On Sun, Jun 14, 2020 at 2:16 PM Kyle Stanley <aeros...@gmail.com> wrote:
>
> > If
> you're fine with invisible context switches, you're probably better
> off with threads, because they're not vulnerable to unexpectedly
> blocking actions (a common culprit being name lookups before network
> transactions - you can connect sockets asynchronously, but
> gethostbyname will block the current thread).
>
> These "unexpectedly blocking actions" can be identified in asyncio's debug
> mode. Specifically, any callback or task step that has a duration greater
> than 100ms will be logged. Then, the user can take a closer look at the
> offending long running step. If it's like socket.gethostbyname() and is a
> blocking IO-bound function call, it can be executed in a thread pool using
> loop.run_in_executor(None, socket.gethostbyname, hostname) to avoid blocking
> the event loop. In 3.9, there's also a roughly equivalent higher-level
> function that doesn't require access to the event loop:
> asyncio.to_thread(socket.gethostbyname, hostname).
>
> With the default duration of 100ms, it likely wouldn't pick up on
> socket.gethostbyname(), but it can rather easily be adjusted via the
> modifiable loop.slow_callback_duration attribute.
>
> Here's a quick, trivial example:
> ```
> import asyncio
> import socket
>
> async def main():
> loop = asyncio.get_running_loop()
> loop.slow_callback_duration = .01 # 10ms
> socket.gethostbyname("python.org")
>
> asyncio.run(main(), debug=True)
> # If asyncio.run() is not an option, it can also be enabled via:
> # loop.set_debug()
> # using -X dev
> # PYTHONASYNCIODEBUG env var
> ```
> Output (3.8.3):
> Executing <Task finished name='Task-1' coro=<main() done, defined at
> asyncio_debug_ex.py:5> result=None created at
> /usr/lib/python3.8/asyncio/base_events.py:595> took 0.039 seconds
>
> This is a bit more involved than it is for working with threads; I just
> wanted to demonstrate one method of addressing the problem, as it's a
> decently common issue. For more details about asyncio's debug mode, see
> https://docs.python.org/3/library/asyncio-dev.html#debug-mode.
>
IOW the solution to the problem is to use threads. You can see here
why I said what I did: threads specifically avoid this problem and the
only way for asyncio to avoid it is to use threads. (Yes, you can
asynchronously do a DNS lookup rather than using gethostbyname, but
the semantics aren't identical, and you may seriously annoy someone
who uses other forms of name resolution. So that doesn't count.) As an
additional concern, you don't always know which operations are going
to be slow. For instance, maybe during testing (with debug=True), your
DNS lookups are always reasonably fast, but then some time after
deployment, you find that they're stalling you out. How much effort is
it to change this over? How many other things are going to be slow,
and can you find them all?
That's why threads are so convenient for these kinds of jobs.
Disadvantages of threads:
1) Overhead. If you make one thread for each task, your maximum
simultaneous tasks can potentially be capped. Irrelevant if each task
is doing things with far greater overhead anyway.
2) Unexpected context switching. Unless you use locks, a context
switch can occur at any time. The GIL ensures that this won't corrupt
Python's internal data structures, but you have to be aware of it with
any mutable globals or shared state.
3) Steven D'Aprano is terrified of them and will rail on you for using threads.
Disadvantages of asyncio:
1) Code complexity. You have to explicitly show which things are
waiting on which others.
2) Unexpected LACK of context switching. Unless you use await, a
context switch cannot occur.
Take your pick. Figure out what your task needs. Both exist for good reasons.
ChrisA
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