CODEWITHSUNDEEP
c++linuxoptimizationprofilingperformance-testing
Medicine
Medicine

Reputation: 2023

Some issues in C++ performance measurement

 timespec start, end;
 clock_gettime(CLOCK_MONOTONIC, &start);
 ---code--lookup in a unordered map <string, int>
 clock_gettime(CLOCK_MONOTONIC, &end);
 int diff = diff_ns(end, start);

Results:
 SAMPLE   MIN(ns)   MAX(ns)   AVG(ns)
           100      1000      3000      1430
           500      1000      2000      1436
          1000         0     16000      1441
          5000         0     15000      1479
         10000         0     26000      1489
         50000         0    363000      1589
        100000         0    110000      1591
        200000         0    804000      1659
        300000         0    118000      1668
        400000      1000    354000      1701
        500000         0   8679000      1712
        600000         0    809000      1701
        700000         0    373000      1704
        800000         0    850000      1716
        900000         0    856000      1736
       1000000         0    817000      1730

Upvotes: 2

Views: 361

Answers (1)

BЈовић
BЈовић

Reputation: 64203

I am running tests in single threaded program...but how to make sure there is no context switch happening, because other processes may also be running on VM

Kill all processes that are not needed, and if you have an option, raise the priority of the profiled process.

Other then that, you can use prof or callgrind to profile your program.

Sometimes I get zero time taken, since I am measuring in nanoseconds, I find it strange, how something can execute in zero nanoseconds?

You are getting 0 ns execution time, because the CPU clock precision is above 10 ms.

Measure the time after more iterations, and you'll get better results.

Do more lookups, and then average the value :

 timespec start, end;
 clock_gettime(CLOCK_MONOTONIC, &start);
 for (int i=0;i<10000;++i)
 ---code--lookup in a unordered map <string, int>
 clock_gettime(CLOCK_MONOTONIC, &end);
 int diff = diff_ns(end, start)/10000;

Like this time spent in clock_gettime will be neglected.

Upvotes: 4

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