Proposal: Abstract References

Mark Miller erights at gmail.com
Wed Oct 22 14:26:43 PDT 2014


On Wed, Oct 22, 2014 at 1:44 PM, Steve Fink <sphink at gmail.com> wrote:

> On 10/22/2014 07:45 AM, Mark S. Miller wrote:
> >
> > * Only objects that have been used as keys in FastWeakMaps would ever
> > have their [[Shadow]] set, so this could also be allocated on demand,
> > given only a bit saying whether it is present. Besides this storage of
> > this bit, there is no other effect or cost on any non-weakmap objects.
> >
> > * Since non-weakmap code doesn't need to test this bit, there is zero
> > runtime cost on non-weakmap code.
> >
> > * Whether an object has been used as a key or not (and therefore
> > whether an extra shadow has been allocated or not), normal non-weak
> > property lookup on the object is unaffected, and pays no additional cost.
>
> Maybe it's because I work on a garbage collector, but I always think of
> the primary cost of WeakMaps as being the GC. The above analysis doesn't
> take GC into account.
>

I should have been more explicit, but GC costs are almost my entire point.
These costs aside, my FastWeakMaps are more expensive in all ways than
SlowWeakMaps, though only by a constant factor, since each FastWeakMap
operation must also perform the corresponding SlowWeakMap operation.



>
> In the straightforward iterative implementation, you record all of the
> live WeakMaps found while scanning through the heap. Then you go through
> them, checking each key to see if it is live. For each such key, you
> recursively mark the value. This marking can discover new live WeakMaps,
> so you iterate to a fixed point.
>

That is when you find yourself doing an ephemeron collection. The point of
the transposed representation is to collect most ephemeron garbage using
conventional collection. Consider

var fastField = new FastWeakMap();
var slowField = new SlowWeakMap();

var transientKey = {};

var fastValue = {};
var slowValue = {};

fastField.set(key, fastValue);
slowField.set(key, slowValue);

transientKey = void 0;
fastValue = void 0;
slowValue = void 0;


At this assume that the old value of transientKey is really garbage, but
that fastField and slowField are still reachable. Clearly, both the old
value of fastValue and the old value of slowValue are now garbage and may
be collected. Let's see what work the collector need to do to collect these.

First comes the conventional mark phase: fastField and slowField both get
marked. slowField is itself a non-transposed weakmap, so when we mark it we
also put it on the queue of weakmaps to be ephemeron collected.
fastField internally is not a weakmap, nor does it point at one. It's only
per-instance state is the token, so this gets marked as well.

Now comes the ephemeron mark phase. The only non-transposed weakmap to be
considered is slowField. The non-transposed weakmap serving as
transientKey's shadow never got marked because transientKey never got
marked. fastValue is only reachable from transientKey's shadow, so it also
never gets marked.

Here's the key important thing: In a generational collector, at this point
we'd typically postpone ephemeron collection. To do so, we would complete
the mark phase conventionally, by simply marking the values held by
slowField. This marks slowValue, causing it to get promoted to the next
older generation. THIS IS EXPENSIVE.

Finally, when we can no longer postpone ephemeron collection, when
ephemeron-marking slowField, we'd notice that transientKey didn't get
marked, so we wouldn't mark slowValue.

The counterpoint is the shadow weakmaps, when engaged in ephemeron
collection must still check whether their keys -- to tokens within the
FastWeakMaps -- are still reachable. Typically they will be. This has two
counter-counterpoints:

* For all practical use patterns we've identified, the WeakMap either lives
longer than its keys, or their lifespan is comparable. When the WeakMap is
known to necessarily live longer than its keys, as for class-private state,
then those shadow properties can even be exempted from the key-mark-check.

* Prior to emphemeron collection, the shadow weakmaps (and the values they
carry) get promoted to older generations only along with the object they
shadow.


> In the current web, this implementation seems to work fine. The worst
> case is O(n^2) in the size of the heap, which is pretty much fatal if
> you ever hit it. But that requires lots of paths through multiple
> WeakMaps, and in practice, it seems WeakMaps aren't being used much.
> I've never seen our WeakMap marking phase show up as a significant cost.
>

Chicken and egg. If WeakMaps are used for private state (and trademarks
and...), they will be used a lot. But they will only be used for those
things if it isn't fatally slow to do so.




>
> For an algorithmically more robust solution, you could add a check
> whenever marking an object. The check would test whether the object is
> (or might be) used as a WeakMap key. This would slow down marking all
> objects, so in practice you want to be clever about avoiding the test.
>

Yeah, I'm very curious about whether this can be made cheap enough that
implementations would be willing to do it. If so, then everything is much
better, whether we transpose the representation or not.



>
> Anyway, my point is that WeakMaps have serious GC ramifications,
> possibly extending to non-key objects, and any performance impact
> analysis of using WeakMaps more extensively is incomplete without
> considering GC costs.
>

Exactly! I should have been clearer that these were the only costs I am
concerned about here. Regarding all other costs, my example code only adds
expense.



>
> > A realistic implementation should seek to avoid allocating the extra
> > shadow objects. However, even if not, we are much better off with the
> > above scheme than we are with the current slow WeakMap.
>
> Perhaps. But multiple WeakMaps introduce the potential for many more
> cycles than a single WeakMap. So I think a final conclusion is premature.


I think this is a good point that I missed in my analysis. We should indeed
think hard about this. Given that, in the end I must agree that a
conclusion is premature. Please let's proceed to build some experiments!


-- 
  Cheers,
  --MarkM
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