On Monday, January 19, 2004, at 06:37 , Gordon Henriksen wrote:
Dan Sugalski wrote:
For a copying collector to work, all the mutators must be blocked, and arguably all readers should be blocked as well.
True of non-moving collectors, too. [...]
Some of what I've written up addresses why. [...] I'll send that section when I get out of the office.
Consider this simple object graph:
C was not marked reachable (although it was) and was thus erroneously collected, leaving a dangling pointer. This problem applies equally to copying and mark-sweep collectors.
Ah, OK, I see. The problem comes in where we've got an object in the transient root set (basically the processor stack and registers) that gets anchored into the base root set (stash, pads, or whatever) after the DOD has traced where it's going into and falls out of the transient root set before the DOD traces over to it.
Race condition. Dammit.
Okay, I'd not wrapped my brain around that possibility, which will make for some interesting DOD tracing, especially on SMP systems. I was *really* hoping a single lock on the arena allocation system that the DOD held onto while tracing would be sufficient, but I see that it isn't.
That means we're going to have to have either a really forgiving DOD system that takes multiple passes before it collects up a PMC or buffer (which still isn't safe) or have some way to force a low-overhead rendezvous.
The obvious rendezvous point is the arena lock, but that's going to see a lot of contention anyway, and we'd as soon not have a single one for speed reasons. Feh.
Okay, we're going to mandate that we have read/write locks, each interpreter pool has one, and mutating vtable entries must get a read lock on the pool read/write lock. Pricey (ick) but isolated to mutators. The DOD gets a write lock on it, which'll block all read/write access so no mutators can be in process while the pool DOD runs.
I think that'll work. The .1j thread spec requires r/w locks, and we can fake them on platforms that don't implement them. Hopefully Nigel's got the Windows scoop so we can see if Win32 has anything like this (which I'd expect it does)
--
Dan
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