Every coordinate read binary-searches the space table for its space's first record, and
every batch of a listing reads a row of it — from the device, per probe, for data that
changes only when the store is folded. It is now held in the driver and keyed on the
**control block's generation**, which a fold raises along with everything else it writes,
plus the image offset, since a fold flips the slot. Nothing has to invalidate it by hand: a
writer that bumps the generation invalidates it, including a writer this driver never sees.
A stale table cannot outlive the image it describes.
Honest about what it bought: nothing measurable in the gate, which is the interesting part.
The gate's store carries about a dozen spaces, so the search it replaces was three or four
probes, and copying a few hundred bytes of table costs about what those probes did — get
0.078 -> 0.077 ms, spaces flat within the run-to-run noise of this bench (which is ~20%).
What changes is the SHAPE: the lookup no longer scales with the number of spaces, and the
box has 21 and grows. That is worth having and it is not the same thing as a measured win,
so it is not claimed as one.
What is still read per call, and is the larger constant: the control block (needed — it is
the validation), the header, the log window, and the allocations for all three. Those are
what the borrow-based version of this cache is for, and they are the next piece.
surface-camera-build
1f7b9fe0de