Task Management

PxTask is a subsystem for managing compute resources for PhysX and APEX. It manages CPU and GPU compute resources, as well as SPU units on PlayStation3, by distributing Tasks to a user-implemented dispatcher and resolving Task dependencies such that Tasks are run in a given order.

Middleware products typically do not want to create CPU threads for their own use. This is especially true on consoles where execution threads can have significant overhead. In the PxTask model, the computational work is broken into jobs that are submitted to the game's thread pool as they become ready to run.

The following classes comprise the PxTask CPU resource management.

TaskManager

A TaskManager manages inter-task dependencies and dispatches ready tasks to their respective dispatcher. There is a dispatcher for CPU tasks, GPU tasks, and SPU tasks assigned to the TaskManager.

TaskManagers are owned and created by the SDK. Each PxScene will allocate its own TaskManager instance which users can configure with dispatchers through either the PxSceneDesc or directly through the TaskManager interface.

CpuDispatcher

The CpuDispatcher is an abstract class the SDK uses for interfacing with the application's thread pool. Typically, there will be one single CpuDispatcher for the entire application, since there is rarely a need for more than one thread pool. A CpuDispatcher instance may be shared by more than one TaskManager, for example if multiple scenes are being used.

PxTask includes a default CpuDispatcher implementation, but we prefer applications to implement this class themselves so PhysX and APEX can efficiently share CPU resources with the application.

Note

The TaskManager will call CpuDispatcher::submitTask() from either the context of API calls (aka: scene::simulate()) or from other running tasks, so the function must be thread-safe.

An implemention of the CpuDispatcher interface must call the following two methods on each submitted task for it to be run correctly:

baseTask->run();        // optionally call runProfiled() to wrap with PVD profiling events
baseTask->release();

The PxExtensions library has default implementations for all dispatcher types, the following code snippets are taken from SampleParticles and SampleBase and show how the default dispatchers are created. mNbThreads which is passed to PxDefaultCpuDispatcherCreate defines how many worker threads the CPU dispatcher will have.:

    PxSceneDesc sceneDesc(mPhysics->getTolerancesScale());
    [...]
    // create CPU dispatcher which mNbThreads worker threads
    mCpuDispatcher = PxDefaultCpuDispatcherCreate(mNbThreads);
    if(!mCpuDispatcher)
        fatalError("PxDefaultCpuDispatcherCreate failed!");
    sceneDesc.cpuDispatcher = mCpuDispatcher;
#ifdef PX_WINDOWS
    // create GPU dispatcher
    pxTask::CudaContextManagerDesc cudaContextManagerDesc;
    mCudaContextManager = pxTask::createCudaContextManager(cudaContextManagerDesc);
    sceneDesc.gpuDispatcher = mCudaContextManager->getGpuDispatcher();
#endif
    [...]
    mScene = mPhysics->createScene(sceneDesc);

Note

Best performance is usually achieved if the number of threads is less than or equal to the available hardware threads of the platform you are running on, creating more worker threads than hardware threads will often lead to worse performance. For platforms with a single execution core, the CPU dispatcher can be created with zero worker threads (PxDefaultCpuDispatcherCreate(0)). In this case all work will be executed on the thread that calls PxScene::simulate(), which can be more efficient than using multiple threads.

Note

CudaContextManagerDesc support appGUID now. It only works on release build. If your application employs PhysX modules that use CUDA you need to use a GUID so that patches for new architectures can be released for your game. You can obtain a GUID for your application from Nvidia. The application should log the failure into a file which can be sent to NVIDIA for support.

CpuDispatcher Implementation Guidelines

After the scene's TaskManager has found a ready-to-run task and submitted it to the appropriate dispatcher it is up to the dispatcher implementation to decide how and when the task will be run.

Often in game scenarios the rigid body simulation is time critical and the goal is to reduce the latency from simulate() to the completion of fetchResults(). The lowest possible latency will be achieved when the PhysX tasks have exclusive access to CPU resources during the update. In reality, PhysX will have to share compute resources with other game tasks. Below are some guidelines to help ensure a balance between throughput and latency when mixing the PhysX update with other work.

  • Avoid interleaving long running tasks with PhysX tasks, this will help reduce latency.
  • Avoid assigning worker threads to the same execution core as higher priority threads. If a PhysX task is context switched during execution the rest of the rigid body pipeline may be stalled, increasing latency.
  • PhysX occasionally submits tasks and then immediately waits for them to complete, because of this, executing tasks in LIFO (stack) order may perform better than FIFO (queue) order.
  • PhysX is not a perfectly parallel SDK, so interleaving small to medium granularity tasks will generally result in higher overall throughput.
  • If your thread pool has per-thread job-queues then queuing tasks on the thread they were submitted may result in more optimal CPU cache coherence, however this is not required.

For more details see the default CpuDispatcher implementation that comes as part of the PxExtensions package. It uses worker threads that each have their own task queue and steal tasks from the back of other worker's queues (LIFO order) to improve workload distribution.

BaseTask

BaseTask is the abstract base class for all PxTask task types. All task run() functions will be executed on application threads, so they need to be careful with their stack usage, use a little stack as possible, and they should never block for any reason.

Task

The Task class is the standard task type. Tasks must be submitted to the TaskManager each simulation step for them to be executed. Tasks may be named at submission time, this allows them to be discoverable. Tasks will be given a reference count of 1 when they are submitted, and the TaskManager::startSimulation() function decrements the reference count of all tasks and dispatches all Tasks whose reference count reaches zero. Before TaskManager::startSimulation() is called, Tasks can set dependencies on each other to control the order in which they are dispatched. Once simulation has started, it is still possible to submit new tasks and add dependencies, but it is up to the programmer to avoid race hazards. You cannot add dependencies to tasks that have already been dispatched, and newly submitted Tasks must have their reference count decremented before that Task will be allowed to execute.

Synchronization points can also be defined using Task names. The TaskManager will assign the name a TaskID with no Task implementation. When all of the named TaskID's dependencies are met, it will decrement the reference count of all Tasks with that name.

APEX uses the Task class almost exclusively to manage CPU resources. The ApexScene defines a number of named Tasks that the modules use to schedule their own Tasks (ex: start after LOD calculations are complete, finish before the PhysX scene is stepped).

LightCpuTask

LightCpuTask is another subclass of BaseTask that is explicitly scheduled by the programmer. LightCpuTasks have a reference count of 1 when they are initialized, so their reference count must be decremented before they are dispatched. LightCpuTasks increment their continuation task reference count when they are initialized, and decrement the reference count when they are released (after completing their run() function)

PhysX 3.0 uses LightCpuTasks almost exclusively to manage CPU resources. For example, each stage of the simulation update may consist of multiple parallel tasks, when each of these tasks has finished execution it will decrement the reference count on the next task in the update chain. This will then be automatically dispatched for execution when its reference count reaches zero.

Note

Even when using LightCpuTasks exclusively to manage CPU resources, the TaskManager startSimulation() and stopSimulation() calls must be made each simulation step to keep the GpuDispatcher synchronized.

The following code snippets show how the crabs' A.I. in SampleSubmarine is run as a CPU Task. By doing so the Crab A.I. is run as a background Task in parallel with the PhysX simulation update.

For a CPU task that does not need handling of multiple continuations LightCpuTask can be subclassed. A LightCpuTask subclass requires that the getName and a run method be defined:

class Crab: public ClassType, public physx::pxtask::LightCpuTask, public SampleAllocateable
{
public:
    Crab(SampleSubmarine& sample, const PxVec3& crabPos, RenderMaterial* material);
    ~Crab();
    [...]

    // Implements LightCpuTask
    virtual  const char*    getName() const { return "Crab AI Task"; }
    virtual  void           run();

    [...]
}

After PxScene::simulate() has been called, and the simulation started, the application calls removeReference() on each Crab task, this in turn causes it to be submitted to the CpuDispatcher for update. Note that it is also possible to submit tasks to the dispatcher directly (without manipulating reference counts) as follows:

pxtask::LightCpuTask& task = &mCrab;
mCpuDispatcher->submitTask(task);

Once queued for execution by the CpuDispatcher, one of the thread pool's worker threads will eventually call the task's run method. In this example the Crab task will perform raycasts against the scene and update its internal state machine:

void Crab::run()
{
    // run as a separate task/thread
    scanForObstacles();
    updateState();
}

It is safe to perform API read calls, such as scene queries, from multiple threads while simulate() is running. However, care must be taken not to overlap API read and write calls from multiple threads. In this case the SDK will issue an error, see Data Access and Buffering for more information.

An example for explicit reference count modification and task dependency setup:

// assume all tasks have a refcount of 1 and are submitted to the task manager
// 3 task chains a0-a2, b0-b2, c0-c2
// b0 shall start after a1
// the a and c chain have no dependencies and shall run in parallel
//
// a0-a1-a2
//      \
//       b0-b1-b2
// c0-c1-c2

// setup the 3 chains
for(PxU32 i = 0; i < 2; i++)
{
    a[i].setContinuation(&a[i+1]);
    b[i].setContinuation(&b[i+1]);
    c[i].setContinuation(&c[i+1]);
}

// b0 shall start after a1
b[0].startAfter(a[1].getTaskID());

// setup is done, now start all task by decrementing their refcount by 1
// tasks with refcount == 0 will be submitted to the dispatcher (a0 & c0 will start).
for(PxU32 i = 0; i < 3; i++)
{
    a[i].removeReference();
    b[i].removeReference();
    c[i].removeReference();
}

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