At 6:37 PM -0500 1/19/04, 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. Or, let me put it this way: non- copying *GC* (the sweep or copy phase) can be threadsafe, but the mark phase is never threadsafe. The method in which marking is not threadsafe is a bit more pathological (i.e., it's not the common case as it is with the copying collector), but a standard tracing DOD cannot be correct when competeting with mutators. It WILL collect non- garbage (those are MUTATORS, remember), and the result WILL be Heizenbugs and crashes.
Some of what I've written up addresses why. It's pretty simple to demonstrate a single case to prove the point, but I don't feel like re-creating the ASCII art right now. :) I'll send that section when I get out of the office.
parrot will have to be able to suspend all threads in the environment. Unfortunate, but really quite unavoidable.
I'm not sure that's the case. What we need to do is suspend metadata mutation--that is, buffers can't be resized while a gc run is in progress. Other than that, if we have guarantees of aligned atomic pointer writes (so we don't have word shearing to deal with) we don't have to stop the mutation of the contents of the buffers themselves.
The only tricky bit comes in with the examination of the root set of other threads--accessing the hardware register contents of another running thread may be... interesting. (Which, I suppose, argues for some sort of volatile marking of the temp variables)
--
Dan
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