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I am trying to time a chunk of code. If I run it from the command line like: Code: taskset -c 0 ./conv Conv Time: 0.014350082 I get a time ...
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  1. #1
    Just Joined!
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    Aug 2011
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    51

    What is being cached?


    I am trying to time a chunk of code. If I run it from the command line like:
    Code:
    taskset -c 0 ./conv
    Conv Time:  0.014350082
    I get a time of ~14ms but there is some variation. So, I put the command above into a script and ran to get an average
    Code:
    taskset -c 0 ./conv
    taskset -c 0 ./conv
    .
    .
    .
    taskset -c 0 ./conv
    The output I get from running the script is:
    Code:
    Conv Time:  0.014284663
    Conv Time:  0.010995277
    Conv Time:  0.008977601
    Conv Time:  0.008802842
    Conv Time:  0.009022121
    Conv Time:  0.008894075
    Conv Time:  0.008738924
    Conv Time:  0.008423263
    Conv Time:  0.008694661
    Conv Time:  0.008902102
    What is causing the increased performance I see?

    If the program is starting and exiting, what could be cached?
    I also ran another version that I moved the program to a new core each time in the script.
    Code:
    taskset -c 0 ./conv
    taskset -c 1 ./conv
    .
    .
    .
    taskset -c 9 ./conv
    That gave an output of:
    Code:
    Conv Time:  0.014503283
    Conv Time:  0.013998719
    Conv Time:  0.013912381
    Conv Time:  0.014000417
    Conv Time:  0.013930824
    Conv Time:  0.013995235
    Conv Time:  0.014705306
    Conv Time:  0.013931455
    Conv Time:  0.014027264
    Conv Time:  0.014017024
    Give that, something is helping the performance if I say on the same core.
    Here is the code below
    Code:
    // Use:
    //         g++ -o conv -O3 conv.cpp -lrt
    // to compile
    
    #include <iostream>
    #include <cstdio>
    #include <sys/time.h>
    
    using namespace std;
    
    
    typedef struct {
        float real;
        float imag;
    } Complex_T;
    
    #define TAPS  228
    #define SAMPS 341
    int main(int argc, char **argv)
    {
    
      Complex_T y[TAPS+SAMPS],y2[TAPS+SAMPS],h[TAPS],x[SAMPS];
      int i(0),j(0);
    
      struct timeval val1,val2;
      struct timespec a,b;
    
      double thetime1,thetime2;
    
      clock_gettime(CLOCK_REALTIME,&a);
    
       for (int k(0); k < 100; k++)
      for ( i = 0; i < TAPS+SAMPS; i++ ) {
        y[i].real = 0;                       // set to zero before sum
        y[i].imag = 0;                       // set to zero before sum
        for ( j = 0; j < TAPS; j++ ){
            if ( i-j < 0 )
               break;
            if ( i-j >= SAMPS )
               continue;
            y[i].real += h[j].real * x[i - j].real;    // convolve: multiply and accumulate
            y[i].imag += h[j].real * x[i - j].imag;    // convolve: multiply and accumulate
            y[i].imag += h[j].imag * x[i - j].real;    // convolve: multiply and accumulate
            y[i].real += -1.0*(h[j].imag * x[i - j].imag);    // convolve: multiply and accumulate
        }
      }
    
      clock_gettime(CLOCK_REALTIME,&b);
    
      thetime1 = (double)((double)a.tv_sec+(double)(a.tv_nsec)/1.0e9);
      thetime2 = (double)((double)b.tv_sec+(double)(b.tv_nsec)/1.0e9);
    
      printf("Conv Time:  %.9f\n",(thetime2-thetime1));
    
      return 0;
    }

  2. #2
    Linux Enthusiast scathefire's Avatar
    Join Date
    Jan 2010
    Location
    Western Kentucky
    Posts
    626
    Even when it exits, that memory is still reserved in case it has to call that program again. Granted, if another program comes along and needs the memory it will be given up, but otherwise the system will hold it.
    linux user # 503963

  3. #3
    Just Joined!
    Join Date
    Aug 2011
    Posts
    51
    But what is it caching? If the program starts again, it should (or not) be in a different piece of memory and then the cache would be dirty.

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