[whatwg] Limit on number of parallel Workers.

[Drew Wilson]

This is a bit of an aside, but section 4.5 of the Web Workers spec no longer
makes any guarantees regarding GC of workers. I would expect user agents to
make some kind of best effort to detect unreachability in the simplest
cases, but supporting MessagePorts and SharedWorkers makes authoritatively
determining worker reachability exceedingly difficult except in simpler
cases (DedicatedWorkers with no MessagePorts or nested workers, for
example). It seems like we should be encouraging developers to call
WorkerGlobalScope.close() when they are done with their
workers, which in the case below makes the number of running threads
less undeterministic.
Back on topic, I believe what Dmitry was suggesting was not that we specify
a specific
limit in the specification, but rather we have some sort of general
agreement on how a UA might handle limits (what should it do when the
limit is reached).
His suggestion of delaying the startup of the worker seems like a better
solution than other approaches like throwing an exception on the Worker
constructor.

-atw

On Tue, Jun 9, 2009 at 6:28 PM, Oliver Hunt <oliver at apple.com> wrote:

> I believe that this will be difficult to have such a limit as sites may
> rely on GC to collect Workers that are no longer running (so number of
> running threads is
> non-deterministic), and in the context of mix source content ("mash-ups") it
> will be difficult for any content source to be sure it isn't going to
> contribute to that
> limit.  Obviously a UA shouldn't crash, but i believe that it is up to the UA to determine how to achieve this -- eg. having a limit to allow a 1:1 relationship between workers and processes will have a much lower limit than an implementation that has a worker per thread model, or an m:n relationship between workers and threads/processes.
>  Having the specification limited simply because one implementation
> mechanism has certain limits when there are many alternative implementation
> models seems like a bad idea.
> I believe if there's going to be any worker related
> limits, it should realistically be a lower limit on the number of workers rather than an upper.
>
> --Oliver
>
>
> On Jun 9, 2009, at 6:13 PM, Dmitry Titov wrote:
>
> Hi WHATWG!
>
> In Chromium, workers are going to have their separate processes, at least
> for now. So we quickly found that "while(true) foo = new Worker(...)"
> quickly consumes the OS resources :-) In fact, this will kill other browsers
> too, and on some systems the unbounded number of threads will effectively
> "freeze" the system beyond the browser.
>
> We think about how to reasonably place limits on the resources consumed by
> 'sea of workers'. Obviously, one could just limit a maxumum number of
> parallel workers available to page or domain or browser. But what do you do
> when a limit is reached? The Worker() constructor could return null or throw
> exception. However, that seems to go against the spirit of the spec since it
> usually does not deal with resource constraints. So it makes sense to look
> for the most sensible implementation that tries best to behave.
>
> Current idea is to let create as many Worker objects as requested, but not
> necessarily start them right away. So the resources are not allocated except
> the thin JS wrapper. As long as workers terminate and the number of them
> decreases below the limit, more workers from the "ready queue" could be
> started. This allows to support implementation limits w/o exposing them.
>
> This is similar to how a 'sea of XHRs' would behave. The test page here<http://www.figushki.com/test/xhr/xhr10000.html> creates
> 10,000 async XHR requests to distinct URLs and then waits for all of them to
> complete. While it's obviosuly impossible to have 10K http connections in
> parallel, all XHRs will be completed, given time.
>
> Does it sound like a good way to avoid the resource crunch due to high
> number of workers?
>
> Thanks,
> Dmitry
>
>
>
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