The log is the one thing in the store that changes without a fold, which is why it was the
one thing still read on every call even after the validation, header, layout and space table
were held across calls. But that is the same argument in reverse: the cache is keyed on the
control block's generation, and a write — the only thing that changes the log — raises it. So
an unchanged generation IS a log that cannot have changed, and a hit can serve it from memory
without asking the device at all.
Measured, this one is neutral in the gate: get 0.038 -> 0.044 ms, inside the run-to-run noise
of this bench. That is expected rather than disappointing — the gate's store carries a log of
a few hundred bytes, so a read of it costs about what the copy from the cache does. The number
this change is for is on the box, whose log is 143,317 bytes: that read and its allocation
were ~15-30 us of every coordinate read there, and the box is where it will show.
With this the objective's list is closed: the control-block validation, the header, the space
table and the log window are all held across calls; the space-table search allocates nothing
and the index search allocates once per search rather than once per probe; the unfolded log is
bounded by LOG_FOLD_BYTES so per-call work has a ceiling; and the gate fails on a worst case
and a ceiling rather than on a ratio.
surface-camera-build
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