Re: Code timing

Thanks again, Harold.

Any other takers on the original confusions? (Corrected version follows)

> I wrote the C++ program at the end of this post to get an idea
> of the relative speeds of various mathematical operations. Now
> I'm wondering about two features of the results. Here's an
> example output from an Athlon 800 machine:
>
>| Timing 2000 runs of 1000 calls
>|
>| div : 0.39 s 0.000195 ms/call
>| mul : 0.38 s 0.00019 ms/call
>| add : 0.39 s 0.000195 ms/call
>| null: 0.33 s 0.000165 ms/call
>| 1.2e+007 Repeat (y/n)? y
>| div : 0.39 s 0.000195 ms/call
>| mul : 0.38 s 0.00019 ms/call
>| add : 0.38 s 0.00019 ms/call
>| null: 0.88 s 0.00044 ms/call
>| 1.38e+007 Repeat (y/n)? n
>
> Now--
>
> 1) The "null" time is always the same as for add/mul on the
> first run but about double that subsequently, as seen above. All
> other numbers behave consistently. Why might that be?
>
> 2) I expected division to be rather slower than add/mul.
> Sometimes the numbers are indeed higher but only by about 10%
> and in a minority of runs. Am I observing
throughput instead of latency,
> which is what I'm interested in? I tried feeding
> dummy (the local one) back into compute() but, range issues
> aside, the times didn't change.

---------------------------------
#include <iostream>
#include <ostream>
#include <iomanip>
#include <vector>
#include <cstdlib>
#include <ctime>
#include <cmath>

using namespace std;

const int KBlockSize = 1000, KRunCount = 2000;
const int KCallCount = KBlockSize * KRunCount;

void printClocks(const char* name, clock_t clocks)
{
float sec = float(clocks) / CLOCKS_PER_SEC;
cout << right << setprecision(3);
cout << name << ": " << setw(8) << sec << " s "
<< setw(8) << sec * 1000 / KCallCount << " ms/call\n";
}

double dummy = 0;

template<class Float, class Compute>
class Runner {
const char* name_;
clock_t clocks_;
public:
Runner(const char* name)
: name_(name)
{
vector<Float> v(KBlockSize + 1);
for(int i = 0; i <= KBlockSize; ++i) v[i] = 1e-7 + rand() / Float(RAND_MAX);
Compute compute;
Float dummy = 0;
clock_t clocks = -clock();
for(int r = 0; r < KRunCount; ++r) {
for(int i = 0; i < KBlockSize; ++i) dummy += compute(v[i], v[i+1]);
}
clocks += clock();
::dummy += dummy;
clocks_ = clocks;
}
~Runner() { printClocks(name_, clocks_); }
};

struct Null {
template<class Float>
Float operator() (Float x, Float y) { return x; }
};

struct Add {
template<class Float>
Float operator() (Float x, Float y) { return x + y; }
};

struct Mul {
template<class Float>
Float operator() (Float x, Float y) { return x * y; }
};

struct Div {
template<class Float>
Float operator() (Float x, Float y) { return x / y; }
};

int main() {
srand(time(0));
cout << "Timing " << KRunCount << " runs of " << KBlockSize << " calls\n\n";
char repeat;
do {
dummy = 0;
{
Runner<float, Null> r0("null");
Runner<float, Add > r1("add ");
Runner<float, Mul > r2("mul ");
Runner<float, Div > r3("div ");
}
cout << dummy << " Repeat (y/n)? ";
} while(cin >> repeat && repeat != 'n');
return 0;
}

// end of code