CUDA Runtime API (PDF) - v10.0.130 (older) - Last updated August 25, 2018 - Send Feedback

## 2. API synchronization behavior

The API provides memcpy/memset functions in both synchronous and asynchronous forms, the latter having an "Async" suffix. This is a misnomer as each function may exhibit synchronous or asynchronous behavior depending on the arguments passed to the function.

## Memcpy

In the reference documentation, each memcpy function is categorized as synchronous or asynchronous, corresponding to the definitions below.

Synchronous

1. All transfers involving Unified Memory regions are fully synchronous with respect to the host.

2. For transfers from pageable host memory to device memory, a stream sync is performed before the copy is initiated. The function will return once the pageable buffer has been copied to the staging memory for DMA transfer to device memory, but the DMA to final destination may not have completed.

3. For transfers from pinned host memory to device memory, the function is synchronous with respect to the host.

4. For transfers from device to either pageable or pinned host memory, the function returns only once the copy has completed.

5. For transfers from device memory to device memory, no host-side synchronization is performed.

6. For transfers from any host memory to any host memory, the function is fully synchronous with respect to the host.

Asynchronous

1. For transfers from device memory to pageable host memory, the function will return only once the copy has completed.

2. For transfers from any host memory to any host memory, the function is fully synchronous with respect to the host.

3. For all other transfers, the function is fully asynchronous. If pageable memory must first be staged to pinned memory, this will be handled asynchronously with a worker thread.

## Memset

The synchronous memset functions are asynchronous with respect to the host except when the target is pinned host memory or a Unified Memory region, in which case they are fully synchronous. The Async versions are always asynchronous with respect to the host.

## Kernel Launches

Kernel launches are asynchronous with respect to the host. Details of concurrent kernel execution and data transfers can be found in the CUDA Programmers Guide.