Proposal: `await.all {...}` for parallelism

Naveen Chawla naveen.chwl at gmail.com
Mon Nov 25 15:53:10 UTC 2019


Hi Tom! Would you mind clarifying the performance hit you are referring to?
I'm seeing that the "synchronous" calls wouldn't be added to the array you
used in your example, so it's not clear to me to which performance hit you
are referring.

Hi Manuel! Would you mind explaining the added value of your proposal? I am
only seeing it being more verbose, but I've not found any added
functionality or benefit from it

On Mon, 25 Nov 2019 at 14:57, manuelbarzi <manuelbarzi at gmail.com> wrote:

> another example, that may "normalize" it a bit more:
>
> ```
> async { // ... = returns a promise
>     const x1 = await|| ...
>     const x2 = await ... (x1)
>     const x3 = await|| ...
>     const x10 = await ... (x2, x3)
>
>     let x4, x5, x6
>
>     async {
>         x4 = await|| ... (x1, x2)
>         x5 = await|| ... (x2, x3)
>         x6 = await ... (x4, x5, x10)
>     }
>
>     let x7, x8, x9
>
>     async {
>         x7 = await|| ... (x4, x6)
>         x8 = await ... (x6, x7)
>         x9 = await|| ... (x5, x6)
>     }
>
>     await ... (x8, x9)
> }
> ```
>
> On Mon, Nov 25, 2019 at 3:19 PM manuelbarzi <manuelbarzi at gmail.com> wrote:
>
>> why not making it work with the addition of a new keyword suffix for
>> parallel awaits (for example, `||`), grouping the mid blocks that require
>> parallelism. playing with your example and going a bit further:
>>
>> ```
>> async {
>>   const v0 = await|| returnsAPromise(); // to be grouped in parallel
>>   for (let i = 0; (i < 100000); i++) {
>>     doesSimpleFastSynchronousMath();
>>   }
>>   const v1 = await|| returnsAnotherPromise(); // to be grouped in
>> parallel
>>   async {
>>       await returnsAnotherPromise1();
>>       const v2 = await|| returnsAnotherPromise2(); // to be grouped in
>> parallel
>>       const v3 = await|| returnsAnotherPromise3(); // to be grouped in
>> parallel
>>       await returnsAnotherPromise4(v2, v3);
>>       const v4 = await returnsAnotherPromise5();
>>   }
>>   await returnsAnotherPromiseX(v0, v1);
>> }
>> ```
>>
>> On Mon, Nov 25, 2019 at 2:06 PM Tom Boutell <tom at apostrophecms.com>
>> wrote:
>>
>>> There is however a performance concern with your code that we should
>>> talk about.
>>>
>>> If I write this:
>>>
>>> await.all {
>>>   returnsAPromise();
>>>   for (let i = 0; (i < 100000); i++) {
>>>     doesSimpleFastSynchronousMath();
>>>   }
>>>   returnsAnotherPromise();
>>> }
>>>
>>> Then Babel will have no choice but to compile this to:
>>>
>>> {
>>>   const promises = [];
>>>   {
>>>     const maybeThenable = returnsAPromise();
>>>     if (maybeThenable && maybeThenable.then) {
>>>       promises.push(maybeThenable);
>>>     }
>>>   }
>>>   promises.push(returnsAPromise());
>>>   for (let i = 0; (i < 100000); i++) {
>>>     const maybeThenable = doesSimpleFastSynchronousMath();
>>>     if (maybeThenable && maybeThenable.then) {
>>>       promises.push(maybeThenable);
>>>     }
>>>   }
>>>   const maybeThenable = returnsAnotherPromise();
>>>   if (maybeThenable && maybeThenable.then) {
>>>     promises.push(maybeThenable);
>>>   }
>>> }
>>> await Promise.all(promises);
>>>
>>> Which could have a significant performance impact on that synchronous
>>> inner loop.
>>>
>>>
>>>
>>> On Mon, Nov 25, 2019 at 7:55 AM Tom Boutell <tom at apostrophecms.com>
>>> wrote:
>>>
>>>> Hey, you're absolutely right! It's OK because it just means things are
>>>> more deterministic before the block exits. It doesn't impact any reasonable
>>>> expectations *during* the block.
>>>>
>>>> I am convinced that your syntax is useful and does not introduce any
>>>> new confusion.
>>>>
>>>> I wonder, then, if it is also possible to implement concurrency limits
>>>> for this properly?
>>>>
>>>> await.all({ concurrency: 5 }) {
>>>>   for (const item of items) {
>>>>     // returns promise
>>>>     item.process();
>>>>   }
>>>> }
>>>>
>>>> This is more challenging because, in our transpilation, we can't just
>>>> bottle up all the promises and call Promise.all at the end. It would be too
>>>> late to manage how many are in process at once, bashing on various API
>>>> limits (:
>>>>
>>>> On Sun, Nov 24, 2019 at 7:43 PM Naveen Chawla <naveen.chwl at gmail.com>
>>>> wrote:
>>>>
>>>>> Hi! It does not change the meaning of the ";" at all. As you may
>>>>> already know, omitting `await` already invokes multiple async function
>>>>> calls in parallel, in current JavaScript, so absolutely no change in that
>>>>> respect.
>>>>>
>>>>> The only thing this `await.all` suggestion does, is ensure that all
>>>>> non-awaited async function calls are completed before proceeding beyond the
>>>>> end of the block.
>>>>>
>>>>> i.e. it adds fairly straightforward and terse deterministic control to
>>>>> otherwise non-deterministic code, without requiring knowledge of
>>>>> destructuring or `Promise.all`.
>>>>>
>>>>> On Sat, 23 Nov 2019 at 13:25, Tom Boutell <tom at apostrophecms.com>
>>>>> wrote:
>>>>>
>>>>>> This is very interesting, but this code:
>>>>>>
>>>>>> await.all {
>>>>>>    x = getXAsync();
>>>>>>    y = getYAsync();
>>>>>> }
>>>>>>
>>>>>> processXAndY(x, y);
>>>>>>
>>>>>> Still carries within it the problem that if I'm looking at just the
>>>>>> middle of the { ... } block — if "await.all" has scrolled offscreen — I'll
>>>>>> be completely wrong about what ";" means. I think that's too much magic.
>>>>>>
>>>>>> Also, in the case of the "for" loop, this doesn't address managing
>>>>>> the level of concurrency. Although it could in theory with a syntax like
>>>>>> await.all({ concurrency: 5 }), I'm not sure if it's practical to implement
>>>>>> that for your general case.
>>>>>>
>>>>>> Actually I'm curious about what the implementation would look like in
>>>>>> general.  If it were babel compiling this, I guess it would have to wrap
>>>>>> every statement not preceded by "await" with a check for whether it returns
>>>>>> a thenable and add it to an array if it does. But with the concurrency
>>>>>> feature it would also have to defer executing the code at all until the
>>>>>> right time as otherwise we're still starting zillions of "processes" at
>>>>>> once.
>>>>>>
>>>>>>
>>>>>> On Sat, Nov 23, 2019 at 5:08 AM Naveen Chawla <naveen.chwl at gmail.com>
>>>>>> wrote:
>>>>>>
>>>>>>> However, if `await.all { ... }` were to mean "wait for all
>>>>>>> non-awaited async function calls made within this block to complete before
>>>>>>> proceeding", as I suggested earlier, I think that could satisfy determinism
>>>>>>> for "await" wherever it is used, and satisfy the original motivation:
>>>>>>>
>>>>>>> ```
>>>>>>> await.all {
>>>>>>>     for (const item of items) {
>>>>>>>         doTheThingAsync(item);
>>>>>>>     }
>>>>>>> }
>>>>>>> ```
>>>>>>>
>>>>>>> Notice I have omitted `await` inside the loop. Like current
>>>>>>> JavaScript, that causes parallel execution, so no change on that front,
>>>>>>> from a determinism perspective. So determinism is not hurt by `await.all`.
>>>>>>> Rather, it guarantees completion before going further.
>>>>>>>
>>>>>>> In an earlier example (paraphrase-coded as I forgot the names):
>>>>>>>
>>>>>>> ```
>>>>>>> let x, y;
>>>>>>>
>>>>>>> await.all {
>>>>>>>    x = getXAsync();
>>>>>>>    y = getYAsync();
>>>>>>> }
>>>>>>>
>>>>>>> processXAndY(x, y);
>>>>>>> ```
>>>>>>>
>>>>>>> I think the benefit of this syntax appears more stark with the
>>>>>>> looped (first) example, as current JavaScript requires building an array in
>>>>>>> the loop to subsequently pass to `Promise.all`, which I think is a little
>>>>>>> more difficult to conceptualize than the `await.all { ... }` way of doing
>>>>>>> it. The 2nd example is arguably better than current JavaScript too,
>>>>>>> particularly because the coder doesn't have to be very smart with
>>>>>>> destructuring in light of understanding the "Promise.all" return type, etc.
>>>>>>> In other words, less cognitive overhead, which I think is a net positive.
>>>>>>>
>>>>>>> On Fri, 22 Nov 2019 at 13:44, Tom Boutell <tom at apostrophecms.com>
>>>>>>> wrote:
>>>>>>>
>>>>>>>> I am very sympathetic to pitches to allow more common cases for
>>>>>>>> promise libraries to be written in an "awaitful" syntax without thinking
>>>>>>>> explicitly about promises.
>>>>>>>>
>>>>>>>> Howeever I think that changing the meaning of the semicolon in a
>>>>>>>> particular context has too much potential for confusion. As others have
>>>>>>>> said, parallel execution is different, and it should look and feel
>>>>>>>> different. The most basic assumption a developer makes (consecutive lines
>>>>>>>> of code run consecutively) is difficult to get away from; that's why we
>>>>>>>> introduced "await" in the first place, to bring back the ability to write
>>>>>>>> deterministic code with consecutive statements. Which sounds like a
>>>>>>>> reasonable ask, when it's put that way. (:
>>>>>>>>
>>>>>>>> I did propose this recently:
>>>>>>>>
>>>>>>>> for (const item of items concurrency 5) {
>>>>>>>>   await  doTheThing(item);
>>>>>>>> }
>>>>>>>>
>>>>>>>> However in this case I'm not talking about consecutive statements,
>>>>>>>> I'm only talking about rules for simultaneously (in the sense of async, not
>>>>>>>> threads) running more than one instance of the block. So I'm not proposing
>>>>>>>> that we change the meaning of the semicolon(s) *within* the block in a way
>>>>>>>> that could mean that if you're looking at half the code in the middle you
>>>>>>>> would be likely to fundamentally misunderstand its operation.
>>>>>>>>
>>>>>>>> I think that risk - that you can't tell what a semicolon means
>>>>>>>> without reference to the outer context - is what makes your proposal a
>>>>>>>> bridge too far for me.
>>>>>>>>
>>>>>>>>
>>>>>>>> --
>>>>>>>>
>>>>>>>> THOMAS BOUTELL | CHIEF TECHNOLOGY OFFICER
>>>>>>>> APOSTROPHECMS | apostrophecms.com | he/him/his
>>>>>>>> _______________________________________________
>>>>>>>> es-discuss mailing list
>>>>>>>> es-discuss at mozilla.org
>>>>>>>> https://mail.mozilla.org/listinfo/es-discuss
>>>>>>>>
>>>>>>>
>>>>>>
>>>>>> --
>>>>>>
>>>>>> THOMAS BOUTELL | CHIEF TECHNOLOGY OFFICER
>>>>>> APOSTROPHECMS | apostrophecms.com | he/him/his
>>>>>>
>>>>>
>>>>
>>>> --
>>>>
>>>> THOMAS BOUTELL | CHIEF TECHNOLOGY OFFICER
>>>> APOSTROPHECMS | apostrophecms.com | he/him/his
>>>>
>>>
>>>
>>> --
>>>
>>> THOMAS BOUTELL | CHIEF TECHNOLOGY OFFICER
>>> APOSTROPHECMS | apostrophecms.com | he/him/his
>>> _______________________________________________
>>> es-discuss mailing list
>>> es-discuss at mozilla.org
>>> https://mail.mozilla.org/listinfo/es-discuss
>>>
>>
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