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Editorial Team
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Asked: 2026-06-11T09:32:10+00:00

The kernel uses: ( –ptxas-options=-v ) 0 bytes stack frame, 0 bytes spill sotes,

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The kernel uses: (--ptxas-options=-v)
0 bytes stack frame, 0 bytes spill sotes, 0 bytes spill loads
ptxas info: Used 45 registers, 49152+0 bytes smem, 64 bytes cmem[0], 12 bytes cmem[16]

Launch with: kernelA<<<20,512>>>(float parmA, int paramB); and it will run fine.
Launch with: kernelA<<<20,513>>>(float parmA, int paramB); and it get the out of resources error. (too many resources requested for launch).

The Fermi device properties: 48KB of shared mem per SM, constant mem 64KB, 32K registers per SM, 1024 maximum threads per block, comp capable 2.1 (sm_21)

I’m using all my shared mem space.
I’ll run out of block register space around 700 threads/block. The kernel will not launch if I ask for more than half the number of MAX_threads/block. It may just be a coincidence, but I doubt it.

  1. Why can’t I use a full block of threads (1024)?
  2. Any guess as to which resource I’m running out of?
  3. I have often wondered where the stalled thread data/state goes between warps. What resource holds these?
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1 Answer

  1. Editorial Team

    When I did the reg count, I commented out the printf’s. Reg count= 45
    When it was running, it had the printf’s coded. Reg count= 63 w/plenty of spill “reg’s”.
    I suspect each thread really has 64 reg’s, with only 63 available to the program.
    64 reg’s * 512 threads = 32K – The maximum available to a single block.

    So I suggest the # of available “code” reg’s to a block = cudaDeviceProp::regsPerBlock – blockDim i.e. The kernel doesn’t have access to all 32K registers.
    The compiler currently limits the # of reg’s per thread to 63, (or they spill over to lmem). I suspect this 63, is a HW addressing limitation.

    So it looks like I’m running out of register space.

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