Sniper with OpenMP program giving 99% idle time for most of the cores.

[isha....@stonybrook.edu]

I am integrating my openmp program with Sniper. I can see that there are some instructions running on each core but the idle time for most of the cores(except 1 or 2) is 99% which means that only 1-2 cores are doing all the work. I am also setting MAX_NUM_THREADS to 32(or higher) but not getting any difference. Do i have to add something else in the openmp code to fix this issue? Or do I need to change the run-sniper flags?

I am running with following command:

../../run-sniper -n 32 -c gainestown --roi --fast-forward --no-cache-warming ./test_openmp 256 32 4

Thanks,

Isha

[Wim Heirman]

Hi Isha,

What OpenMP runtime and compiler are you using? You'll probably need to set OMP_NUM_THREADS. When using the Intel runtime, you can also set KMP_AFFINITY=verbose to see how many threads are being created and to what cores they are being bound. You can also check the dumpstats.py output for the threads.instructions_by_core statistics, they will have one column per thread and one row per core, and allow you to see for each thread how many instructions it executed on each core.

Regards,

Wim

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[isha....@stonybrook.edu]

Hi Wim,

Thank you for the reply. I have set the environment variable but this does not help. I have copy pasted the simple openmp code, makefile and the sim.out snapshot. Its weird that I am not getting proper parallelism even for this simple parallel for. Would appreciate any help.

  int main()

  {

    const int size = 25600;

    double sinTable[size];

    SimRoiStart();

    #pragma omp parallel for

    for(int n=0; n<size; ++n)

      sinTable[n] = n;

    SimRoiEnd();

        return 0;

}

I am using the following configuration in the Makefile:

export OMP_NUM_THREADS=10000

TARGET=abc

include ../shared/Makefile.shared

$(SNIPER_CFLAGS) += -fopenmp

$(TARGET): $(TARGET).o

        $(CC) $(TARGET).o -std=c99 -lm $(SNIPER_LDFLAGS) -fopenmp -o $(TARGET)

run_$(TARGET):

        ../../run-sniper -n 4 -c gainestown --roi --fast-forward ./abc

CLEAN_EXTRA=viz

Sim.out screenshot: 

[Wim Heirman]

What is in sim.out? Is it empty or did you not paste it?

I'm not too sure what will happen if you export OMP_NUM_THREADS from the Makefile. Can you run it like this on the command line:

OMP_NUM_THREADS=4  ../../run-sniper -n 4 -c gainestown --roi -- ./abc

Is there any other console output that may hint at some error? I'm sure setting OMP_NUM_THREADS to 10000 is way too high and will crash the application before it does any parallel work.

-Wim

[isha....@stonybrook.edu]

Hi Wim,

Thank you for the reply. I tried pasting the screenshot for sim.out in previous post but there must be some problem. sim.out basically shows just 1 core with idle time as -0.7% and all other as 100%. I tried running the command(../../run-sniper -n 4 -c gainestown --roi --fast-forward ./abc) from the console but it fails with the error: Application received fatal signal 11 at eip 0x400565. 

However when I run it using make above shown sim.out is generated. I think the number of threads etc shouldn't be a problem. Probably we are missing on some flags that sniper uses to run openmp program in parallel. Not sure though.

[Zi Yi Xu]

Hi, I am wondering how you solved this problem. I am encountering similar issues. I think that maybe we should compile the application using Sniper flags, although I am not sure which flags to use. Thanks!