Accessing cuda device memory when the cuda kernel is running

Question

I have allocated memory on device using cudaMalloc and have passed it to a kernel function. Is it possible to access that memory from host before the kernel finishes its executi

Answer 1

The only way I can think of to get a memcpy to kick off while the kernel is still executing is by submitting an asynchronous memcpy in a different stream than the kernel. (If you use the default APIs for either kernel launch or asynchronous memcpy, the NULL stream will force the two operations to be serialized.)

But because there is no way to synchronize a kernel's execution with a stream, that code would be subject to a race condition. i.e. the copy engine might pull from memory that hasn't yet been written by the kernel.

The person who alluded to mapped pinned memory is into something: if the kernel writes to mapped pinned memory, it is effectively "copying" data to host memory as it finishes processing it. This idiom works nicely, provided the kernel will not be touching the data again.