Is CUvideoctxlock redundant?

Gaming and Visualization Technologies Video Processing & Optical Flow
,
1

My understanding from the runtime/driver documentation regarding cuda contexts is that they can exist on multiple threads and can safely be push popped without any synchronization/locks. However the documentation inside nvcuvid.h appears to disagree

//! Context-locking: to facilitate multi-threaded implementations, the following 4 functions
//! provide a simple mutex-style host synchronization. If a non-NULL context is specified
//! in CUVIDDECODECREATEINFO, the codec library will acquire the mutex associated with the given
//! context before making any cuda calls.
//! A multi-threaded application could create a lock associated with a context handle so that
//! multiple threads can safely share the same cuda context:
//!  - use cuCtxPopCurrent immediately after context creation in order to create a 'floating' context
//!    that can be passed to cuvidCtxLockCreate.
//!  - When using a floating context, all cuda calls should only be made within a cuvidCtxLock/cuvidCtxUnlock section.
//!
//! NOTE: This is a safer alternative to cuCtxPushCurrent and cuCtxPopCurrent, and is not related to video
//! decoder in any way (implemented as a critical section associated with cuCtx{Push|Pop}Current calls).

it sounds like this was a way to get around the single context per thread limitation (pre CUDA 4.0 I think) but should be unecessary now is that correct?

If so would it also be unecessary to use this with CUVIDDECODECREATEINFO::vidLock

    CUvideoctxlock vidLock;                    /**< IN: If non-NULL, context lock used for synchronizing ownership of 
                                                    the cuda context. Needed for cudaVideoCreate_PreferCUDA decode     */

I am asking because this lock is still used inside the samples (NvDecoder.cpp:279).

1 Like
2

Hi @cudawarped and welcome to the NVIDIA developer forums!

After some consideration I think the CUDA category will be better suited to answer your question on this.

I’ll keep on watching the thread in case I am mistaken and we need to pull in specific video expertise.

Thanks!

3

Moved it back as requested, let’s see we can find both video and CUDA experts to comment!

4

Is there any update? Can Nvidia codec developers make a comment? There is a bunch of threading related questions on the forum that have been left unanswered for years.

Is the practice of using lock outdated post CUDA 4.0? What is the recommended way of doing multithreaded encoding and decoding with the codec API?

2 Likes