Sharing Resources Efficiently

Objects, allocated at the context level, are shared between devices in the context. For example, both buffers and images created with regular clCreateBuffer/ clCreateImage2D are effectively shared by default, so no implicit copying/synchronization happens between devices or between the devices and the host on Map/Unmap calls.

NOTE: Shared memory objects cannot be written concurrently by different command queues. Use explicit synchronization of the write access with OpenCL* synchronization objects, such as events.

You can also avoid implicit copying when you share data with the host, as explained in the “Mapping Memory Objects” section (see Related Documents).

NOTE: You need to properly align the pointer to the memory referenced with CL_MEM_USE_HOST_PTR; otherwise explicit copying might happen between host and devices on Map/Unmap.

The following table demonstrates the cases were the application can benefit from zero-copy sharing with host using CL_MEM_USE_HOST_PTR:

Memory Object


CPU-Only Context

Processor Graphics

Only Context

CPU+Processor Graphics

Shared Context


True sharing when hostptr is aligned


Always True Sharing

True Sharing

when hostptr is aligned

to 128 Bytes

Image True Sharing when hostptr is aligned to the image’s element size No True Sharing

True sharing

when hostptr is aligned to 4K

Other resources that are shared automatically across all devices, include program and kernel objects.

See Also

Synchronization Caveats (suggested next topic)
Mapping Memory Objects
Related Documents
The OpenCL* 1.1 Specification at portal [PDF]
Overview Presentations of the OpenCL* Standard at portal [Online Article]
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