What's different between the normal memory object and OpenCL's pipe?

Question

Pipe is one of the OpenCL 2.0's new features, and this feature has been demonstrated in the AMDAPPSDK's producer/consumer example. I've read some articles abut pipe's use cases and they're all like the producer/consumer way.

My question is, the same functionality can be achieved by creating a global memory space/object and passing the pointer to 2 kernel functions given that OpenCL 2.0 provides the shared virtual memory. So what's the difference between a pipe object and a global memory object? Or is it invented just for optimization?

Answer 1

It is as useful as std::vector and std::queue. One is useful to store data, while the other is useful to store packets.

Packets are indeed data, but it is much easier to handle them as small units rather than a big block.

Pipes in OpenCL allow you to consume these small packets in a kernel, without having to deal with the indexing + storing + pointers + forloops hell that would happen if you manually implement a pipe mechanism yourself in the kernel.

Answer 2

Pipes are useful for example when each work item can generate variable number of outputs. Prior to OpenCL 2.0 this was difficult to handle.

Pipes may reside in faster memory (vendor specific) i.e. Altera recommends using pipes to exchange data between kernels instead of using global memory.

Answer 3

Pipes are designed to transfer data from one kernel to another kernel/s without the need to store/load data in/from global or host memory. This is essentially a FIFO on the FPGA device. So, the speed of access of the data are much faster than that of through DDR or host memory. This is probably the reason to use FPGA as an accelerator.

Sometimes the DDRs are used to share data between kernels as well. One example is that a SIMD kernel want to share some data with a single task kernel with requirement on input data sequence. As, Pipes will run out of order in a SIMD way.

Other than the Pipes, you can use Altera channels for more function support. But this is not portable to other OpenCL devices.

Hope this can help. :)