GPGPU IGAD 2014/2015. Lecture 4. Jacco Bikker

Transcription

1 GPGPU IGAD 2014/2015 Lecture 4 Jacco Bikker

2 Today: Demo time! Parallel scan Parallel sort Assignment

3 Demo Time

4 Parallel scan What it is: in: out: C++: out[0] = 0 for ( i = 1; i < n; i++ ) out[i] = in[i-1] + out[i-1];

5 Parallel scan What it is good for: Building block for many parallel algorithms: Output to array of variable number of elements per thread Summed area tables Compaction

6 Variable output Verlet fluid solver: Go over the cells, and create an array of all particle pairs. Process this array in a second pass (which will have full GPU utilization). Each cell will emit 0..MAXPARTICLES-1 entries in the output array. wi wi wi wi wi wi wi wi warp

7 Summed Area Tables What it is: A table containing, for each pixel P of an image, the sum of all pixels between (0,0) and P. Using a SAT, we can calculate an arbitrary-width box filter in O(1):

8 Compaction What it is: When in a multi-pass algorithm not all data requires the same number of passes, compaction ensures that subsequent passes have full warps. What it is good for: Whitted-style ray tracing. wi wi wi wi wi wi wi wi warp 0

9 Parallel scan: Algorithm for ( d = 1; d < log 2 n; d++ ) for all k in parallel do if k >= 2 d x[k] += x[k 2 d-1 ] O(n log n)

10 Algorithm (2) Down-sweep Up-sweep O(n)

11 Today: Demo time! Parallel scan Parallel sort Assignment

12 Parallel sort Selection sort: kernel void Sort( global int* in, global int* out ) { int i = get_global_id( 0 ); int n = get_global_size( 0 ); int ikey = in[i]; // compute position of in[i] in output int pos = 0; for( int j = 0; j < n; j++ ) { int jkey = in[j]; // broadcasted bool smaller = (jkey < ikey) (jkey == ikey && j < i); pos += (smaller)? 1 : 0; } out[pos] = ikey; }

13 Parallel sort Merge sort:

14 Parallel sort Parallel merge sort: Main operation: merge if (*a < *b) *d++ = *a++; else *d++ = *b++;

15 Parallel sort Parallel merge sort: Main operation: merge while (a < a_end && b < b_end) if (*a < *b) *d++ = *a++; else *d++ = *b++; while (a < a_end) *d++ = *a++; while (b < b_end) *d++ = *b++;

16 Parallel merge What it is: Given two sorted sequences a and b, produce sorted sequence c: a: b: c: Note: position of a i in c is i + f( b, a i ) where f is the number of elements in b smaller than a i. Since b is sorted, finding f( b, x ) can be done using a binary search.

17 Sorting Networks

18 Sorting Networks

19 Bitonic Sort

20 Bitonic Sort kernel void Sort( global uint* data, const uint stage, const uint passofstage, const uint width, const uint direction ) { uint sortdir = direction; const uint idx = get_global_id( 0 ); const uint pairdist = 1 << (stage - passofstage); const uint leftid = (idx % pairdist) + (idx / pairdist) * 2 * pairdist; const uint rightid = leftid + pairdist; const uint A = data[leftid]; const uint B = data[rightid]; sortdir = ((idx >> stage) & 1) == 1? (1 - sortdir) : sortdir; const uint greater = A > B? A : B; const uint lesser = A > B? B : A; data[leftid] = sortdir? lesser : greater; data[rightid] = sortdir? greater : lesser; }

21 Today: Demo time! Parallel scan Parallel sort Updated Template Assignment

22 Template v3 #define CHECKCL(r) CheckCL( r, FILE, LINE ) float GetTime(); void StartTimer(); float GetDuation();

23 Template v3 static cl_int getplatformid( cl_platform_id* platform ) { char chbuffer[1024]; cl_uint num_platforms, devcount; cl_platform_id* clplatformids; cl_int error; *platform = NULL; CHECKCL( error = clgetplatformids( 0, NULL, &num_platforms ) ); if (num_platforms == 0) CHECKCL( -1 ); clplatformids = (cl_platform_id*)malloc( num_platforms * sizeof( cl_platform_id ) ); error = clgetplatformids( num_platforms, clplatformids, NULL ); #ifdef USE_CPU_DEVICE cl_uint devicetype[2] = { CL_DEVICE_TYPE_CPU, CL_DEVICE_TYPE_CPU }; char* deviceorder[2][3] = { { "", "", "" }, { "", "", "" } }; #else cl_uint devicetype[2] = { CL_DEVICE_TYPE_GPU, CL_DEVICE_TYPE_CPU }; char* deviceorder[2][3] = { { "NVIDIA", "AMD", "" }, { "", "", "" } }; #endif...

24 Template v3 glteximage2d( texturetype, 0, GL_RGBA32F, width, height, 0, GL_RGB, GL_FLOAT, data ); gltexparameteri( texturetype, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE ); gltexparameteri( texturetype, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE ); gltexparameteri( texturetype, GL_TEXTURE_MIN_FILTER, GL_NEAREST ); gltexparameteri( texturetype, GL_TEXTURE_MAG_FILTER, GL_NEAREST ); gltexparameteri( GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE ); gltexparameteri( GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE ); gltexparameteri( GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST ); gltexparameteri( GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST ); glteximage2d( GL_TEXTURE_2D, 0, GL_RGBA32F, width, height, 0, GL_RGB, GL_FLOAT, data );

25 Template v3 class Buffer { public: enum { DEFAULT = 0, TEXTURE }; // constructor / destructor Buffer() : hostbuffer( 0 ) {} Buffer( unsigned int N, unsigned int t = DEFAULT ); ~Buffer(); cl_mem* GetDevicePtr() { return &devicebuffer; } unsigned int* GetHostPtr() { return hostbuffer; } void CopyToDevice(); void CopyFromDevice(); void CopyTo( Buffer* buffer ); cl_int ParallelScan(); cl_int ParallelSort();...

26 Today: Demo time! Parallel scan Parallel sort Assignment

27 Assignment

28 Assignment Some options: Fluid simulation with surface reconstruction Cloth simulation Flocking / Boids Library of sorting functions for varying data sets, with analysis Ray traced shadows for rasterizer Mesh compression / decompression

29 Next week: Development tools Debugging Random numbers The End (for now)

30 Bonus material

31 Merge Sort in OpenCL kernel void Sort( global const int* in, global int* out, local int* aux ) { int i = get_local_id(0); // index in workgroup int wg = get_local_size(0); // workgroup size = block size, power of 2 int offset = get_group_id(0) * wg; in += offset; out += offset; // move in, out to block start aux[i] = in[i]; // load block in aux[wg] barrier(clk_local_mem_fence); // make sure AUX is entirely up to date // now we will merge sub-sequences of length 1,2,...,wg/2 for( int length = 1; length < wg; length <<=1 ) { uint ikey = aux[i]; int ii = i & (length - 1); // index in our sequence in 0..length-1 int sibling = (i - ii) ^ length; // beginning of the sibling sequence int pos = 0; for (int inc = length; inc > 0; inc >>=1 ) // increment for dichotomic search { int j = sibling + pos + inc - 1; uint jkey = aux[j]; bool smaller = (jkey < ikey) ( jkey == ikey && j < i ); pos += (smaller)? Inc : 0; pos = min( pos, length ); } int bits = 2 * length - 1; // mask for destination int dest = ((ii + pos) & bits) (i & ~bits); // dest idx in merged sequence barrier(clk_local_mem_fence); aux[dest] = ikey; barrier(clk_local_mem_fence); } out[i] = aux[i]; // write output }