making Multinomial sampling slightly faster

lib/THC/THCTensorRandom.cuh

@@ -228,17 +228,15 @@ sampleMultinomialOnce(long* dest,
 
 template <typename T>
 __global__ void
-sampleMultinomialWithReplacement(curandStateMtgp32* state,
+sampleMultinomialWithReplacement(T* uniform_idx,
                                  int totalSamples,
                                  long* dest,
                                  long distributions,
                                  int categories,
                                  T* normDistPrefixSum) {
   // At the moment, each warp computes one sample value in the binary
   // search due to divergence. It seems possible to compute multiple
-  // values and limit divergence though later on. However, no matter
-  // what, all block threads must participate in the curand_uniform
-  // call to update the generator state.
+  // values and limit divergence though later on.
 
   // The block determines the distribution for which we generate a point
   for (long curDist = blockIdx.x;
@@ -250,7 +248,7 @@ sampleMultinomialWithReplacement(curandStateMtgp32* state,
       int sample = sampleBase + threadIdx.y;
 
       // All threads participate in this
-      T r = ScalarConvert<float, T>::to(curand_uniform(&state[blockIdx.x]));
+      T r = uniform_idx[sample];
 
       if (threadIdx.x == 0 && sample < totalSamples) {
         // Find the bucket that a uniform sample lies in
@@ -284,6 +282,7 @@ sampleMultinomialWithoutReplacement(curandStateMtgp32* state,
 
   // The block and warp determines the distribution for which we
   // generate a point
+  T zero = ScalarConvert<int, T>::to(0);
   for (long curDistBase = blockIdx.x * blockDim.y;
        curDistBase < distributions;
        curDistBase += gridDim.x * blockDim.y) {
@@ -292,7 +291,7 @@ sampleMultinomialWithoutReplacement(curandStateMtgp32* state,
 
     // All threads must participate in this
     T r = ScalarConvert<float, T>::to(curand_uniform(&state[blockIdx.x]));
-
+    
     if (threadIdx.x == 0 && curDist < distributions) {
       // Find the bucket that a uniform sample lies in
       int choice = binarySearchForMultinomial<T>(
@@ -305,7 +304,7 @@ sampleMultinomialWithoutReplacement(curandStateMtgp32* state,
 
       // Without replacement, so update the original probability so it
       // is not considered a second time
-      origDist[curDist * categories + choice] = ScalarConvert<int, T>::to(0);
+      origDist[curDist * categories + choice] = zero;
     }
   }
 }

lib/THC/generic/THCTensorRandom.cu

@@ -202,13 +202,21 @@ THC_API void THCTensor_(multinomial)(struct THCState *state,
       // distribution concurrently.
       dim3 grid(numDist < MAX_NUM_BLOCKS ? numDist : MAX_NUM_BLOCKS);
 
+
+      //Create the matrix of uniformly sampled numbers
+      THCTensor *uniform_idx = THCTensor_(newWithSize1d)(state, n_sample);
+      THCTensor_(uniform)(state, uniform_idx, 0, 1);
+
+
       sampleMultinomialWithReplacement
         <<<grid, block, 0, THCState_getCurrentStream(state)>>>(
-          gen->gen_states,
-          n_sample,
-          THCudaLongTensor_data(state, self),
-          numDist, numCategories,
-          THCTensor_(data)(state, prefixSum));
+	   THCTensor_(data)(state, uniform_idx),
+	   n_sample,
+	   THCudaLongTensor_data(state, self),
+	   numDist, numCategories,
+	   THCTensor_(data)(state, prefixSum));
+
+      THCTensor_(free)(state, uniform_idx);
     } else {
       // Sample without replacement
 
@@ -237,13 +245,13 @@ THC_API void THCTensor_(multinomial)(struct THCState *state,
         // recalculate our distribution
         sampleMultinomialWithoutReplacement
           <<<grid, block, 0, THCState_getCurrentStream(state)>>>(
-            gen->gen_states,
-            n_sample,
-            sample,
-            THCudaLongTensor_data(state, self),
-            numDist, numCategories,
-            THCTensor_(data)(state, origDist),
-            THCTensor_(data)(state, prefixSum));
+	     gen->gen_states,
+	     n_sample,	     
+	     sample,	     
+	     THCudaLongTensor_data(state, self),
+	     numDist, numCategories,
+	     THCTensor_(data)(state, origDist),
+	     THCTensor_(data)(state, prefixSum));
       }
     }
 

test/multinomial.lua

@@ -0,0 +1,52 @@
+local tester = torch.Tester()
+
+cmd = torch.CmdLine()
+cmd:text()
+cmd:text()
+cmd:text('Testing alias multinomial on cuda')
+cmd:text()
+cmd:text('Options')
+cmd:option('--compare',false,'compare with cutorch multinomial')
+cmd:text()
+
+-- parse input params
+params = cmd:parse(arg)
+
+require 'cutorch'
+local function checkMultinomial()
+   local n_class = {10, 100, 1000}
+   local n_sample = {10, 100, 1000, 10000}
+   local n_dist = 100
+   for _, curr_n_class in pairs(n_class) do
+      for _, curr_n_sample in pairs(n_sample) do
+	 print("")
+	 print("Benchmarking multinomial with "..curr_n_class.." classes and "..curr_n_sample.." samples")
+	 torch.seed()
+	 local probs = torch.CudaDoubleTensor(n_dist, curr_n_class):uniform(0,1)
+	 local a = torch.Timer()
+	 local cold_time = a:time().real
+	 a:reset()
+	 cutorch.synchronize()
+	 a:reset()
+	 for i = 1,10 do
+	    torch.multinomial(probs, curr_n_sample, true)
+	    cutorch.synchronize()
+	 end
+	 print("[CUDA] : torch.multinomial draw: "..(a:time().real/10).." seconds (hot)")
+      end
+      torch.seed()
+      local probs = torch.CudaDoubleTensor(3, curr_n_class):uniform(0,1)
+      for i =1,3 do
+	 probs[i]:div(probs[i]:sum())
+      end
+      local output = torch.multinomial(probs, 5000000, true)
+      local counts = torch.Tensor(3, curr_n_class):zero()
+      for i=1,3 do
+	 output[i]:long():apply(function(x) counts[{i, x}] = counts[{i, x}] + 1 end)
+	 counts[i]:div(counts[i]:sum())
+      end
+      tester:eq(probs:double(), counts, 0.01, "probs and counts should be approximately equal for n_class = "..curr_n_class)
+   end
+end
+tester:add(checkMultinomial)
+tester:run()