Return freed activations to the OS on Linux; recommend jemalloc (A+B)

MIMALLOC_PURGE_DELAY=0 only bites on Windows (bundled mimalloc). On a Linux
server PyTorch uses glibc ptmalloc, which keeps a BiRefNet@2048 forward's freed
activations in the arena, so RSS ratchets up across requests and the env var is
a no-op there.

A. bgfilter/memtune.py: release_freed_memory() calls glibc malloc_trim(0) at
   runtime (no-op on Windows/musl/other allocators). Wired into service.process
   (after each request) and the CLI batch loop (after each image), the two
   long-lived paths where RSS accumulates. Single-image CLI exits, so it is left
   alone.
B. DEPLOY(_ZH): document preloading jemalloc via LD_PRELOAD + MALLOC_CONF (also
   improves CPU throughput) as the production alternative, with MALLOC_ARENA_MAX/
   MALLOC_TRIM_THRESHOLD_ as an allocator-free fallback. Under jemalloc/tcmalloc
   malloc_trim simply no-ops.

Also fixes the stale off-by-default cross-check heading in DEPLOY.md.

Verified: modules import; release_freed_memory() returns False (no-op) on Windows.

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

bgfilter/memtune.py

@@ -0,0 +1,44 @@
+from __future__ import annotations
+
+"""Cross-platform release of freed allocator memory after a heavy inference.
+
+Both of the process allocators this pipeline runs under retain large freed
+blocks instead of returning them to the OS, so a BiRefNet@2048 forward's
+activations keep inflating RSS long after the tensors are gone:
+
+- Windows PyTorch bundles mimalloc; its lazy purge is defused at import time by
+  ``MIMALLOC_PURGE_DELAY=0`` (set in ``bgfilter/__init__.py`` before torch loads).
+- Linux/glibc PyTorch uses ptmalloc, which keeps freed arenas committed;
+  ``malloc_trim(0)`` hands those pages back and is called at runtime via
+  :func:`release_freed_memory` after each image.
+
+Neither hook exists on the other's platform, so this stays a no-op there (and on
+musl, which has no ``malloc_trim``, or under a preloaded jemalloc/tcmalloc that
+manages its own release). See DEPLOY for the recommended jemalloc LD_PRELOAD.
+"""
+
+import ctypes
+import ctypes.util
+import sys
+
+_malloc_trim = None
+if sys.platform.startswith("linux"):
+    try:
+        _libc = ctypes.CDLL(ctypes.util.find_library("c") or "libc.so.6", use_errno=True)
+        _malloc_trim = _libc.malloc_trim  # glibc only; AttributeError on musl
+        _malloc_trim.argtypes = [ctypes.c_size_t]
+        _malloc_trim.restype = ctypes.c_int
+    except (OSError, AttributeError):
+        _malloc_trim = None
+
+
+def release_freed_memory() -> bool:
+    """Return freed heap pages to the OS; True if a release was actually issued.
+
+    Cheap (sub-millisecond) and safe to call after every processed image. No-op
+    where no supported hook is available (Windows, musl, non-glibc allocators).
+    """
+    if _malloc_trim is None:
+        return False
+    _malloc_trim(0)
+    return True