]   number of continue calls                            100000003
> > [0]   Instruction per cycle:                             1.55227
> > [0]   Total Elapsed Time:                                36060 sec 0 usec
> > [0]   Total Retirement Time:                             1522 sec 670804 
> > usec
> > [0]   Approximate cycle per sec:                         1786.52
> > [0]   Approximate instructions per sec:                  2773.15
> > [0]   This processor's Simics overhead (retire/elapsed):   4.22%
> > [0]   Average number of instructions per continue          1.00
> > [1] *** Runtime statistics:
> > [1]   Total number of instructions                        255117523
> > [1]   Total number of cycles                               64421882
> > [1]   number of continue calls                            255117523
> > [1]   Instruction per cycle:                             3.96011
> > [1]   Total Elapsed Time:                                36060 sec 0 usec
> > [1]   Total Retirement Time:                             3740 sec 262608 
> > usec
> > [1]   Approximate cycle per sec:                         1786.52
> > [1]   Approximate instructions per sec:                  7074.79
> > [1]   This processor's Simics overhead (retire/elapsed):  10.37%
> > [1]   Average number of instructions per continue          1.00
> > [2] *** Runtime statistics:
> > [2]   Total number of instructions                        256142056
> > [2]   Total number of cycles                               64421882
> > [2]   number of continue calls                            256142056
> > [2]   Instruction per cycle:                             3.97601
> > [2]   Total Elapsed Time:                                36060 sec 0 usec
> > [2]   Total Retirement Time:                             3713 sec 458821 
> > usec
> > [2]   Approximate cycle per sec:                         1786.52
> > [2]   Approximate instructions per sec:                  7103.2
> > [2]   This processor's Simics overhead (retire/elapsed):  10.30%
> > [2]   Average number of instructions per continue          1.00
> > [3] *** Runtime statistics:
> > [3]   Total number of instructions                         40012500
> > [3]   Total number of cycles                               64421882
> > [3]   number of continue calls                             40012500
> > [3]   Instruction per cycle:                             0.621101
> > [3]   Total Elapsed Time:                                36060 sec 0 usec
> > [3]   Total Retirement Time:                             600 sec 825168 
> > usec
> > [3]   Approximate cycle per sec:                         1786.52
> > [3]   Approximate instructions per sec:                  1109.61
> > [3]   This processor's Simics overhead (retire/elapsed):   1.67%
> > [3]   Average number of instructions per continue          1.0
> >
> >     ----- Original Message -----
> >     *From:* Dan Gibson <mailto:degibson@xxxxxxxx>
> >     *To:* Lei Yang <mailto:lya755@xxxxxxxxxxxxxxxxxxxx> ; Gems Users
> >     <mailto:gems-users@xxxxxxxxxxx>
> >     *Sent:* Wednesday, March 28, 2007 8:42 AM
> >     *Subject:* Re: [Gems-users] IPC and Ruby cycles
> >
> >     If you're using Opal, then Opal will calculate its IPC for you.
> >     If you're not using Opal, then the IPC is a meaningless measure,
> >     since Simics is an extremely idealized in-order processor model.
> >
> >     If IPC is a metric that you absolutely MUST know, change
> >     SIMICS_RUBY_MULTIPLIER to 1 and double all your ruby latencies (to
> >     achieve an approximately identical timing). Then use RUBY_CYCLES
> >     to calculate IPC.
> >
> >     Regards,
> >     Dan
> >
> >     Lei Yang wrote:
> > >     Dear list,
> > >      I noticed that with SIMICS_RUBY_MULTIPLIER set to 2, the
> > >     Ruby_cycles shown in Ruby stats is always half of the cycle count
> > >     shown by simics. When calculating the IPC, which cycle count
> > >     should I use, Ruby_cycles or the cycle count of Simics? I'm quite
> > >     curious why there is a SIMICS_RUBY_MULTIPLIER parameter and
> > >     what's its purpose. As always, thanks a lot for your comments!
> > >      Lei
> > >     ------------------------------------------------------------------------
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