Support disabling automatic sync on task switch

src/array.jl

@@ -453,6 +453,23 @@ function Base.unsafe_convert(::Type{CuDeviceArray{T,N,AS.Global}}, a::DenseCuArr
                                a.maxsize - a.offset*Base.elsize(a))
 end
 
+## synchronization behavior
+
+"""
+    unsafe_disable_task_sync!(arr::CuArray)
+
+By default `CuArray`s are implicitly synchronized when they are used on different CUDA streams.
+A `CuArray` that is used on multiple Julia tasks will be used by different streams
+and thus will cause a synchronization between multiple Julia tasks.
+
+This `unsafe_disable_task_sync` disables synchronization when stream are being switched.
+"""
+function unsafe_disable_task_sync!(arr::CuArray)
+    return arr.data[].task_sync = false
+end
+function unsafe_enable_task_sync!(arr::CuArray)
+    return arr.data[].task_sync = true
+end
 
 ## memory copying
 

src/memory.jl

@@ -509,10 +509,13 @@ mutable struct Managed{M}
   # whether the memory has been captured in a way that would make the dirty bit unreliable
   captured::Bool
 
-  function Managed(mem::AbstractMemory; stream=CUDA.stream(), dirty=true, captured=false)
+  # whether memory accessed from another task causes implicit syncrhonization
+  task_sync::Bool
+
+  function Managed(mem::AbstractMemory; stream = CUDA.stream(), dirty = true, captured = false, task_sync = true)
     # NOTE: memory starts as dirty, because stream-ordered allocations are only
     #       guaranteed to be physically allocated at a synchronization event.
-    new{typeof(mem)}(mem, stream, dirty, captured)
+    return new{typeof(mem)}(mem, stream, dirty, captured, task_sync)
   end
 end
 
@@ -566,7 +569,13 @@ function Base.convert(::Type{CuPtr{T}}, managed::Managed{M}) where {T,M}
 
   # accessing memory on another stream: ensure the data is ready and take ownership
   if managed.stream != state.stream
-    maybe_synchronize(managed)
+    # Synchronize the array if task_sync is enabled
+    managed.task_sync && maybe_synchronize(managed)
+    # We must still switch the stream, since we are about to set ".dirty=true"
+    # and otherwise subsequent operations will synchronize against the wrong stream.
+    # XXX: What to do when an array is used on multiple tasks concurrently?
+    #      We could have the situation that we will synchronize against the "wrong" stream
+    #      But that would mean that we need a mapping from task to stream per Managed object...
     managed.stream = state.stream
   end