Stop %gui asyncio from polling every millisecond

ipykernel/kernelbase.py

@@ -543,6 +543,14 @@ def enter_eventloop(self):
             self.log.info("Exiting as there is no eventloop")
             return
 
+        from .eventloops import loop_asyncio
+
+        if eventloop is loop_asyncio:
+            # The kernel already runs on asyncio, so there is nothing to poll.
+            # Polling every millisecond would only burn CPU (#1532).
+            self.log.info("Kernel already runs on asyncio; no polling needed")
+            return
+
         async def advance_eventloop():
             # check if eventloop changed:
             if self.eventloop is not eventloop:

tests/test_eventloop.py

@@ -93,6 +93,26 @@ def do_thing():
     loop_asyncio(kernel)
 
 
+@windows_skip
+async def test_asyncio_gui_does_not_poll(kernel):
+    # The kernel already runs on asyncio, so `%gui asyncio` has nothing to poll (#1532).
+    calls = []
+    call_later = kernel.io_loop.call_later
+
+    def counting_call_later(delay, callback, *args, **kwargs):
+        calls.append(callback)
+        return call_later(delay, callback, *args, **kwargs)
+
+    kernel.io_loop.call_later = counting_call_later
+    try:
+        kernel.eventloop = loop_asyncio
+        await asyncio.sleep(0.1)
+    finally:
+        kernel.eventloop = None
+        del kernel.io_loop.call_later
+    assert len(calls) <= 1
+
+
 @windows_skip
 def test_enable_gui(kernel):
     enable_gui("tk", kernel)