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Data Access from Multiple Threads¶
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For efficiency reasons, PhysX does not internally lock access to its data structures by the application, so be careful when calling the API from multiple application threads. The rules are as follows:
- API interface methods marked 'const' are read calls, other API interface methods are write calls.
- API read calls may be made simultaneously from multiple threads.
- Objects in different scenes may be safely accessed by different threads.
- Different objects outside a scene may be safely accessed from different threads. Be aware that accessing an object may indirectly cause access to another object via a persistent reference (such as joints and actors referencing one another, an actor referencing a shape, or a shape referencing a mesh.)
Access patterns which do not conform to the above rules may result in data corruption, deadlocks, or crashes. Note in particular that it is not legal to perform a write operation on an object in a scene concurrently with a read operation to an object in the same scene. The checked build contains code which tracks access by application threads to objects within a scene, to try and detect problems at the point when the illegal API call is made.
Scene Locking¶
Each PxScene object provides a multiple reader, single writer lock that can be used to control access to the scene by multiple threads. This is useful for situations where the PhysX scene isshared between more than one system, for example APEX and a game engine's physics code. The scene lock provides a way for these systems to coordinate with each other.
It is not mandatory to use the lock. If all access to the scene is from a single thread, using the lock adds unnecessary overhead. Even if you are accessing the scene from multiple threads, you may be able to synchronize the threads using a simpler or more efficient application-specific mechanism that guarantees your application meets the above conditions. However, using the scene lock has two potential benefits:
- If the PxSceneFlag::eREQUIRE_RW_LOCK is set, the checked build will issue a warning for any API call made without first acquiring the lock, or if a write call is made when the lock has only been acquired for read,
- The APEX SDK uses the scene lock to ensure that it shares the scene safely with your application.
There are four methods for for acquiring / releasing the lock:
Additionally there is an RAII helper class to manage these locks, see PxSceneLock.h.
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Locking Semantics¶
There are precise rules regarding the usage of the scene lock:
- Multiple threads may read at the same time.
- Only one thread may write at a time, no thread may write if any threads are reading.
- If a thread holds a write lock then it may call both read and write API methods.
- Re-entrant read locks are supported, meaning a lockRead() on a thread that has already acquired a read lock is permitted. Each lockRead() must have a paired unlockRead().
- Re-entrant write locks are supported, meaning a lockWrite() on a thread that has already acquired a write lock is permitted. Each lockWrite() must have a paired unlockWrite().
- Calling lockRead() by a thread that has already acquired the write lock is permitted and the thread will continue to have read and write access. Each lock*() must have an associated unlock*() that occurs in reverse order.
- Lock upgrading is not supported - a lockWrite() by a thread that has already acquired a read lock is not permitted. Attempting this in checked builds will result in an error, in release builds it will lead to deadlock.
- Writers are favored - if a thread attempts a lockWrite() while the read lock is acquired it will be blocked until all readers leave. If new readers arrive while the writer thread is blocked they will be put to sleep and the writer will have first chance to access the scene. This prevents writers being starved in the presence of multiple readers.
- If multiple writers are queued then the first writer will receive priority, subsequent writers will be granted access according to OS scheduling.
Note: PxScene::release() automatically attempts to acquire the write lock, it is not necessary to acquire it manually before calling release().
Locking Best Practices¶
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It is often useful to arrange your application to acquire the lock a single time to perform multiple operations. Torrent downloading software list. This minimizes the overhead of the lock, and in addition can prevent cases such as a sweep test in one thread seeing a rag doll that has been only partially inserted by another thread.
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Clustering writes can also help reduce contention for the lock, as acquiring the lock for write will stall any other thread trying to perform a read access.