async_q.get() can block the event loop under high load about janus HOT 3 CLOSED

I don't think this is the problem.
janus doesn't switch the loop on await async_q.get() if queue is not empty.
Your example fetches many items one by one in while loop until gets queue-empty state and waits for new data.
Usually asyncio code does IO operations: performs HTTP requests, communicates with database etc.
Once IO is not ready (e.g. socket read buffer has no data to fetch, write buffer is overloaded etc. etc.) the task is suspended.
I expect you do have IO in your real while loop, q.get() is not the only possible yield point.

P.S.
await asyncio.sleep(0) should be enough.

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You're perfectly right. Calling await asyncio.sleep(0) fixes the issue. To be honest I expected any async to always yield to the scheduler to give it time for other tasks, but it seems priorities get messed up. In fact I don't call any I/O there right now as I use it as a synchronization point between other tasks.

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In fact await doesn't force context switch exactly for performance reason.
It is a possible switch point, not mandatory. await asyncio.sleep(0) forces the switch.
Like old good file.read() doesn't assume OS thread switching but time.sleep(0) does.

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