perf(mpiarray): avoid redistribute copy on single process

Author: ljgray

caput/mpiarray.py

@@ -739,6 +739,9 @@ def redistribute(self, axis: int) -> "MPIArray":
         if self.axis == axis or self.comm is None:
             return self
 
+        if self.comm.size == 1:
+            return MPIArray.wrap(self.local_array, axis, self.comm)
+
         # Check to make sure there is enough memory to perform the redistribution.
         # Must be able to allocate the target array and 2 buffers. We allocate
         # slightly more space than needed to be safe
@@ -761,72 +764,62 @@ def redistribute(self, axis: int) -> "MPIArray":
         csize = self.comm.size
         crank = self.comm.rank
 
-        if csize == 1:
-            if arr.shape[self.axis] == self.global_shape[self.axis]:
-                # We are working on a single node.
-                target_arr[:] = arr
-            else:
-                raise ValueError(
-                    f"Global shape {self.global_shape} is incompatible with local "
-                    f"array shape {self.shape}"
-                )
-        else:
-            # Get the start and end of each subrange of interest
-            _, sac, eac = mpiutil.split_all(self.global_shape[axis], self.comm)
-            _, sar, ear = mpiutil.split_all(self.global_shape[self.axis], self.comm)
-            # Split the soruce array into properly sized blocks for sending
-            blocks = np.array_split(arr, np.insert(eac, 0, sac[0]), axis)[1:]
-            # Create fixed-size contiguous buffers for sending and receiving
-            buffer_shape = list(target_arr.shape)
-            buffer_shape[self.axis] = max(ear - sar)
-            buffer_shape[axis] = max(eac - sac)
-            # Pre-allocate buffers and buffer type
-            recv_buffer = np.empty(buffer_shape, dtype=self.dtype)
-            send_buffer = np.empty_like(recv_buffer)
-            buf_type = self._prep_buf(send_buffer)[1]
-
-            # Empty slices for target, send buf, recv buf
-            targetsl = [slice(None)] * len(buffer_shape)
-            sendsl = [slice(None)] * len(buffer_shape)
-            recvsl = [slice(None)] * len(buffer_shape)
-            # Send and recv buf have some fixed axis slices per rank
-            sendsl[self.axis] = slice(ear[crank] - sar[crank])
-            recvsl[axis] = slice(eac[crank] - sac[crank])
-
-            mpistatus = mpiutil.MPI.Status()
-
-            # Cyclically pass messages forward to i adjacent rank
-            for i in range(csize):
-                send_to = (crank + i) % csize
-                recv_from = (crank - i) % csize
-
-                # Write send data into send buffer location
-                sendsl[axis] = slice(eac[send_to] - sac[send_to])
-                send_buffer[tuple(sendsl)] = blocks[send_to]
-
-                self.comm.Sendrecv(
-                    sendbuf=(send_buffer, buf_type),
-                    dest=send_to,
-                    sendtag=(csize * crank + send_to),
-                    recvbuf=(recv_buffer, buf_type),
-                    source=recv_from,
-                    recvtag=(csize * recv_from + crank),
-                    status=mpistatus,
-                )
+        # Get the start and end of each subrange of interest
+        _, sac, eac = mpiutil.split_all(self.global_shape[axis], self.comm)
+        _, sar, ear = mpiutil.split_all(self.global_shape[self.axis], self.comm)
+        # Split the soruce array into properly sized blocks for sending
+        blocks = np.array_split(arr, np.insert(eac, 0, sac[0]), axis)[1:]
+        # Create fixed-size contiguous buffers for sending and receiving
+        buffer_shape = list(target_arr.shape)
+        buffer_shape[self.axis] = max(ear - sar)
+        buffer_shape[axis] = max(eac - sac)
+        # Pre-allocate buffers and buffer type
+        recv_buffer = np.empty(buffer_shape, dtype=self.dtype)
+        send_buffer = np.empty_like(recv_buffer)
+        buf_type = self._prep_buf(send_buffer)[1]
+
+        # Empty slices for target, send buf, recv buf
+        targetsl = [slice(None)] * len(buffer_shape)
+        sendsl = [slice(None)] * len(buffer_shape)
+        recvsl = [slice(None)] * len(buffer_shape)
+        # Send and recv buf have some fixed axis slices per rank
+        sendsl[self.axis] = slice(ear[crank] - sar[crank])
+        recvsl[axis] = slice(eac[crank] - sac[crank])
+
+        mpistatus = mpiutil.MPI.Status()
+
+        # Cyclically pass and receive array chunks across ranks
+        for i in range(csize):
+            send_to = (crank + i) % csize
+            recv_from = (crank - i) % csize
+
+            # Write send data into send buffer location
+            sendsl[axis] = slice(eac[send_to] - sac[send_to])
+            send_buffer[tuple(sendsl)] = blocks[send_to]
+
+            self.comm.Sendrecv(
+                sendbuf=(send_buffer, buf_type),
+                dest=send_to,
+                sendtag=(csize * crank + send_to),
+                recvbuf=(recv_buffer, buf_type),
+                source=recv_from,
+                recvtag=(csize * recv_from + crank),
+                status=mpistatus,
+            )
 
-                if mpistatus.error != mpiutil.MPI.SUCCESS:
-                    logger.error(
-                        f"**** ERROR in MPI SEND/RECV "
-                        f"(rank={crank}, "
-                        f"target={send_to}, "
-                        f"receive={recv_from}) ****"
-                    )
+            if mpistatus.error != mpiutil.MPI.SUCCESS:
+                logger.error(
+                    f"**** ERROR in MPI SEND/RECV "
+                    f"(rank={crank}, "
+                    f"target={send_to}, "
+                    f"receive={recv_from}) ****"
+                )
 
-                # Write buffer into target location
-                targetsl[self.axis] = slice(sar[recv_from], ear[recv_from])
-                recvsl[self.axis] = slice(ear[recv_from] - sar[recv_from])
+            # Write buffer into target location
+            targetsl[self.axis] = slice(sar[recv_from], ear[recv_from])
+            recvsl[self.axis] = slice(ear[recv_from] - sar[recv_from])
 
-                target_arr[tuple(targetsl)] = recv_buffer[tuple(recvsl)]
+            target_arr[tuple(targetsl)] = recv_buffer[tuple(recvsl)]
 
         return dist_arr