The backlog threshold of a level backs off by doublings as backlog passes
find little to free, but only passes that had candidates count. A program
whose mature passes have no candidates while its heap grows keeps the base
threshold, a quarter of the live heap, while those same passes widen the
level's growth fraction. When it then reads the structure it built, the
mature buffer fills with live objects and the first backlog pass walks a
quarter of the live heap to find nothing. In tsc-ts's program build on
Sentry, that pass ran inside the loader walk and took 9 to 18 ms.
Until a pass at a level has had candidates, the backlog threshold now uses
the level's growth fraction doublings. The first pass with candidates
judges the level, and the threshold follows its own yield from then on.
Chris Tate
7e0106722b