I’m using OpenCL and have ATI 4850 card. It has:
- CL_DEVICE_MAX_COMPUTE_UNITS: 10
- CL_DEVICE_MAX_WORK_ITEM_DIMENSIONS: 3
- CL_DEVICE_MAX_WORK_GROUP_SIZE: 256
- CL_DEVICE_MAX_WORK_ITEM_SIZES:(256, 256, 256)
- CL_DEVICE_AVAILABLE: 1
- CL_DEVICE_NAME: ATI RV770
How many tasks can it execute simultaneously?
Is it CL_DEVICE_MAX_COMPUTE_UNITS * CL_DEVICE_MAX_WORK_ITEM_SIZES = 2560?
To be more specific: a single core processor can execute only one task in the one moment, dual-core can execute 2 tasks… How many tasks can execute my GPU at one moment? Or rephrased: How many processors does my GPU have?
The RV770 has 10 SIMD cores, each consisting of 16 shader cores, each consisting of 5 ALUs (VLIW5 architecture). A total of 800 ALUs that can do parallel computations. I don’t think there’s a way to get all these numbers out of OpenCL. I’m also not sure what you would equate to a CPU core. Perhaps a shader core? You can read about VLIW at Wikipedia. It’s an interesting design.
If you say a CPU core is only executing one “task” at any given time, even though it has multiple ALUs working in parallel, then I guess you can say the RV770 would be working on 160 tasks. But with the differences in how different chips work, I think “core” and “task” can become difficult to define. A CPU with hyperthreading can even execute two sets of code at the same time. With OpenCL I don’t believe it is possible yet to execute more than one kernel at any given time – unless recent driver updates have changed that.
Anyway, I think it is more important to present your work to the GPU in a way that gives the best performance. Unfortunately there’s no way to find the best work group size other than experimenting. At least not that I know of. One help is that if the drivers support OpenCL 1.1 you can query the CL_KERNEL_PREFERRED_WORK_GROUP_SIZE_MULTIPLE and set your work size to a multiple of that. Otherwise, going for a multiple of 64 is probably a safe bet.