It shipped at 4, which made it the binding limit for every install. The limiter
is meant to be each engine's own `*_instances` -- the pool hands out one instance
per page and an engine is not reentrant -- while a RunPod fleet can hold
max_pods x scale_up_inflight_per_pod pages in flight: 64 x 16 = 1024 on the
Digesta install. A ceiling of 4 capped that at a quarter of ONE pod.

Measured on one A40, same page, same install:

    N     surya req/s  p50     chandra pg/s  p50
    1     0.93         1.1s    0.21          4.6s
    8     3.79         2.0s    1.31          6.1s
    16    7.11         2.2s    1.88          8.4s      <- both peak
    24    5.89         4.1s    2.09         10.6s
    32    5.44 +13 err 3.5s    1.44 +21 err  7.5s
    48    ALL FAILED

Both peak at 16 and both fail outright by 32 -- a cliff, not a degradation.

Default raised 4 -> 64: four pods' worth at the measured knee, far above what a
single-box install can feed, so it stops being the limiter while staying finite.
NOT higher, because every admitted page holds a rasterised bitmap (~18 MB for A4
at 250 DPI): 64 in flight is ~1.2 GB, 1024 would be ~18 GB of page images before
any engine does work. Trading a throughput limit for an OOM is not an upgrade.

docs/ocr.md carries the table and spells out the three settings that bound this,
which are easy to confuse: the per-model scale-out threshold, the per-engine
instance count (the real limiter, and for the SUBPROCESS engines it is what the
box can host -- a surya worker is a separate interpreter at ~750 MB), and this
ceiling.

Also recorded there: surya's req/s is BLOCKS, not pages, since it calls once per
layout block; and its worker protocol is synchronous, so a worker handles one
page at a time and concurrency is the process count. In served mode that worker
only scales an image and makes an HTTP call, so making it concurrent -- or
skipping the subprocess entirely, as chandra does -- is what would let surya use
a large fleet.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PxYHWbCAjjjD7A1rhFKtaW