Question

I’m currently trying to code a certain dynamic programming approach for a vehicle routing problem. At a certain point, I have a partial route that I want to add to a minmaxheap in order to keep the best 100 partial routes at a same stage. Most of the program runs smooth but when I actually want to insert a partial route into the heap, things tend to go a bit slow. That particural code is shown below:

 clock_t insert_start, insert_finish, check1_finish, check2_finish;

insert_start = clock();
check2_finish = clock();

if(heap.get_vector_size() < 100) {
    check1_finish= clock();
    heap.insert(expansion);
    cout << "node added" << endl;
}
else {
    check1_finish = clock();
    if(expansion.get_cost() < heap.find_max().get_cost() ) {
        check2_finish = clock();
        heap.delete_max();
        heap.insert(expansion);
        cout<< "worst node deleted and better one added"   <<endl;
    }
    else {
        check2_finish = clock();
        cout << "cost too high check"<<endl;
    }
}

number_expansions++;

cout << "check 1 takes " << check1_finish - insert_start << " ms" << endl;
cout << "check 2 takes " << check2_finish - check1_finish << "ms " << endl;

insert_finish = clock();

cout << "Inserting an expanded state into the heap takes " << insert_finish - insert_start << " clocks" << endl;

A typical output is this:

cost too high check 
check1 takes 0 ms 
check2 takes 0ms 
Inserting an expanded state into the heap takes 0 clocks

cost too high check 
check1 takes 0 ms 
check2 takes 0ms 
Inserting an expanded state into the heap takes 16 clocks

cost too high check 
check1 takes 0 ms 
check2 takes 0ms 
Inserting an expanded state into the heap takes 0 clocks

I know it’s hard to say something about the code when this block uses functions that are implemented elsewhere but I’m flabbergasted as to why this sometimes takes less than a ms and sometimes takes up to 16 ms. The program should execute this block thousands of times so these small hiccups are really slowing things down enormously.

My only guess is that something happens with the vector in the heap class that stores all these states but I reserve place for a 100 items in the constructor using vector::reserve so I don’t see how this could still be a problem.

Thanks!

Answer 1

Preempting. Your program may be preempted by the operating system, so some other program can run for a bit.

Also, it’s not 16 ms. It’s 16 clock ticks: http://www.cplusplus.com/reference/clibrary/ctime/clock/

If you want ms, you need to do:

cout << "Inserting an expanded state into the heap takes "
     << (insert_finish - insert_start) * 1000 / CLOCKS_PER_SEC
     << " ms " << endl;

Finally, you’re setting insert_finish after printing out the other results. Try setting it immediately after your if/else block. The cout command is a good time to get preempted by another process.