Question

I have a single-threaded linux app which I would like to make parallel. It reads a data file, creates objects, and places them in a vector. Then it calls a compute-intensive method (.5 second+) on each object. I want to call the method in parallel with object creation. While I’ve looked at qt and tbb, I am open to other options.

I planned to start the thread(s) while the vector was empty. Each one would call makeSolids (below), which has a while loop that would run until interpDone==true and all objects in the vector have been processed. However, I’m a n00b when it comes to threading, and I’ve been looking for a ready-made solution.

QtConcurrent::map(Iter begin,Iter end,function()) looks very easy, but I can’t use it on a vector that’s changing in size, can I? And how would I tell it to wait for more data?

I also looked at intel’s tbb, but it looked like my main thread would halt if I used parallel_for or parallel_while. That stinks, since their memory manager was recommended (open cascade’s mmgt has poor performance when multithreaded).

/**intended to be called by a thread
\param start the first item to get from the vector
\param skip how many to skip over (4 for 4 threads)
*/
void g2m::makeSolids(uint start, uint incr) {
  uint curr = start;
  while ((!interpDone) || (lineVector.size() > curr)) {
    if (lineVector.size() > curr) {
      if (lineVector[curr]->isMotion()) {
        ((canonMotion*)lineVector[curr])->setSolidMode(SWEPT);
        ((canonMotion*)lineVector[curr])->computeSolid();
      }
      lineVector[curr]->setDispMode(BEST);
      lineVector[curr]->display();

      curr += incr;
    } else {
      uio::sleep(); //wait a little bit for interp
    }
  }
}

EDIT: To summarize, what’s the simplest way to process a vector at the same time that the main thread is populating the vector?

Answer 1

It’s hard to tell if you have been thinking about this problem deeply and there is more than you are letting on, or if you are just over thinking it, or if you are just wary of threading.

Reading the file and creating the objects is fast; the one method is slow. The dependency is each consecutive ctor depends on the outcome of the previous ctor – a little odd – but otherwise there are no data integrity issues so there doesn’t seem to be anything that needs to be protected by mutexes and such.

Why is this more complicated than something like this (in crude pseudo-code):

while (! eof)
{
    readfile;
    object O(data);
    push_back(O);
    pthread_create(...., O, makeSolid);
}


while(x < vector.size())
{
    pthread_join();
    x++;
}

If you don’t want to loop on the joins in your main then spawn off a thread to wait on them by passing a vector of TIDs.

If the number of created objects/threads is insane, use a thread pool. Or put a counter is the creation loop to limit the number of threads that can be created before running ones are joined.