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From "Xiao-Feng Li" <xiaofeng...@gmail.com>
Subject Re: [drlvm] Class unloading support - tested one approach
Date Thu, 02 Nov 2006 07:39:03 GMT
Robin, good idea.

I understand the main difference between your design and Aleksey's
proposal 1 is, the tracing in your design stops as vtable, but
Aleksey's continues to classloader. On the other hand, your approach
requires an extra step to sweep the vtables in order to determine the
classloaders' reachability.

If this is true, why not just let the tracing to continue as a
complete step to determine the classloaders' reachability?

Another difference is to mark the reachability with an unconditional
write instead of a bit mask write. I think this can be applied to
either approach.


On 11/1/06, Robin Garner <robin.garner@anu.edu.au> wrote:
> 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

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