test_reload's registry-under-a-writer case failed about one run in five on a
loaded machine and never on an idle one. The refusals were always unread=1:
one slot, on the best of eight walks, that would not copy. In that mode the
table cannot even move — re-noting live blocks kills nothing, the dead count
stays at zero, and the compaction trigger never fires — so the reader was
refusing a table that sat perfectly still for it.
flan_reg_snap re-read a slot's counter sixty-four times with nothing between
the tries. That is right for the writer it was written for, which holds the
counter odd for seven stores. It is hopeless against a writer the scheduler
took the core from mid-write, which holds it odd for a quantum: the reader
burns all sixty-four looks inside a fraction of that, and the looking is what
keeps the core the writer needs. Hence load-only.
The walk-level retry already had the answer written up at length — a spin takes
a core from the thread being waited on. The per-slot retry never got it. One
pause helper now, shared by both: eight bare looks for the running writer, then
the same 250us step. A refusal means the table would not hold still.
480 contended runs of regfull after, no refusals; 46 of 240 before.
Joseph Ferano
8f05ece441