HLSL Get number of threadGroups and numthreads in code

Question

my question concerns ComputeShader, HLSL code in particular. So, DeviceContext.Dispath(X, Y, Z) spawns X * Y * Z groups, each of which has x * y * z individual threads set in attribute [numthreads(x,y,z)]. The question is, how can I get total number of ThreadGroups dispatched and number of threads in a group? Let me explain why I want it - the amount of data I intend to process may vary significantly, so my methods should adapt to the size of input arrays. Of course I can send Dispath arguments in constant buffer to make it available from HLSL code, but what about number of threads in a group? I am looking for methods like GetThreadGroupNumber() and GetThreadNumberInGroup(). I appreciate any help.

Answer 1

The number of threads in a group is simply the product of the numthreads dimensions. For example, numthreads(32,8,4) will have 32*8*4 = 1024 threads per group. This can be determined statically at compile time.

The ID for a particular thread-group can be determined by adding a uint3 input argument with the SV_GroupId semantic.

The ID for a particular thread within a thread-group can be determined by adding a uint3 input argument with the SV_GroupThreadID semantic, or uint SV_GroupIndex if you prefer a flattened version.

As far as providing information to each thread on the total size of the dispatch, using a constant buffer is your best bet. This is analogous to the graphics pipeline, where the pixel shader doesn't naturally know the viewport dimensions.

It's also worth mentioning that if you do find yourself in a position where each thread needs to know the overall dispatch size, you should consider restructuring your algorithm. In general, it's better to dispatch a variable numbers of thread groups, each with a fixed amount of work, rather than dispatching a fixed number of threads with a variable amount of work. There are of course exceptions but this will tend provide better utilization of the hardware.