[html5] JSON parsing in Web Worker

Ricardo Tomasi ricardobeat at gmail.com
Tue Dec 28 11:51:20 PST 2010

I assume you are using a single worker (if not, you could parse the chunks on a timer to avoid freezing). Isn't the longer processing time countered by the fact that the UI is not frozen anymore?

Anyway, a sample of the code you're using would be nice, we can't talk about performance without an implementation.

-- Ricardo

Sent from my iPad

On 28/12/2010, at 17:21, Igor Minar <iiminar at gmail.com> wrote:

> Drew,
> I tested Safari 5.0.2 (6533.18.5) and while it's one of the faster
> browsers out there, my tests show that parsing 650kb json string takes
> 3x longer when I use webworker than when I parse it in the main
> thread.
> Parsing alone, take equivalent amount of time, it's the async
> messaging and mainly transfer of data from the worker that adds 2x
> overhead.
> I use JSON.parse to do the parsing, and while this method is snappy,
> with payloads bigger than 500kb, I can make the UI freeze just long
> enough to make it noticeable.
> I think what I really want is for JSON.parse to by implemented as
> async and executed in it's own thread. I would then just pass in a
> callback that would handled the parsed object when it's ready. Web
> workers get pretty close to allowing me to do something similar, but
> the messaging overhead is killing all the benefits I'm getting from
> the async parsing in worker thread.
> /i
> On Tue, Dec 28, 2010 at 10:51 AM, Drew Wilson <atwilson at chromium.org> wrote:
>> Hi Igor,
>> Objects passed via message ports (including the intrinsic port for dedicated
>> workers) are cloned. I can't speak for other implementations, but in WebKit
>> I believe cloned objects aren't JSON encoded/decoded, but instead there is
>> another native mechanism for cloning these objects that will likely be
>> faster than JSON encoding.
>> That said, I'm not sure that "parsing large JSON files" is the best
>> WebWorker use case, depending on how you're doing the parsing and how large
>> the files are.
>> -atw
>> On Tue, Dec 28, 2010 at 10:35 AM, Igor Minar <iiminar at gmail.com> wrote:
>>> Hello,
>>> I'm exploring the possibilities of using web workers for parsing large
>>> JSON files outside of the main UI thread.
>>> I found several references that this could be one of the use cases for
>>> web workers (e.g. oreilly's intro to web workers [1]). However, the
>>> more I read about webworkers, the less attractive they are for this
>>> purpose, mainly because of how data is passed from worker to the main
>>> thread.
>>> Please correct me if I'm wrong, but my understanding is that any data
>>> that is returned in the message from the worker, is copied rather than
>>> shared and it seems that this is often implemented by serializing the
>>> data into a json string and then deserializing it in the main script.
>>> Is this right? Because if it is, then what's the point of parsing the
>>> json string in worker thread, just to serialize it and then parse it
>>> again in the main thread.
>>> I'd love to be wrong about this because the concept of workers looks
>>> like a perfect match for my use case (parsing large json payloads
>>> quickly without affecting the UI), but my trivial microbenchmarks show
>>> that the overhead of passing the data to, as well as from the
>>> webworker is just too big to use it for this purpose.
>>> thanks,
>>> Igor
>>> [1] http://answers.oreilly.com/topic/1358-introducing-the-web-workers-api/
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