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Firefox Javascript Engine Becomes Single Threaded
An anonymous reader writes with news about work on Mozilla's Javascript engine. Quoting Mozilla engineer Luke Wagner's blog: "With web workers in separate runtimes, there were no significant multi-threaded runtime uses remaining. Furthermore, to achieve single-threaded compartments, the platform features that allowed JS to easily ship a closure off to another thread had been removed since closures fundamentally carry with them a reference to their original enclosing scope. Even non-Mozilla SpiderMonkey embeddings had reportedly experienced problems that pushed them toward a similar shared-nothing design. Thus, there was little reason to maintain the non-trivial complexity caused by multi-threading support. There are a lot of things that 'would be nice' but what pushed us over the edge is that a single-threaded runtime allows us to hoist a lot data currently stored per-compartment into the runtime. This provides immediate memory savings."
I am going to assume you are aware of how those functions work (specifically that they cannot interrupt the current thread if it is busy; they are only handled if the thread is idle) and that they themselves are not multi-threaded or have anything to do with multi-threading. It's not 100% clear from your post, though.
Multi-threading in JS is handled by web workers.
The same way it's always been implemented. setTimeout is event driven; it adds an event to the event queue to be executed at a later time. Once your code returns, the browser can spin the event loop again. The timer event will come up in due course and the browser will reenter the js engine to call your function.
Which is why they always cream the competition in the benchmarks? Seriously, the only time I've ever seen it waste memory was during a session where Silverlight crashed. In general it tends to use very little in the way of memory.
OTOH, given your post, I can only assume that you're using lynx, tons of extensions or are some sort of troll.
Sorry, "leaks memory like a BP pipeline" sounds like the best description for a browser which seems to absolutely refuse to free up RAM used by old images loaded by Javascript that have since been kicked off the page. I can set up a timer to reload an image every, say, half hour* (think "weather report precipitation map" or "webcam image") on a machine that should be up 24/7, come in the next day, and have Firefox's "This page has a script that is not responding" popup because the OS was too busy thrashing swap after physical RAM filled up and Firefox thought it was the script's fault. It's not often I see the Mem and Swap meters in GKrellm2 solidly maxed out. For debug purposes, I can have it reload that image every five seconds and watch the memory steadily creep up every five seconds without ever doing anything resembling GC. Of course, if I close that tab, the memory returns instantly.
Now, you might say that for a kiosk that should remain up 24/7 like that, I should consider a different means of presenting the data. And ultimately, I did consider a different means. Because neither Chrome/Chromium nor Opera have this problem. Using the exact same script on both browsers, once it reloaded the image, the old one was booted out of memory immediately, or at least quickly enough that any extra memory use was marginal and incidental, and certainly not to the point where it would suck down all of swap like Firefox did. In fact, this script is still running on a kiosk here, it has been for a couple weeks straight now, and there's no memory wasting in sight. Firefox wouldn't have lasted the first night without manually reloading the entire page.
So yes. It's Firefox. Firefox leaks memory. A lot. It does this due to very poor cache decisions and inferior GC techniques. Period. This has been a known problem for some time; a cursory glance through Stack Overflow will find numerous questions regarding this exact situation and Firefox, none of which have conclusive answers besides "stop using Firefox". And the only common thread in all of them is Firefox. The problem is Firefox. Firefox is the problem. It leaks memory.
*: Note that this is using the trick of appending the image's URL with a dummy timestamp variable to trick Firefox into not just loading the old image from cache despite pragmas and meta tags telling it not to. Point still stands, though: Chrome/Chromium and Opera understand enough to unload the previous image from RAM with the exact same script and usage.
What, the hell are you talking about?
Didn't you read this slashdot article: http://developers.slashdot.org/story/12/01/17/1338225/notes-on-reducing-firefoxs-memory-consumption
Here is one of the relevant parts from the Firefox developer:
"Finally, Firefox 4 had a new HTML5 parser. It had a bug which meant that every time you set an innerHTML property on an element, some memory would leak. And that’s a pretty common operation on many websites. "
Please think before you post again.
Of the 38 bugs listed there, about 5 were not memory leaks according to the subject. How many of those were because of addons?
Then clicking on the first few bugs:
- ONE GUY had addons and the dev requested a no-addon test. No reply.
- This one look more promising but when the dev said "To find out it's a real memory leak or not, run 'prstat' command in a terminal.
If the vaule of 'RSS' field of firefox row keeps increasing, it's a memory leak."... No reply.
- ONE GUY had problem spamming the same URL into the URL bar. "Maybe have to close it" sounds like he wasn't even sure. Not a typical use case. (don't spam the URL bar with the same URL.
- Agan one guy.
- "The numbers you quote are not exceptional at all. They're probably caused by the malware-database that is updated in the background"
- "The fix in bug 426236 fixes all the leaks, but it doesn't touch controllers..." fixed in 2008, but left open "just-in-case"
Maybe you should actually look at the things before linking them?
In all seriousness, they need to do something about the extensions. Refuse to host leaky ones or something. Extensions can't be Firefox's killer feature if they make it eat all of your RAM.
We are so on this. In fact, add-ons are the majority of what we talk about at MemShrink these days.
In theory, leak checking is now part of the addons.mozilla.org review process, so new add-ons will all undergo a (very basic) leak check before they're approved. I'm sure we'll have to tweak as time goes on, but it's a start.
http://jlebar.com/2011/11/13/The_carrot%2C_the_stick%2C_and_the_wrench%3A_Add-on_leaks_are_everyone's_problem..html