Ensure we only have one copy of the fence

mlx/backend/metal/distributed.cpp

@@ -29,10 +29,10 @@ void AllReduce::eval_gpu(
 
   auto& in = inputs[0];
 
-  Fence f{stream()};
+  auto f = std::make_shared<Fence>(stream());
 
   if (in.event().valid()) {
-    f.update_gpu(in);
+    f->update_gpu(in);
   }
 
   auto& out = outputs[0];
@@ -41,7 +41,7 @@ void AllReduce::eval_gpu(
   } else {
     out.set_data(allocator::malloc_or_wait(out.nbytes()));
   }
-  f.wait_gpu(out);
+  f->wait_gpu(out);
 
   auto task = [in = in,
                out = unsafe_weak_copy(out),
@@ -49,7 +49,7 @@ void AllReduce::eval_gpu(
                reduce_type = reduce_type_,
                group = group()]() mutable {
     if (in.event().valid()) {
-      f.wait();
+      f->wait();
     }
     switch (reduce_type) {
       case Sum:
@@ -59,7 +59,7 @@ void AllReduce::eval_gpu(
       default:
         throw std::runtime_error("Only all reduce sum is supported for now");
     }
-    f.update();
+    f->update();
   };
   scheduler::enqueue(detail::communication_stream(), std::move(task));
 }
@@ -74,22 +74,22 @@ void AllGather::eval_gpu(
 
   out.set_data(allocator::malloc_or_wait(out.nbytes()));
 
-  Fence f{stream()};
+  auto f = std::make_shared<Fence>(stream());
 
   if (in.event().valid()) {
-    f.update_gpu(in);
+    f->update_gpu(in);
   }
-  f.wait_gpu(out);
+  f->wait_gpu(out);
 
   auto task = [in = in,
                out = unsafe_weak_copy(out),
                f = std::move(f),
                group = group()]() mutable {
     if (in.event().valid()) {
-      f.wait();
+      f->wait();
     }
     distributed::detail::all_gather(group, in, out);
-    f.update();
+    f->update();
   };
   scheduler::enqueue(detail::communication_stream(), std::move(task));
 }
@@ -103,9 +103,9 @@ void Send::eval_gpu(
   auto& in = inputs[0];
 
   // Encode a signal event for the input
-  Fence f{stream()};
+  auto f = std::make_shared<Fence>(stream());
   if (in.event().valid()) {
-    f.update_gpu(in);
+    f->update_gpu(in);
   }
 
   auto& out = outputs[0];
@@ -118,7 +118,7 @@ void Send::eval_gpu(
                group = group(),
                dst = dst_]() mutable {
     if (in.event().valid()) {
-      f.wait();
+      f->wait();
     }
     distributed::detail::send(group, out, dst);
   };
@@ -135,8 +135,8 @@ void Recv::eval_gpu(
 
   out.set_data(allocator::malloc_or_wait(out.nbytes()));
 
-  Fence f{stream()};
+  auto f = std::make_shared<Fence>(stream());
-  f.wait_gpu(out);
+  f->wait_gpu(out);
 
   // Schedule an async recv on the comm stream
   auto task = [out = unsafe_weak_copy(out),
@@ -144,7 +144,7 @@ void Recv::eval_gpu(
                group = group(),
                src = src_]() mutable {
     distributed::detail::recv(group, out, src);
-    f.update();
+    f->update();
   };
   scheduler::enqueue(detail::communication_stream(), std::move(task));
 }

mlx/backend/metal/fence.cpp

@@ -34,6 +34,12 @@ Fence::Fence(const Stream& stream) : stream_(stream) {
   }
 }
 
+Fence::~Fence() {
+  if (fence_ != nullptr && use_fast_) {
+    cpu_value()[0] = INT_MAX;
+  }
+}
+
 void Fence::wait_gpu(array& x) {
   gpu_count_++;
   auto& d = metal::device(stream_.device);

mlx/backend/metal/fence.h

@@ -20,7 +20,11 @@ namespace mlx::core {
  */
 class Fence {
  public:
+  Fence(const Fence&) = delete;
+  Fence& operator=(const Fence&) = delete;
+
   Fence(const Stream& stream);
+  ~Fence();
 
   void update_gpu(const array& x);
   void wait_gpu(array& x);

mlx/backend/metal/kernels/fence.metal

@@ -29,7 +29,10 @@ constexpr constant metal::thread_scope thread_scope_system =
 [[kernel]] void fence_update(
     volatile coherent(system) device uint* timestamp [[buffer(0)]],
     constant uint& value [[buffer(1)]]) {
-  timestamp[0] = value;
+  uint tmp = timestamp[0];
+  if (tmp < value) {
+    timestamp[0] = value;
+  }
   metal::atomic_thread_fence(
       metal::mem_flags::mem_device,
       metal::memory_order_seq_cst,