[gem5-users] Simulated vs Committed Ops

[Farhad Yusufali via gem5-users]

Hello all,

I’m trying to figure out the difference between two stats - simOps and 
system.cpu.commit.committedOps. I’m using a 4-wide single DerivO3CPU CPU, 
running serial workloads (i.e. no threads). In 
src/cpu/o3/cpu.cc<http://cpu.cc>, in the instDone function, I found the 
following code that is executed every time an instruction is committed:

    thread[tid]->numOp++;
    thread[tid]->numOps++;
    committedOps[tid]++;

As you can see, both committedOps and numOp is incremented once for every op 
committed. In the same file, in the totalOps function, the numOp values across 
different threads are accumulated to return a total op count for the entire 
core:

 template <class Impl>
Counter
FullO3CPU<Impl>::totalOps() const
{
    Counter total(0);

    ThreadID size = thread.size();
    for (ThreadID i = 0; i < size; i++) {
        total += thread[i]->numOp;
    }

    return total;
}

Finally, in src/cpu/base.hh, in the BaseCPU class, there is a function, 
numSimulatedOps that coalesces op counts across CPUs:

 static Counter numSimulatedOps()
 {
    Counter total = 0;

    int size = cpuList.size();
    for (int i = 0; i < size; ++i)
        total += cpuList[i]->totalOps();

    return total;
 }

This total value is used to generate the simOps stat.

To summarize, every time an op is commited, both committedOps and numOp are 
incremented. committedOps is dumped out using the 
system.cpu.commit.committedOps stat, while numOp is accumulated across both 
threads and CPUs and then dumped out using the simOps stat. Since I’m using a 
single CPU with a single thread, I would expect both stats to be identical, 
however they end up being vastly different (sometimes by over a factor of 10) 
(simOps is usually larger).

Does anyone have any insight into why this could be happening?

Thanks,
Farhad

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