Cap cpu-fast lane at attn_query_chunk 512 for the 8 GB server

Config-only tuning of the temporary CPU lane (no pipeline/code change). Query chunk 512 trims ViTMatte's attention transient ~0.9 GB at 2048x2048 (8.0 -> 7.1 GB peak working set) with bitwise-identical output; 256 gives nothing more. The remaining ~7.1 GB floor is attributed to ViTMatte's detail-capture decoder (backbone stage peaks 4.6 GB, decoder 7.1 GB, pymatting only 0.08 GB), so it is not further reducible by attention chunking -- noted for a possible future decoder-tiling pass. Accepted as-is for the 8 GB box.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

configs/cpu-fast.yaml

@@ -4,11 +4,22 @@
 # per 1.5 MP image on CPU. It needs ONLY these two weights provisioned --
 # BiRefNet_HR-matting (cross-check) and anime-seg are never loaded.
 #
+# Tuned for an 8 GB box (~7 GB for the service). Measured peak working set:
+#   2048x1152 (2.4 MP) ~5.6 GB | 2048x2048 (4.2 MP) ~7.1 GB.
+# The 2048x2048 peak is a floor set by ViTMatte's own dense compute, not the
+# attention (shrinking the chunk below 512 does not lower it further). Set
+# MemoryMax=8G with a hard 4 MP input cap (BGFILTER_MAX_IMAGE_PIXELS default),
+# or cap inputs tighter if you need more headroom.
+#
 # Anything omitted here inherits the built-in defaults (identical to default.yaml).
 
 model:
   device: cpu
   precision: fp32          # fp32 only: this lane targets non-bf16 CPUs
+  # Query chunk of 512 (vs the 2048 default) trims ViTMatte's attention transient
+  # ~0.9 GB at 2048x2048 (8.0 -> 7.1 GB) for ~1 s more compute; 256 gives nothing
+  # more. Exact/bitwise-identical output -- purely a memory-vs-speed knob.
+  attn_query_chunk: 512
 
 segmentation:
   enabled: true