Actually, just thinking about how I would implement this in JikesRVM, I
would use the reachability based algorithm, but piggyback on the
existing GC mechanisms:
- Allocate a byte (or word) in each vtable for the purpose of tracking
class reachability.
- Periodically, at a time when no GC is running (even the most
aggressive concurrent GC algorithms have these, I believe), zero this
flag across all vtables. This is the beginning of a class-unloading epoch.
- During each GC, when the GC is fetching the GC map for an object,
*unconditionally* write a value to the class reachability byte. It may
make sense for this byte to be in either the first cache-line of the
vtable, or the cache line that points to the GC map - just make sure the
mark operation doesn't in general fetch an additional cache line.
- At a point in the sufficiently far future, when all reachable objects
are known to have been traced by the GC, sweep the vtables and check the
reachability of the classloaders.
The features of this approach are:
- Minimal additional work at GC time. The additional write will cause
some additional memory traffic, but a) it's to memory that is already
guaranteed to be in L1 cache, and b) it's an unconditional independent
write, and c) multiple writes to common classes will be absorbed by the
write buffer.
- Space cost of at most 1 word per vtable.
- This works whether vtables are objects or VM structures
- If the relationship between a class and a vtable is not 1:1, this only
need affect the periodic sweep process, which should be infrequent and
small compared to a GC.
- shouldn't need a stop-the-world at any point.
I've implemented and tested the GC-relevant part of this in JikesRVM,
and the GC time overhead appears to be just under 1% in the MMTk
MarkSweep collector.
cheers,
Robin
