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Re: Thread-Safe DOM // was Re: do not deprecate synchronous XMLHttpRequest

From: Boris Zbarsky <bzbarsky@mit.edu>
Date: Wed, 11 Feb 2015 15:33:57 -0500
Message-ID: <54DBBCB5.1050302@mit.edu>
To: public-webapps@w3.org
On 2/11/15 3:04 PM, Brendan Eich wrote:
> If you want multi-threaded DOM access, then again based on all that I
> know about the three open source browser engines in the field, I do not
> see any implementor taking the huge bug-risk and opportunity-cost and
> (mainly) performance-regression hit of adding barriers and other
> synchronization devices all over their DOM code. Only the Servo project,
> which is all about safety with maximal hardware parallelism, might get
> to the promised land you seek (even that's not clear yet).

A good start is defining terms.  What do we mean by "multi-threaded DOM 
access"?

If we mean "concurrent access to the same DOM objects from both a window 
and a worker, or multiple workers", then I think that's a no-go in Servo 
as well, and not worth trying to design for: it would introduce a lot of 
spec and implementation complexity that I don't think is warranted by 
the use cases I've seen.

If we mean the much more modest "have a DOM implementation available in 
workers" then that might be viable.  Even _that_ is pretty hard to do in 
Gecko, at least, because there is various global state (caches of 
various sorts) that the DOM uses that would need to either move into TLS 
or become threadsafe in some form or something...  Again, various specs 
(mostly DOM and HTML) would need to be gone over very carefully to make 
sure they're not making assumptions about the availability of such 
global shared state.

> We should add lighter-weight workers and immutable data structures

I should note that even some things that could be immutable might 
involved a shared cache in current implementations (e.g. to speed up 
sequential indexed access into a child list implemented as a linked 
list)...  Obviously that sort of thing can be changed, but your bigger 
point that there is a lot of risk to doing that in existing 
implementations remains.

-Boris
Received on Wednesday, 11 February 2015 20:34:27 UTC

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