We were delegating too much work. The original
algorithm description said that for each insert,
"1" unit of merge work has to be done
*at each level* … implying that if nothing needs
doing at a level, that "not done work" does not
add to work done elsewhere. This fix gets us back
to that situation (by always subtracting at least
2^TOP_LEVEL from the presented work amount), while
maintaining the (beneficial) effect of chunking
merge work at at anything but the last level.
Effectively, this reduces the maximum amount of
merge work done, also reducing our worst case
latency.
Now that we understand this, we can refactor the
algorithm to delegate "DoneWork", because then
each level can determine the total work, and see
if any work is left "for me". That's next.
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