CRR (M): 7127706: G1: re-enable survivors during the initial-mark pause

[Tony Printezis]

Hi all,

Can I have a couple of code reviews for this change that re-enables the 
use of survivor regions during the initial-mark pause?

http://cr.openjdk.java.net/~tonyp/7127706/webrev.0/

 From the CR:

We could scan the survivors as we're copying them, however this will 
require more work during the initial-mark GCs (and in particular: 
special-case code in the fast path).

A better approach is to let the concurrent marking threads scan the 
survivors and mark everything reachable from them a) before any more 
concurrent marking work is done (so that we can just mark the objects, 
without needing to push them on a stack, and let the "finger" algorithm 
discover them) and b) before the next GC starts (since, if we copy them, 
we won't know which of the new survivors are the ones we need to scan).

This approach has the advantage that it does not require any extra work 
during the initial-mark GCs and all the work is done by the concurrent 
marking threads. However, it has the disadvantage that the survivor 
scanning might hold up the next GC. In most cases this should not be an 
issue as GCs take place at a reasonably low rate. If it does become a 
problem we could consider the following:

- like when the GC locker is active, try to extend the eden to give a 
bit more time to the marking threads to finish scanning the survivors
- instead of waiting for the marking threads, a GC can take over and 
finish up scanning the remaining survivors (typically, we have more GC 
threads than marking threads, so the overhead will be reduced)
- if we supported region pinning, we could pin all the regions that were 
not scanned by the time the GC started so that the marking threads can 
resume scanning them after the GC completes

Implementation notes:

I introduced the concept of a "snapshot regions" in the ConcurrentMark 
which is a set of regions that need to be scanned at the start of a 
concurrent cycle. Currently, these can only be survivors but maybe we 
can use the same concept for something else in the future.

Tony