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Date: Fri, 9 Feb 2018 08:12:49 +0800
Cc: Aaron Piotrowski <aaron@trowski.com>,
 PHP Internals <internals@lists.php.net>,
 Dmitry Stogov <dmitry@zend.com>
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To: Niklas Keller <me@kelunik.com>
Feedback-ID: bizesmtp:lvht.net:qybgforeign:qybgforeign4
Subject: Re: [PHP-DEV] [RFC] Fiber support (again)
From: i@lvht.net (Haitao Lv)



> On Feb 9, 2018, at 06:22, Niklas Keller <me@kelunik.com> wrote:
>=20
>>=20
>>> - How do you determine when a fiber has returned? Looking at the =
source,
>> it appears Fiber::status() must be used, comparing against constants.
>> Separate methods similar to Generator would be better IMO. e.g.:
>> Fiber::alive(), Fiber::suspended(), Fiber::running()
>>=20
>> Offering methods like Fiber::alive, Fiber::running makes no =
difference to
>> check the Fiber::status() return value. This is just a style issue. =
And as
>> a language feature,
>> Fiber only offer the essential API and let other works to the user =
land.
>=20
>=20
> The language should offer a sane API, not the absolute minimum =
required to
> work for these things.

The Ruby's Fiber do offer a live? method but does not have a getStatus =
method.
The Lua's coroutine only offer a status method.

So do we really need to offer three additional helper method? Or what is =
your
advice about these API?

>=20
>>> - What about throwing exceptions into a fiber?
>>=20
>> Currently does not support throw exception into the fiber. User land =
code
>> could check
>> the value of Fiber::yield and throw exception themselves. The Ruby's =
Fiber
>> and Lua's
>> coroutine also does not support such api as well.
>=20
>=20
> And throw the exception where? That means async code with fibers can't
> really handle errors?

Actually you can transfer any thing to Fiber by the resume method. And =
you can
check the return value of Fiber::yield to handle error.

Fiber is designed as a primitive, low level, and lightweight feature. =
User land
code seldom not need to use them directly in your normal code.
So the following is not a big problem,

$a =3D Fiber::yield(...);
if ($a =3D=3D=3D false) {
   throw new Exception(...);
}

And both the Ruby and Lua does not offer such API as well.
>=20
>>=20
>>>=20
>>> - Using Fiber::resume() to initialize the fiber and resume feels
>> awkward. Separate methods again would be better here, perhaps
>> Fiber::init(...$args) and Fiber::resume($send).
>>=20
>> All Fiber created with a suspended status. So make resume to do both =
the
>> init and resume
>> do make sense.
>>=20
>=20
> It does't make sense to me. Reading the example in the README and
> understanding why the first resume() takes two arguments instead of =
one
> took me quite some minutes.

This Ruby's Fiber and Lua's coroutine using one resume API to init and =
resume
the coroutine. I do not think a dedicate is really required.

The generator cannot be init by it's send method. And if you want to =
implement
coroutine feature(without stack) by it, you have to write code

function run() {
    if ($this->beforeFirstYield) {
        $this->beforeFirstYield =3D false;
        return $this->coroutine->current();
    } else {
        $retval =3D $this->coroutine->send($this->sendValue);
        $this->sendValue =3D null;
        return $retval;     =20
    }  =20
}

It is verbose.

See =
https://nikic.github.io/2012/12/22/Cooperative-multitasking-using-coroutin=
es-in-PHP.html

>=20
>> Please see Ruby Fiber API https://ruby-doc.org/core-2.2.0/Fiber.html.
>>=20
>>>=20
>>> - What happens if the sub1() function in the RFC is invoked outside =
of a
>> fiber?
>>=20
>> You will get a Fatal Error like
>> Fatal error: Uncaught Error: Cannot call Fiber::yield out of Fiber
>>=20
>>> - I think a keyword here would be beneficial, even if it has a minor =
BC
>> impact. Fibers could then be written like generators. `await` or =
`emit` as
>> a keyword perhaps? This would be a less verbose API, feel less =
magical (a
>> static method call that actually pauses execution feels out of =
place), and
>> would allow Fibers to be returned from methods, named functions, etc =
with
>> less boilerplate.
>>=20
>> Wishing this to be accepted by the community in the PHP 7.3, so no =
keyword
>> is accepted.
>> And if the community cannot accept, the Fiber can be still =
distributed as
>> a standalone
>> extension. So we cannot depend on a new keyword.
>=20
>=20
> Right, if it's a standalone extension it can't use a new keyword, but =
as a
> language feature it totally can.

In my opinion, using a keyword or call a method is just a coding style =
problem.
Introducing a new keyword does not offer any benefit by makes a minor =
BC.

Both Ruby's Fiber and Lua's coroutine does not required a dedicate =
keyword.

> Looking at the current README, there are two issues that must be =
completely
> solved IMO before accepting this:
>=20
>> Each Fiber has a separate 4k stack. You can use the fiber.stack_size =
ini
> option to change the default stack size. You can also use the second
> argument of Fiber::__construct to set the stack size on fly.
>=20
> Resizing of the stack should happen automatically, just like =
generators
> resize automatically.

This size is the init stack size. It means when a Fiber created, it will =
get a
dedicate stack of 4k size. When the fiber use all the stack space, zend =
vm will
allocate additional space for feature call frame, automatically.

The default size is 4k means that every fiber requires at least 4k =
memory to use
as there own stack. But user can change this by php.ini and the =
construct argument
to reduce the memory footprint.

>=20
>> Fiber::yield cannot be used in internal callback
>=20
> This also seems problematic and will make fibers quite less useful,
> especially as these yields can happen anywhere down the stack.
>=20

This do be a problem. But not so much big problem.

You cannot use Fiber::yield like

$f =3D new Fiber(function () {
  array_map(function ($i) {
    Fiber::yield($i);
  }, [1, 2, 3]);
});
$f->resume();
$f->resume();

Because when zend execute the array_map, it will push a new frame onto =
the c stack.
When zend execute Fiber::yield, it only backup it's php stack, and it's =
c stack
will be overwrites.

However, you can use Fiber::yield like

$f =3D new Fiber(function () {
   Fiber::yield(1); // will cause by resume, by an internal callback
});

ExtEventLoop::onRead($fd, function () { // this is an internal call
   $f->resume();
});=20

> Regards, Niklas