Keep track of per-cycle mutator/collector allocs. Fix mutator/collector alloc region overlap in traversal.

[Aleksey Shipilev]

On 03/09/2018 03:29 PM, Roman Kennke wrote:
> Turns out I introduced a bug when extracting it from my traversal-degen
> work. I made it such that the ShenandoahConcurrentThread (can we maybe
> rename it to ShenandoahControlThread or something please?!) resets the
> counters. However, this runs concurrently with Java allocs pre-cycle, so
> it will be racy. This little change moves the resetting to the op_init_
> phases, so that it happens during STW instead.
> 
> Differential:
> http://cr.openjdk.java.net/~rkennke/traversal-alloc-region/webrev.01.diff/
> Full patch:
> http://cr.openjdk.java.net/~rkennke/traversal-alloc-region/webrev.01/

I think this is a simple solution, but its simplicity is deceptive.

See, this solution:
 1. Adds pause time, because we walk regions during the pause to reset counters now;
 2. Turns the statistics counters into functionality-critical bits of data, and e.g. their racy
updates would cause functional bugs;
 3. Makes weird tests on allocation fast-path;
 4. Undermines current free set performance mechanics. Current code relies on density of bitmaps
performance-wise: if we cannot allocate in the region, we move the bitmap boundaries right away.
Current patch requires scanning the freeset for fitting regions when GC and mutator allocs meet each
other, because there can now be full regions of different type within the boundaries.

The better solution, in my mind, would be biasing the freeset regions to the particular type of
allocation. That is, freeset can maintain two bitmaps and two sets of boundaries instead of one, and
use those for mutator/GC allocations. Once one bitmapped-type is depleted, the allocation path may
decide to "borrow" from another-type bitmap, if the borrowed region is empty, or even borrow
non-empty region if runtime option allows that (and that option would be "false" for traversal). It
solves all four problems with the patch above.

Thanks,
-Aleksey