Hi,
I've been trying to figure out an efficient way of instrumenting certain specific array accesses in my program. For each of those array accesses, I need to determine whether it resulted in a request to the LLC and if so, whether it hit in the LLC or missed.
Example:
Consider the following program:
int main(int argv, char** argc) {
// array is int* A
for (int i = 0; i < N; i++) {
sum += A[i] // Need to instrument each such access separately
}
}
In this regard, I've been trying to utilize the MSRs. My initial approach was to read/write to /dev/cpu/*/msr to configure the MSRs and get the counts.
However, this method does not seem to have the granularity required to instrument single memory accesses - because the code to read and write from /dev/cpu/*/msr itself seems to generate cache misses.
So, my current approach is to use the rdmsr and wrmsr instructions - by embedding them as asm code within my program. However, I'm hitting an error at the wrmsr instruction and I'm not sure how to debug it. Note that there is no error message - the program just halts at the point of wrmsr.
I'm running the code on a Nehalem processor (Intel(R) Xeon(R) CPU X5550 )
So, following are the questions I have:
(i) Is my approach of using rdmsr and wrmsr to determine if a given array access was an LLC hit/miss a valid one?
(ii) Could you post some sample asm rdmsr and wrmsr instructions so that I could use them as a template. Maybe I'm not calling it the right way. This is my sample code:
hi=0; lo=0xb;
asm volatile("wrmsr"::"c"(0x38d),"a"(lo),"d"(hi)); // The program just halts at this point
Thanks in advance,
Jithin