How to collect individual results of the threads within a block? - cuda

Question

In my Kernel, the threads are processing a small part of an array in global memory.
After processing I would also like to set a flag indicating that the result of the calculation is zero for all threads within a block:
__global__ void kernel( int *a, bool *blockIsNull) {
int tid = blockIdx.x * blockDim.x + threadIdx.x;
int result = 0;
// {...} Here calculate result
a[tid] = result;
// some code here, but I don't know, that's my question...
if (condition)
blockIsNull[blockIdx.x] = true; // if all threads have returned result==0
}
Each individual thread owns the information. But I don't find an efficient way to collect it.
For example, I could have a counter in shared memory that is atomically incremented by each thread when result==0. So when the counter reaches blockDim.x it means that all threads have returned zero. Althought not tested, I am afraid that this solution will have a negative impact on performance (atomic functions are slow).
A zero result does not occur very often, so it is very unlikely to have zeros for all threads within a block. I would like to find a solution that has little impact on the performance in the general case.
What would be your recommendation ?

Answer 1

It sounds like you want to perform a block level reduction of the condition value across a block. Just about all CUDA hardware supports a set of very useful warp voting primitives. You could use the __all() warp vote to determine that each warp of threads satisfied the condition, and then use __all() again to check whether all warps satisfy the condition. In code, it might look like this:
__global__ void kernel( int *a, bool *blockIsNull) {
// assume that threads per block is <= 1024
__shared__ volatile int blockcondition[32];
int laneid = threadIdx.x % 32;
int warpid = threadIdx.x / 32;
// Set each condition value to non zero to begin
if (warpid == 0) {
blockcondition[threadIdx.x] = 1;
}
__syncthreads();
//
// your code goes here
//
// warpcondition holds the vote from each warp
int warpcondition = __all(condition);
// First thread in each warp loads the warp vote to shared memory
if (laneid == 0) {
blockcondition[warpid] = warpcondition;
}
__syncthreads();
// First warp reduces all the votes in shared memory
if (warpid == 0) {
int result = __all(blockcondition[threadIdx.x] != 0);
// first thread stores the block result to global memory
if (laneid == 0) {
blockIsNull[blockIdx.x] = (result !=0);
}
}
}
[ Huge disclaimer: written in browser, never compiled or tested, use at own risk ]
This code should (I think) work for any number of threads per block up to 1024. You could, if required, adjust the size of blockcondition to a smaller value if you were confident of an upper block size limit less than 1024. Probably the smartest way would be to use C++ templating and make the warp count limit a template parameter.