如何使程序以这种方式工作？

Question

So i have a program that does these calculations with numbers. The program is threaded, and the number of threads are specified from the user.

I will give a close example

static void *program_thread(void *thread)
{

    bool somevar = true;

    if(somevar)
    {
        work = getwork();
    }

    dowork(work);

    if(condition1 blah blah)
        somevar = false; /* disable getwork */

    if(condition2)
       somevar = true; /* condition was either met or not met, so we request 
new work either way */  

}

Then with pthreads(and i will skip some code) i do

int main(blah)
{
    if (pthread_create(&thr->pth, NULL, program_thread, thread_number)) {
        printf("%s","program thread create failed");
        return 1;
    }
}

Now i will start explaining. The number of threads created are specified from the user, so i do a for loop and create as many threads as i need. Each thread calls

work = getwork();

Thus getting independant work to do, however the CPU is slow for this kind of job. It tries to compute something by trying 2^32 numbers(which is from 1 to 4 294 967 296)

But my CPU can only do around 3 million numbers per second, and by the time it reaches 4 billion numbers, it's restarted(for new work).

So i then thought of a better method. Instead of each thread getting totally different work, all the threads should get the same work and split the numbers they need to try.

The problem is, that i can't controll what work it get's, so i must fetch

work = getwork();

Before initiating the threads. The question is HOW? Using pthread_create obviously...but then what?

Answer 1

You get more than one way to do it:

• split your work package into smaller parts (thus, your getWork returns a new, smaller work)
• store your work in a common place, that you access from your thread using a reader-writer pattern

• from the pthread API , the 4th parameter is given to your thread, you can do something like the following code:

   Work = getWork(); if (pthread_create(&thr->pth, NULL, program_thread, (void*) &work))...

  And your program_thread function would be like that

  static void *program_thread(void *pxThread) { Work* pWork = (Work*) pxThread; ...

  Of course, you need to check the validaty of the pointer and common stuff (in my example, I created it on stack which is most probably a bad idea). Note that your code is givig a thread_number as a pointer, which is usually a bad idea. If you want to have more information transfered to your thread, simply hide it into a structure.

I'm not sure I fully understood your issue, but this could give you some hints most probably. Please note also that when doing multithreading, you need to take into account specific issues like race conditions, concurrent access and more complex lifecycle of objects...