Cap glibc malloc arenas to keep non-Go RSS bounded (closes #23)

After the earlier memory work, SQLite's live heap is bounded but process
RSS still climbed about 70 MiB/min, nearly all anonymous and outside the
Go runtime and outside SQLite's own accounting, without any SQLITE_NOMEM.
The cause is glibc: the SQLite C library allocates and frees millions of
small page-cache chunks from many threads, and glibc keeps each arena's
freed chunks resident. With arenas uncapped it creates up to eight per
core, so on a large host RSS grows with the core count.

Set MALLOC_ARENA_MAX=2 in the image to bound the retained memory. Writes
are already serialized, so the two-arena cap costs no throughput. Update
the README Memory section.

Model: opus-4-8

Dockerfile

@@ -83,6 +83,14 @@ ENV XDG_DATA_HOME=/var/lib
 # XDG_DATA_HOME above.
 ENV GOMEMLIMIT=1536MiB
 
+# Cap glibc's malloc arenas. The SQLite C library allocates and frees millions
+# of small page-cache chunks from many threads; glibc otherwise creates up to
+# eight arenas per core (hundreds on a large host) and keeps each arena's freed
+# chunks resident, so process RSS climbs far above SQLite's live heap and never
+# comes back down. Two arenas keep that retained memory bounded; database writes
+# are already serialized, so the lost allocator concurrency costs nothing here.
+ENV MALLOC_ARENA_MAX=2
+
 # Expose HTTP port
 EXPOSE 8080
 

README.md

@@ -169,6 +169,7 @@ Configuration is handled via environment variables and OS-specific paths:
 | `PORT` | `8080` | HTTP server port |
 | `DEBUG` | (empty) | Set to `routewatch` for debug logging |
 | `GOMEMLIMIT` | `1536MiB` (in the Docker image) | Go soft memory limit; see Memory |
+| `MALLOC_ARENA_MAX` | `2` (in the Docker image) | glibc malloc arena cap; see Memory |
 
 State directory (database location):
 - macOS: `~/Library/Application Support/routewatch/`
@@ -183,6 +184,13 @@ Budget:
 - Go heap: a 1.5 GiB soft limit (`GOMEMLIMIT=1536MiB`, set in the image).
 - SQLite: at most 640 MiB of page cache across the connection pool (64 MiB per
   connection, 10 connections) and a 1.5 GiB hard heap limit for the C library.
+- glibc allocator: the SQLite C library runs on glibc `malloc`, which frees
+  page-cache chunks back to per-arena free lists rather than to the kernel, so
+  process RSS tracks the high-water mark of those arenas, not SQLite's live
+  heap. glibc creates up to eight arenas per core, so on a many-core host the
+  retained memory — and thus RSS — grows with the core count. The image sets
+  `MALLOC_ARENA_MAX=2` to bound it; the two-arena cap costs nothing here because
+  database writes are already serialized.
 - About 0.2 GiB for everything else in the runtime.
 
 Run the container with a memory limit of 5 GiB and swap disabled:
@@ -195,7 +203,9 @@ or the equivalent in your deployment tool. This leaves headroom above the
 ceilings for spikes and the kernel page cache.
 
 Override the Go soft limit by setting `GOMEMLIMIT` in the environment (for
-example `-e GOMEMLIMIT=1GiB`); this replaces the image default.
+example `-e GOMEMLIMIT=1GiB`); this replaces the image default. `MALLOC_ARENA_MAX`
+can be overridden the same way, but raising it lets RSS climb again on a
+many-core host.
 
 What happens at each limit:
 - Go soft limit: as the heap approaches `GOMEMLIMIT`, the runtime runs garbage