total number of registers

Question

I wanted to ask.We say that using --ptxas-options=-v doesn't give the exact number of registers that our program uses.

1) Then , how am I going to supply the occupancu calculator with registers per thread and shared memory per block?

2) In my program I use also thrust calls which generate ptx code.I am having 2 kernels but I can see the thrust functions to produce ptx as well.So , I am taking into account these numbers also when I am counting the total number of registers I use? (I think yes!) (the same applies for the shared memory)

Answer 1

1) Then , how am I going to supply the occupancy calculator with registers per thread and shared memory per block?

The only other thing needed should be rounding up (if necessary) the output of ptxas to an even granularity of register allocation, which varies by device (see Greg's answer here) I think the common register allocation granularities are 4 and 8, but I don't have a table of register allocation granularity by compute capability.

I think shared memory also has an allocation granularity. Since the max number of threadblocks per SM is limited anyway, this should only matter (for occupancy) if your allocation/usage is within a granular amount of exceeding the limit for however many blocks you are otherwise limited to.

I think in most cases you'll get a pretty good feel by using the numbers from ptxas without rounding. If you feel you need this level of accuracy in the occupancy calculator, asking a nice directed question like "what are the allocation granularities for registers and shared memory for various GPUs" may get someone like Greg to give you a crisp answer.

2) In my program I use also thrust calls which generate ptx code.I am having 2 kernels but I can see the thrust functions to produce ptx as well.So , I am taking into account these numbers also when I am counting the total number of registers I use? (I think yes!) (the same applies for the shared memory)

Fundamentally I believe this thinking is incorrect. The only place I could see where it might matter is if you are running concurrent kernels, and I doubt that is the case since you mention thrust. The only figures that matter for occupancy are the metrics for a single kernel launch. You do not add threads, or registers, or shared memory across different kernels, to calculate resource usage. When a kernel completes execution, it releases its resource usage, at least for these resource types (registers, shared memory, threads).