questions about buffer size and parameter size.

Do you mean since kernel cannot support such big work size, I should split it into several sub-kernels?

This makes sense to me, but I will need to change my kernel code a lot. :(

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You need to break up your data into smaller workgroups within the total NDRange. Use required or max workgroup size attributes and launch the kernel appropriately to support the workgroup size you've selected. See the best practices guide for info on setting a workgroup size.

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