Build · 22 May 2026
Mean fanout measured at 2.60
One to three reads per lookup, which is what makes a memory-mapped artefact viable.
Specifics
The numbers, and where they come from
| Quantity | Value | Basis |
|---|---|---|
| Mean fanout | 2.60 | Measured |
| Reads per lookup | 1 to 3 | Measured |
| Corpus | 40,000 held-out sentences | Counted |
| Worst case observed | 5 reads | Measured |
The measurement
Why fanout is the number that matters
Fanout is how many index nodes a lookup touches before it resolves. On a memory-mapped artefact each of those is potentially a page fault, so the mean determines whether Folio is viable on a machine with a disk rather than only on one with the whole artefact in RAM.
Measured at 2.60 across the held-out set. Ninety-four per cent of lookups resolve in three reads or fewer.
What it does not tell you
A mean is not a guarantee
The mean is comfortable and the tail is what will hurt you. Six per cent of lookups take four reads or more, and on a cold page cache those are the ones a latency budget notices.
This is why the response figure is quoted with the artefact already mapped, and why that qualifier appears every time the figure does.