JSThreadsAndGC

Motivation

• More robust thread safety API
• Reduce GC pauses
• Minor performance win from eliminating object locking

Threading. In short, SM's support for sharing objects among threads has not gotten the maintenance attention it needs. It's accumulating bugs. At the same time, it is now widely acknowledged that shared-everything threads are a problematic programming model. Better ideas (like HTML5 Workers) are emerging. We need new JSAPI support for shared-nothing threads. I'll propose something in this newsgroup later this week.

GC. At the same time our GC performance is not where we want it. There are still direct optimization opportunities. Beyond that, we want to be able to perform GC on a single browser tab, for shorter pause times when only one tab is active.

What these two things have in common is that they both involve dividing the JSRuntime's object graph into separate, mostly-isolated "regions", such that every object is in a region, and an ordinary object cannot have a direct reference to an object in a different region. (We will support special "wrapper" objects that provide transparent access to objects in other regions.)

The changes we need to make for single-tab GC will simultaneously make it easier to add assertions enforcing the new rules about sharing objects across threads. Conveniently, in the browser, wrapper objects already exist in almost all the right places--it's a critical security boundary in the browser, and the wrappers impose various security policies.

Plans

• Pursue opportunities to make GC faster without API changes. For example, string and object destruction (not finalizers) could be moved to the background thread.
• Use membranes to separate threads and GC heaps, as well as for security. This means making the membranes more robust, eliminating all leaks.
• Implement per-region garbage collection.
• Eventually: implement "worker modules" so addons have a non-broken way to use threads.

We have no immediate plans to implement cross-thread proxying. I'm not sure it would be sufficient for existing embeddings that share objects among threads; and it seems no one else needs it. The debugger needs to be able to touch objects across all threads, but proxying objects transparently across threads may not be the right answer there either. (jimb's first impression was that it is not.) I don't think enough is known about the new debugger's eventual design. By mid-March 2010 we'll know more.

There are 122 files using nsIThread in addons.mozilla.org. Locking wrappers are a possible solution for these guys, but it's likely they would still need source changes, in which case we should just encourage their authors to migrate to postMessage.

Tasks

• Igor: optimize GC and move finalization to background
• jorendorff: shell workers. tests for what we think the new contract should be.
• jorendorff: design whatever new JSAPI we need to support "regions"
• jorendorff: add assertions that test our assumption that objects aren't shared.
• jorendorff: ask bz about same-origin "regions" already implemented somewhere in Gecko ("window target algorithm")
• jorendorff: ask gwagner for his GC benchmarks so we have a baseline
• ???: review existing code that shares objects across threads

What is sharing objects across threads now?

• DocShell scripting cross-thread
• multithreaded xpcom
• UrlClassifier
• Personal Security Manager

(Proxy autoconfig is main-thread only.)

References

Preventing Capability Leaks in Secure JavaScript Subsets. Matthew Finifter, Joel Weinberger, and Adam Barth. To appear: Proc. of the 17th Network and Distributed System Security Symposium (NDSS 2010).