Shared memory mutex with CUDA - adding to a list of items - cuda

Question

My problem is the following: I have an image in which I detect some points of interest using the GPU. The detection is a heavyweight test in terms of processing, however only about 1 in 25 points pass the test on average. The final stage of the algorithm is to build up a list of the points. On the CPU this would be implemented as:
forall pixels x,y
{
if(test_this_pixel(x,y))
vector_of_coordinates.push_back(Vec2(x,y));
}
On the GPU I have each CUDA block processing 16x16 pixels. The problem is that I need to do something special to eventually have a single consolidated list of points in global memory. At the moment I am trying to generate a local list of points in shared memory per block which eventually will be written to global memory. I am trying to avoid sending anything back to the CPU because there are more CUDA stages after this.
I was expecting that I could use atomic operations to implement the push_back function on shared memory. However I am unable to get this working. There are two issues. The first annoying issue is that I am constantly running into the following compiler crash: "nvcc error : 'ptxas' died with status 0xC0000005 (ACCESS_VIOLATION)" when using atomic operations. It is hit or miss whether I can compile something. Does anyone know what causes this?
The following kernel will reproduce the error:
__global__ void gpu_kernel(int w, int h, RtmPoint *pPoints, int *pCounts)
{
__shared__ unsigned int test;
atomicInc(&test, 1000);
}
Secondly, my code which includes a mutex lock on shared memory hangs the GPU and I dont understand why:
__device__ void lock(unsigned int *pmutex)
{
while(atomicCAS(pmutex, 0, 1) != 0);
}
__device__ void unlock(unsigned int *pmutex)
{
atomicExch(pmutex, 0);
}
__global__ void gpu_kernel_non_max_suppress(int w, int h, RtmPoint *pPoints, int *pCounts)
{
__shared__ RtmPoint localPoints[64];
__shared__ int localCount;
__shared__ unsigned int mutex;
int x = blockIdx.x * blockDim.x + threadIdx.x;
int y = blockIdx.y * blockDim.y + threadIdx.y;
int threadid = threadIdx.y * blockDim.x + threadIdx.x;
int blockid = blockIdx.y * gridDim.x + blockIdx.x;
if(threadid==0)
{
localCount = 0;
mutex = 0;
}
__syncthreads();
if(x<w && y<h)
{
if(some_test_on_pixel(x,y))
{
RtmPoint point;
point.x = x;
point.y = y;
// this is a local push_back operation
lock(&mutex);
if(localCount<64) // we should never get >64 points per block
localPoints[localCount++] = point;
unlock(&mutex);
}
}
__syncthreads();
if(threadid==0)
pCounts[blockid] = localCount;
if(threadid<localCount)
pPoints[blockid * 64 + threadid] = localPoints[threadid];
}
In the example code at this site, the author manages to successfully use atomic operations on shared memory, so I am confused as to why my case does not function. If I comment out the lock and unlock lines, the code runs ok, but obviously incorrectly adding to the list.
I would appreciate some advice about why this problem is happening and also perhaps if there is a better solution to achieving the goal, since I am concerned anyway about the performance issues with using atomic operations or mutex locks.

Answer 1

I suggest using prefix-sum to implement that part to increase parallelism. To do that you need to use a shared array. Basically prefix-sum will turn an array (1,1,0,1) into (0,1,2,2,3), i.e., will calculate an in-place running exclusive sum so that you'll get per-thread write indices.
__shared__ uint8_t vector[NUMTHREADS];
....
bool emit = (x<w && y<h);
emit = emit && some_test_on_pixel(x,y);
__syncthreads();
scan(emit, vector);
if (emit) {
pPoints[blockid * 64 + vector[TID]] = point;
}
prefix-sum example:
template <typename T>
__device__ uint32 scan(T mark, T *output) {
#define GET_OUT (pout?output:values)
#define GET_INP (pin?output:values)
__shared__ T values[numWorkers];
int pout=0, pin=1;
int tid = threadIdx.x;
values[tid] = mark;
syncthreads();
for( int offset=1; offset < numWorkers; offset *= 2) {
pout = 1 - pout; pin = 1 - pout;
syncthreads();
if ( tid >= offset) {
GET_OUT[tid] = (GET_INP[tid-offset]) +( GET_INP[tid]);
}
else {
GET_OUT[tid] = GET_INP[tid];
}
syncthreads();
}
if(!pout)
output[tid] =values[tid];
__syncthreads();
return output[numWorkers-1];
#undef GET_OUT
#undef GET_INP
}

Answer 2

Based on recommendations here, I include the code that I used in the end. It uses 16x16 pixel blocks. Note that I am now writing the data out in one global array without breaking it up. I used the global atomicAdd function to compute a base address for each set of results. Since this only gets called once per block, I did not find too much of a slow down, while I gained a lot more convenience by doing this. I'm also avoiding shared buffers for the input and output of prefix_sum. GlobalCount is set to zero prior to the kernel call.
#define BLOCK_THREADS 256
__device__ int prefixsum(int threadid, int data)
{
__shared__ int temp[BLOCK_THREADS*2];
int pout = 0;
int pin = 1;
if(threadid==BLOCK_THREADS-1)
temp[0] = 0;
else
temp[threadid+1] = data;
__syncthreads();
for(int offset = 1; offset<BLOCK_THREADS; offset<<=1)
{
pout = 1 - pout;
pin = 1 - pin;
if(threadid >= offset)
temp[pout * BLOCK_THREADS + threadid] = temp[pin * BLOCK_THREADS + threadid] + temp[pin * BLOCK_THREADS + threadid - offset];
else
temp[pout * BLOCK_THREADS + threadid] = temp[pin * BLOCK_THREADS + threadid];
__syncthreads();
}
return temp[pout * BLOCK_THREADS + threadid];
}
__global__ void gpu_kernel(int w, int h, RtmPoint *pPoints, int *pGlobalCount)
{
__shared__ int write_base;
int x = blockIdx.x * blockDim.x + threadIdx.x;
int y = blockIdx.y * blockDim.y + threadIdx.y;
int threadid = threadIdx.y * blockDim.x + threadIdx.x;
int valid = 0;
if(x<w && y<h)
{
if(test_pixel(x,y))
{
valid = 1;
}
}
int index = prefixsum(threadid, valid);
if(threadid==BLOCK_THREADS-1)
{
int total = index + valid;
if(total>64)
total = 64; // global output buffer is limited to 64 points per block
write_base = atomicAdd(pGlobalCount, total); // get a location to write them out
}
__syncthreads(); // ensure write_base is valid for all threads
if(valid)
{
RtmPoint point;
point.x = x;
point.y = y;
if(index<64)
pPoints[write_base + index] = point;
}
}