Telemetry puts nexrad_decode_png at 1.76 s/call × 37,847 calls/h
— ~18.5 h of CPU across the cluster every real hour, the single
biggest CPU consumer in the system. The map-click path
(fetch_rain_cells) already cached decoded frames by 5-min
bucket, but the per-contact CommonVolumeRadarWorker path
(fetch_decoded_frame) went straight to network + decode on every
call. Backfill means many contacts share a 5-min window, so the
same 66 MB frame was being decoded dozens of times.
Wire fetch_decoded_frame through NexradCache keyed on the
rounded timestamp. Add a 20-entry size cap in NexradCache so
backfill processing contacts in random timestamp order can't
grow the ETS table to hundreds of GB. Each frame is ~66 MB, so
20 = ~1.3 GB worst case, well under typical pod memory.
Expected impact: cuts sustained decode load by an order of
magnitude depending on backfill temporal locality; map-click
path is unchanged.
Graham McIntire
bbf6481603