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Re: Sync API for workers

From: Olli Pettay <Olli.Pettay@helsinki.fi>
Date: Thu, 06 Sep 2012 09:30:19 +0300
Message-ID: <504842FB.7040702@helsinki.fi>
To: Jonas Sicking <jonas@sicking.cc>
CC: Glenn Maynard <glenn@zewt.org>, Andrea Marchesini <amarchesini@mozilla.com>, David Bruant <bruant.d@gmail.com>, "public-webapps@w3.org" <public-webapps@w3.org>
On 09/06/2012 09:12 AM, Jonas Sicking wrote:
> On Wed, Sep 5, 2012 at 11:02 PM, bugs@pettay.fi <bugs@pettay.fi> wrote:
>> On 09/06/2012 08:31 AM, Jonas Sicking wrote:
>>> On Wed, Sep 5, 2012 at 8:07 PM, Glenn Maynard <glenn@zewt.org> wrote:
>>>> On Wed, Sep 5, 2012 at 2:49 AM, Jonas Sicking <jonas@sicking.cc> wrote:
>>>>> The problem with a "Only allow blocking on children, except that
>>>>> window can't block on its children" is that you can never block on a
>>>>> computation which is implemented in the main thread. I think that cuts
>>>>> out some major use cases since todays browsers have many APIs which
>>>>> are only implemented in the main thread.
>>>> You can't have both--you have to choose one of 1: allow blocking upwards,
>>>> 2:
>>>> allow blocking downwards, or 3: allow deadlocks.  (I believe #1 is more
>>>> useful than #2, but each proposal can go both ways.  I'm ignoring more
>>>> complex deadlock detection algorithms that can allow both #1 and #2, of
>>>> course, since that's a lot harder.)
>>> Indeed. But I believe #2 is more useful than #1. I wasn't proposing
>>> having both, I was proposing only doing #2.
>>> It's actually technically possible to allow both #1 and #2 without
>>> deadlock detection algorithms, but to keep things sane I'll leave that
>>> as out of scope for this thread.
>>> [snip]
>>>> I think that's by far the most
>>>> interesting category of use cases raised for this feature so far, the
>>>> ability to implement sync APIs from async APIs (or several async APIs).
>>> That is certainly an interesting use case. I think another interesting
>>> use case is being able to write synchronous APIs in workers whose
>>> implementation uses APIs that are only available on the main thread.
>>> That's why I'm not interested in only blocking on children, but rather
>>> only blocking on parents.
>>>>> The fact that all the examples that people have used while we have
>>>>> been discussing synchronous messaging have spun event loops in
>>>>> attempts to deal with messages that couldn't be handled by the
>>>>> synchronous poller makes me very much think that so will web
>>>>> developers.
>>>> getMessage doesn't spin the event loop.  "Spinning the event loop" means
>>>> that tasks are run from task queues (such as asynchronous callbacks)
>>>> which
>>>> might not be expecting to run, and that tasks might be run recursively;
>>>> none
>>>> of that that happens here.  All this does is block until a message is
>>>> available on a specified port (or ports), and then returns it--it's just
>>>> a
>>>> blocking call, like sync XHR or FileReaderSync.
>>> The example from Olli's proposal 3 does what effectively amounts to
>>> "spinning an event loop". It pulls out a bunch of events from the
>>> normal event loop and then manually dispatches them in a while loop.
>>> The behavior is exactly the same as spinning the event loop (except
>>> that non-message tasks doesn't get dispatchet).
>> It is just dispatching events.
>> The problems we (Gecko) have had with event loop spinning in main thread
>> relate mainly to the problems where "unexpected" events are dispatched while
>> running the loop, as an example user input events or events coming from
>> network.
>> getMessage/waitForMessage does not have that problem.
> I'm not sure what you mean by "just dispatching events". That's
> exactly what event loop spinning is.

No. waitForMessage example I wrote down just dispatches DOM events in a loop.
That is a synchronous operation and you know exactly which events you're
about to dispatch.
If you run the generic event loop, you also end up running
timers and getting input from network and user etc. and you can't
controls those.

> Why are the gecko events any more "unexpected" than the message events
> that the example dispatches.
We don't want to block certain events in Gecko (like user input to chrome).
Blocking events in worker code is ok.

> The network events or user input events
> that we had are all events created by gecko code. The messages that
> might get dispatches by the worker code can easily also be network
> events or user events which are sent to the worker for processing.
> Web pages can have just as much inconsistent state while deep in call
> stacks as we do.
That is true...

> If they at that point call into a library which
> starts pulling messages off of the task queue and dispatches them,
> they'll run into the same problems as we've had.

...but then it is up to the library to handle the case properly and
dispatch events async.


> / Jonas
Received on Thursday, 6 September 2012 06:31:22 UTC

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