Bug 721515 will add two new global arrays that persist in between CCs to remember where bits have been set. These should be added to the memory reporter, probably to the DOM memory reporter. I'm not sure how large they will be in practice, but maybe a few thousand each.

Bug 721515 doesn't add any persistent arrays. The arrays are alive either from the first CanSkip to the end of forgetSkippable, or from CanSkipInCC to Unlink.

So they only maintain state across a single forgetSkippable? (Or a single CC in the latter case.) It can still be good to add them in, just in case somebody breaks something later. Maybe not a high priority then.

Yes, they are just for single forgetSkippable or cycleCollect.

Well, it would still be good to have it, eventually. I have a similar reporter for the cycle collector graph, even though it is cleared in between cycle collections, because it is held in a persistent global structure. But yeah this is probably not a big deal. What do you think, Nick? This is a globally held data structure that in practice should only going to be alive for a few milliseconds. Should I bother?

Well, the "cycle-collector" number can get really big (multiple MBs) if you repeatedly hit "Update" in about:memory. Could the same thing happen with this buffer?

Usually these buffers are 1 kB, but can get

up to few kilobytes. Well, in pathological cases even more.