Avoid GPU synchronization in serial MPI runs

src/communication.jl

@@ -299,21 +299,38 @@ function progress(::AbstractCommManager)
 end
 
 function start!(A, cm::CommManagerBuffered)
-    setbuffer!(cm.sendbufferdevice, A, cm.pattern.sendindices)
-    KernelAbstractions.synchronize(get_backend(cm))
+    if !isempty(cm.sendrequests)
+        setbuffer!(cm.sendbufferdevice, A, cm.pattern.sendindices)
+    end
+
+    if !isempty(cm.sendrequests) || !isempty(cm.recvrequests)
+        KernelAbstractions.synchronize(get_backend(cm))
+    end
+
+    if !isempty(cm.recvrequests)
+        MPI.Startall(cm.recvrequests)
+    end
 
-    MPI.Startall(cm.recvrequests)
-    MPI.Startall(cm.sendrequests)
+    if !isempty(cm.sendrequests)
+        MPI.Startall(cm.sendrequests)
+    end
 
     return
 end
 
 function finish!(A, cm::CommManagerBuffered)
-    MPI.Waitall(cm.recvrequests)
-    MPI.Waitall(cm.sendrequests)
+    if !isempty(cm.recvrequests)
+        MPI.Waitall(cm.recvrequests)
+    end
+
+    if !isempty(cm.sendrequests)
+        MPI.Waitall(cm.sendrequests)
+    end
 
-    A = viewwithghosts(A)
-    getbuffer!(A, cm.recvbufferdevice, cm.pattern.recvindices)
+    if !isempty(cm.recvrequests)
+        A = viewwithghosts(A)
+        getbuffer!(A, cm.recvbufferdevice, cm.pattern.recvindices)
+    end
 
     return
 end