Asyncio with Events - large delay on await

[alexphobby]

Hello,

I have noticed some issue.
If i use asyncio and i await for event.wait(), i can get very large delays.

My dirty fix: make a new task with:

while True:
        await asyncio.sleep(0)

and i get very good timing, from 2000ms:9500ms to 1ms

Machine: esp32 c6, latest firmware (did encountered this on S3 before, when i gave up on Events :( )

[peterhinch]

Interesting. I use events extensively and haven't experienced this. The problem and your fix suggests to me that there is something unusual in the way your application is designed: most applications have one or more tasks which run "in the background" with only a few waiting on events. Is it possible that, in your application, all running tasks are waiting on events?

You may have uncovered a bug in asyncio. It would be good if you could write a minimal reproducer which illustrates this behaviour - perhaps using events waiting on soft timer callbacks. If so, you or I could raise an issue.

[alexphobby]

You are right, my program is waiting for events that are set by interrupt, rather than polling.
I do a :
asyncio.run(main())
where
async def main(): t_blink = asyncio.create_task(blinkOnboardLed()) #blink every few seconds(i think this fires the events) t_runLights = asyncio.create_task(runLights()) t_asyncTick = asyncio.create_task(asyncTick()) #await asyncio.sleep(0) loop as the workaroud
the delay can be seen here:
async def runLights(): global last_motion,dimSetPoint run_time = 10000 while True: print("waiting for evt_light") await evt_lights.wait() print(f"lights on, motion: {time.ticks_ms() - last_motion}ms ago")

And on IRQ i deactivate IRQ's, set event, set last_motion and activate IRQs

I am changing a project based on timers in pi pico w to esp32 C3/C6 that are poor on timers(only 2)

Hope that it helps.

[peterhinch]

Please format code by enclosing it with three backtick (`) characters before and after: that will preserve indentation. The above code sample is incomplete - ideally I'd like to see a code sample which I can paste at an ESP32 REPL to study the problem.

[peterhinch]

I have managed to replicate this problem using the following script on an ESP32-S3:

import asyncio
from machine import Timer
from time import ticks_diff, ticks_us

evt = asyncio.Event()
latency = 0
def cb(t):
    global latency
    latency = ticks_us()
    evt.set()

async def foo():  # Must run this task for evt to be recognised
    while True:
        await asyncio.sleep(0)

async def main():
    t = Timer(1, freq=1, callback=cb)
    asyncio.create_task(foo())
    while True:
        await evt.wait()
        evt.clear()
        print(f"Latency = {ticks_diff(ticks_us(), latency)}μs")

asyncio.run(main())

This runs as written, but if you fail to start the foo task, the print statement does not run.

I will investigate further, to see if this has been reported previously; if not, to figure out the reason and raise an issue. I'll report back.

In terms of your application, running a foo task is entirely harmless, using minimal resources.

[peterhinch]

It seems this issue has been fixed. I updated the firmware on the ESP32-S3 from 1.25 to 1.27.0 and the following script now runs OK:

import asyncio
from machine import Timer
from time import ticks_diff, ticks_us

evt = asyncio.Event()
latency = 0
def cb(t):
    global latency
    latency = ticks_us()
    evt.set()

async def main():
    t = Timer(1, freq=1, callback=cb)
    while True:
        await evt.wait()
        evt.clear()
        print(f"Latency = {ticks_diff(ticks_us(), latency)}μs")

asyncio.run(main())

Please try this update.