Parallel GC does not scan large allocation area (-A)
effectively as it does not do work stealing from nursery
That leads to large imbalance when only one of threads
overflows allocation area: most of GC threads finish
quickly (as there is not much to collect) and sit idle
waiting while single GC thread finishes scan of single
allocation area for that thread.
The patch enables work stealing for (equivalent of -qb0)
allocation area of -A32M or higher.
Tested on a highlighting-kate package from Trac Trac #9221
On 8-core machine the difference is around 5% faster
of wall-clock time. On 24-core VM the speedup is 20%.
Signed-off-by: Sergei Trofimovich <email@example.com>