Multithreading.md

Multithreading: Thread-Workers

The source code of the sample containing Vulkan and OpenGL source code is not especially designed to be multi-threaded.

To be more specific, the only declaration that suggests multi-threading is NThreadLocalVar template: to allow TLS (Thread Local Storage) to happen...

All the rest of the multi-threading happens in the main sample file gl_vk_bk3dthreaded.cpp.

In other words, some methods of OpenGL or Vulkan renderers become multithreaded because they got wrapped by dedicated Class, making them multi-threaded.

Thread-Workers job assignment

To assign a job to a worker, you must declare a specific Class, where:

• the constructor will become the receiver for the function arguments
• the worker will start his job through a specific method: Invoke()

This approach allows to prepare function arguments so that they are ready for later use: when the thread will finally be kicked-off by the thread-worker manager. Generic example:

class TskXXX : public TaskBase
{
private:
    int arg1;
    int arg2;
public:
    TskUpdateCommandBuffer(int _arg1, int _arg2)
    {
        arg1 = _arg1; arg2 = _arg2;
    }
    virtual void Invoke()
    {
        s_pCurRenderer->SomeMethod(arg1, arg2);
    }
};

To execute this job, we can queue workers for TskXXX as follow:

for(int n=0; n<100; n++)
{
    // worker will be deleted by the default method Done()
    TskXXX *tskXXX = new TskXXX(10, 2);
    g_mainThreadPool->pushTask(tskXXX);
}

g_mainThreadPool is the main thread Pool manager that god initialized as follow:

g_mainThreadPool = new ThreadWorkerPool(NUMTHREADS, false, false, NWTPS_ROUND_ROBIN, std::string("Main Worker Pool"));

Workers for command-buffer creation

for more details, one of the most important part of multi-threading in this sample is in refreshCmdBuffers()

Here is what multi-threaded command-buffer updates do:

• walk through the 3D model and split it in equal parts (almost...)
• push a Worker for the command-buffer creation of this part ( g_mainThreadPool->pushTask(tskUpdateCommandBuffer) )
• workers will execute in specific thread: what the worker-manager (g_mainThreadPool) will chose for you
• each worker will signal an event object when it finished the command-buffer creation
• the main thread in the meantime will have to wait for all to be done: looping into all the event objects
• Once secondary command-buffers are ready, the main thread will put them together in the primary command-buffer: . This task is not supposed to take time