> Which shouldn't be such a problem if you're already writing an OS?
I guess that could've used more context. I was trying to point out that in the larger landscape of GC algorithms, the one used by .NET is not that advanced, even though it's better than the GC used by common scripting languages.
> Also AIUI the only "hard part" of C4 is making sure objects don't get modified while they're being moved (that's what the barrier's there to detect) - which isn't a problem if everything's immutable.
Would absolutely everything be immutable? How would you implement lazy evaluation with the correct time complexity?
The expensive part of any garbage collector with short pause times is bound to be the read barrier, which is still needed with only immutable data. If the GC is compacting and wants to relocate an object, it has to rewrite all references to point to the new copy of the object.
C4 uses a read barrier to enforce the invariant that all references point to the up-to-date copy on a load of a reference, not just on a use of a reference. It relies on its ability to immediately update all references to reuse compacted pages during compaction, which means that gratuitous amounts of free memory are not required (a common problem with moving collectors).
You might be able to come up with alternate collector design that uses some of the same ideas as C4 while exploiting immutability, but if you loosen the healing read barrier you weaken your ability to reuse memory and put a bound on memory growth, since a reference to an old copy might always escape to the next round of collection.
I guess that could've used more context. I was trying to point out that in the larger landscape of GC algorithms, the one used by .NET is not that advanced, even though it's better than the GC used by common scripting languages.
> Also AIUI the only "hard part" of C4 is making sure objects don't get modified while they're being moved (that's what the barrier's there to detect) - which isn't a problem if everything's immutable.
Would absolutely everything be immutable? How would you implement lazy evaluation with the correct time complexity?
The expensive part of any garbage collector with short pause times is bound to be the read barrier, which is still needed with only immutable data. If the GC is compacting and wants to relocate an object, it has to rewrite all references to point to the new copy of the object.
C4 uses a read barrier to enforce the invariant that all references point to the up-to-date copy on a load of a reference, not just on a use of a reference. It relies on its ability to immediately update all references to reuse compacted pages during compaction, which means that gratuitous amounts of free memory are not required (a common problem with moving collectors).
You might be able to come up with alternate collector design that uses some of the same ideas as C4 while exploiting immutability, but if you loosen the healing read barrier you weaken your ability to reuse memory and put a bound on memory growth, since a reference to an old copy might always escape to the next round of collection.