WeakMap not the weak needed for zombie views

Till Schneidereit till at tillschneidereit.net
Mon Jul 7 02:05:51 PDT 2014

I largely agree with your arguments, but one point is actually more of a
counter argument to having weakrefs:

On Mon, Jul 7, 2014 at 10:41 AM, Katelyn Gadd <kg at luminance.org> wrote:

> Similarly, it's important to realize that while some use cases for
> weakrefs are about managing native resources or doing other 'automatic
> cleanup' behaviors, many use cases are simply about ensuring that the
> GC can free up large graphs of dead objects *as soon as memory
> pressure strikes* instead of waiting until the (likely fragile or
> slow) user-space collector gets around to running and collecting
> user-space objects. Memory pressure is something the browser and JS VM
> have knowledge of that userspace doesn't know about - if a graph is
> effectively dead but can't easily have its references cleaned up
> automatically (as can happen in complex object layouts, where you
> would normally use refcounting or some other mechanism), it's possible
> it could remain 'alive' for a long period of time without weakrefs,
> eating up valuable heap, moving between GC generations, and slowing
> GCs.

While this is true, I think that, as others have argued in the discussion
thread I linked to and elsewhere, weakrefs are a bad solution for this. The
GC cannot distinguish between different types of resources and their
freshness, so they'll just blow away everything they can. In most
real-world cases, you'd want to take into account both how frequently and
how recently a resource is/was accessed. And, of equal importance, how
expensive it is to re-create. You can easily have a large, easily
re-created buffer that you'd want to dump at the slightest hint of memory
pressure (maybe even to prevent costly GCs from running?), while at the
same time you have small-ish objects that are expensive to re-create, so
you'd only do so under serious memory pressure.

All in all, I think the platform should expose tiered memory pressure
notifications, regardless of whether weakrefs are introduced for other
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