Fix GPU sort for large arrays (#1285)

* Fix GPU sort for large arrays

mlx/backend/metal/kernels/sort.h

@@ -522,13 +522,13 @@ template <
     bool ARG_SORT,
     short BLOCK_THREADS,
     short N_PER_THREAD>
-[[kernel, max_total_threads_per_threadgroup(BLOCK_THREADS)]] void
-mb_block_partition(
+[[kernel]] void mb_block_partition(
     device idx_t* block_partitions [[buffer(0)]],
     const device val_t* dev_vals [[buffer(1)]],
     const device idx_t* dev_idxs [[buffer(2)]],
     const constant int& size_sorted_axis [[buffer(3)]],
     const constant int& merge_tiles [[buffer(4)]],
+    const constant int& n_blocks [[buffer(5)]],
     uint3 tid [[threadgroup_position_in_grid]],
     uint3 lid [[thread_position_in_threadgroup]],
     uint3 tgp_dims [[threads_per_threadgroup]]) {
@@ -543,23 +543,29 @@ mb_block_partition(
   dev_vals += tid.y * size_sorted_axis;
   dev_idxs += tid.y * size_sorted_axis;
 
-  // Find location in merge step
-  int merge_group = lid.x / merge_tiles;
-  int merge_lane = lid.x % merge_tiles;
+  for (int i = lid.x; i <= n_blocks; i += tgp_dims.x) {
+    // Find location in merge step
+    int merge_group = i / merge_tiles;
+    int merge_lane = i % merge_tiles;
 
-  int sort_sz = sort_kernel::N_PER_BLOCK * merge_tiles;
-  int sort_st = sort_kernel::N_PER_BLOCK * merge_tiles * merge_group;
+    int sort_sz = sort_kernel::N_PER_BLOCK * merge_tiles;
+    int sort_st = sort_kernel::N_PER_BLOCK * merge_tiles * merge_group;
 
-  int A_st = min(size_sorted_axis, sort_st);
-  int A_ed = min(size_sorted_axis, sort_st + sort_sz / 2);
-  int B_st = A_ed;
-  int B_ed = min(size_sorted_axis, B_st + sort_sz / 2);
+    int A_st = min(size_sorted_axis, sort_st);
+    int A_ed = min(size_sorted_axis, sort_st + sort_sz / 2);
+    int B_st = A_ed;
+    int B_ed = min(size_sorted_axis, B_st + sort_sz / 2);
 
-  int partition_at = min(B_ed - A_st, sort_kernel::N_PER_BLOCK * merge_lane);
-  int partition = sort_kernel::merge_partition(
-      dev_vals + A_st, dev_vals + B_st, A_ed - A_st, B_ed - B_st, partition_at);
+    int partition_at = min(B_ed - A_st, sort_kernel::N_PER_BLOCK * merge_lane);
+    int partition = sort_kernel::merge_partition(
+        dev_vals + A_st,
+        dev_vals + B_st,
+        A_ed - A_st,
+        B_ed - B_st,
+        partition_at);
 
-  block_partitions[lid.x] = A_st + partition;
+    block_partitions[i] = A_st + partition;
+  }
 }
 
 template <

mlx/backend/metal/sort.cpp

@@ -177,6 +177,8 @@ void multi_block_sort(
   array dev_vals_out = dev_vals_1;
   array dev_idxs_out = dev_idxs_1;
 
+  int n_thr_per_group = (n_blocks + 1) < 1024 ? (n_blocks + 1) : 1024;
+
   for (int merge_tiles = 2; (merge_tiles / 2) < n_blocks; merge_tiles *= 2) {
     dev_vals_in = ping ? dev_vals_1 : dev_vals_0;
     dev_idxs_in = ping ? dev_idxs_1 : dev_idxs_0;
@@ -199,8 +201,9 @@ void multi_block_sort(
       compute_encoder.set_input_array(dev_idxs_in, 2);
       compute_encoder->setBytes(&size_sorted_axis, sizeof(int), 3);
       compute_encoder->setBytes(&merge_tiles, sizeof(int), 4);
+      compute_encoder->setBytes(&n_blocks, sizeof(int), 5);
 
-      MTL::Size group_dims = MTL::Size(n_blocks + 1, 1, 1);
+      MTL::Size group_dims = MTL::Size(n_thr_per_group, 1, 1);
       MTL::Size grid_dims = MTL::Size(1, n_rows, 1);
 
       compute_encoder.dispatchThreadgroups(grid_dims, group_dims);

mlx/ops.cpp

@@ -1785,15 +1785,6 @@ array sort(const array& a, int axis, StreamOrDevice s /* = {} */) {
     throw std::invalid_argument(msg.str());
   }
 
-  // TODO: Fix GPU kernel
-  if (a.shape(axis) >= (1u << 21) && to_stream(s).device.type == Device::gpu) {
-    std::ostringstream msg;
-    msg << "[sort] GPU sort cannot handle sort axis of >= 2M elements,"
-        << " got array with sort axis size " << a.shape(axis) << "."
-        << " Please place this operation on the CPU instead.";
-    throw std::runtime_error(msg.str());
-  }
-
   return array(
       a.shape(), a.dtype(), std::make_shared<Sort>(to_stream(s), axis), {a});
 }

python/tests/test_ops.py

@@ -1840,6 +1840,15 @@ class TestOps(mlx_tests.MLXTestCase):
             self.assertTrue(np.array_equal(c_np, c_mx))
             self.assertEqual(b_mx.dtype, c_mx.dtype)
 
+        # Test very large array
+        if mx.default_device() == mx.gpu:
+            a_np = np.random.normal(20, 20, size=(2**22)).astype(np.float32)
+            a_mx = mx.array(a_np)
+
+            b_np = np.sort(a_np)
+            b_mx = mx.sort(a_mx)
+            self.assertTrue(np.array_equal(b_np, b_mx))
+
     def test_partition(self):
         shape = (3, 4, 5)
         for dtype in ("int32", "float32"):