Zephyr port: async event loop implementation starves CPU
When using asyncio in MicroPython, the event loop implementation (asyncio.run) ends up polling Python objects in a queue. The polling code does this (extmod/modselect.c:poll_poll_internal):
...
mp_uint_t start_tick = mp_hal_ticks_ms();
mp_uint_t n_ready;
for (;;) {
// poll the objects
n_ready = poll_map_poll(&self->poll_map, NULL);
if (n_ready > 0 || (timeout != (mp_uint_t)-1 && mp_hal_ticks_ms() - start_tick >= timeout)) {
break;
}
MICROPY_EVENT_POLL_HOOK
}
...
The for(;;) loop above is a busy wait loop. In the Zephyr port, it consumes a lot of CPU time and it starves other threads. Even when MICROPY_EVENT_POOL_HOOK is set to k_yield (and thus the loop gives control back to the OS scheduler after each iteration) the code will still take most of the CPU time. Setting MICROPY_EVENT_POOL_HOOK to something like k_msleep(100) (which waits 100ms before running another iteration of the loop) fixes the starving issue, but it delays the Python thread for no good reason.
I have a possible solution for this, but please let me know first if there is interest to move this forward, since lately I got the impression that the Zephyr port isn't exactly a "fist class citizen" in the world of MicroPython ports.
uasyncio : missing RuntimeError: This event loop is already running
I've just spent far too long debugging, what is essentially the following code:
#!/usr/bin/env python
try:
import asyncio
except ImportError:
import uasyncio as asyncio
loop = asyncio.get_event_loop()
async def foo():
print('foo()')
async def main():
loop.run_until_complete(foo())
while True:
print('app_loop()')
await asyncio.sleep(1)
if __name__ == '__main__':
loop.run_until_complete(main())
I know what you're thinking.... why run_until_complete inside an async function.
I completely agree, and now that I have the root cause, it's easy to fix.
However: this was nested inside a relatively large code-base, and I had no trace-back leading me to the root cause.
Finding this required a lot of cutting code until the problem went away, which wasn't helped by it being in 2 places.
Run on CPython
When run on Python 3.9.9, it raises.
Traceback (most recent call last):
File "<stdin>", line 2, in <module>
File "/usr/lib/python3.9/asyncio/base_events.py", line 647, in run_until_complete
return future.result()
File "<stdin>", line 2, in main
File "/usr/lib/python3.9/asyncio/base_events.py", line 623, in run_until_complete
self._check_running()
File "/usr/lib/python3.9/asyncio/base_events.py", line 583, in _check_running
raise RuntimeError('This event loop is already running')
RuntimeError: This event loop is already running
Run on micropython
When the above code is executed in micropython I get the following output.
MicroPython v1.17 on 2021-09-02; PYBD-SF6W with STM32F767IIK
Type "help()" for more information.
>>>
paste mode; Ctrl-C to cancel, Ctrl-D to finish
=== # ... above code pasted here
===
foo()
app_loop()
>>>
Execution took 1 second, no exception raised.