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Julio Biason 2 years ago
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      content/thoughts/a-mental-models-for-async-rust.md

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content/thoughts/a-mental-models-for-async-rust.md

@ -85,5 +85,38 @@ fn main() {
(Yeah, I did all in a single file. Sue me.)
[async is channel, not spawn]
Quick explanation: Here, there are two threads, one for the producer and one
for the consumer; the MPSC (Multiple Producer, Single Consumer) channel is the
communication channel between those two. With the exception that it has only
one consumer, it is pretty close to the way I learnt producer/consumer
architecture at uni (not in Rust, but C, using mutexes).
## Where it falls apart
Now, the first thing one would do, knowing about "greenthreads" and
`tokio::spawn` is just to replace `thread::spawn` for the Tokio API and think
it is done.
And that's where my initial mental model broke down.
'Cause, you see, async is not the threads; **async is the channel**. And by
that I mean that each task is put in a queue (the event loop queue) and then
processed. Instead of just having data (in our example, an integer), it has
data and code; the code is the task and the data is whatever Rust moves into
the task.
## I saw multitasking correctly
One thing I believe I did right was to "mentalize" the way the event loop works
akin to Windows 3.11, which was really prone to become completely unresponsive
from time to time. The reason for that is that the event loop keeps running
things till someone says "Ok, I give up my time and someone else can run their
stuff" -- a.k.a. "cooperative multitasking".
Async works akin to cooperative multitasking, but instead of having the
developer add a bunch of "event loop, you can run something else" statements,
this is done in I/O layer, for one simple reason: Your code would, normally,
block on those, and the event loop will take care of running this in a
non-blocking fashion.
##

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