[Python-ideas] The async API of the future: yield-from

Greg Ewing greg.ewing at canterbury.ac.nz
Sun Oct 21 01:21:05 CEST 2012

Yuval Greenfield wrote:
> block() removes the current task from the ready_list, but is the current 
> task guaranteed to be my task?

Yes, block() always operates on the currently running task.

> If so, then I'd never run again after the 
> yield in acquire(), that is unless a gracious other player unblocks me.

Yes, the unblocking needs to be done by another task, or by
something outside the task system such as an I/O callback.

> I'm not sure it makes sense for scheduler functions to store waiting 
> tasks in a queue owned by the app and invisible from the scheduler. This 
> can cause *invisible deadlocks* such as:
> schedule(philosopher("Socrates", 8, 3, 1, forks[0], forks[2]), "Socrates")
> schedule(philosopher("Euclid", 5, 1, 4, forks[2], forks[0]), "Euclid")

Deadlocks are a potential problem in any system involving
concurrency, and have to be dealt with on a case-by-case

Simply having the scheduler know where all the tasks are
will not prevent deadlocks. It might make it possible for the
scheduler to *detect* deadlocks, but you still have to do
something about them.

Having said that, I'm thinking about writing a more elaborate
version of my scheduler that does keep track of which queue a
task is waiting on, mainly so that tasks can be cancelled

> Is there a coroutine strategy for tackling these challenges? Or will I 
> just get better at overcoming them with practice?

If you've been using threads all your life as you say, then
you're probably already pretty good at dealing with them. All
of the same techniques apply.


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