docs: guidance for 512MB and 1GB boards

Documents the memory ceiling on small boards and, more usefully, what
running into it actually looks like: sshd accepting connections and
closing them before the banner, the web UI still responding normally,
clean ping, a dark panel, and a wrong clock after the next boot. None of
those read as "out of memory", which makes the failure hard to identify
from the symptoms.

Cross-referenced from SSH_UNAVAILABLE_AFTER_INSTALL.md, since "I can't
SSH in any more" is how most people will first meet this.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/LOW_MEMORY_BOARDS.md

@@ -0,0 +1,104 @@
+# Running on Low-Memory Boards
+
+Applies to the Pi Zero 2 W (512 MB), Pi 3 / 3B+ (1 GB), and the 1 GB Pi 4.
+If your board has 2 GB or more you can skip this document.
+
+## The failure this prevents
+
+The display process is the largest thing on the board. On a 1 GB Pi 3B+ with
+around 20 plugins enabled it settles near **600 MB of 905 MB usable**, leaving
+under 200 MB of headroom for everything else.
+
+When that headroom runs out, the board does not crash cleanly. `fork()` starts
+failing, and because a new process is needed to do almost anything, the
+symptoms look nothing like "out of memory":
+
+| What you see | Why |
+|---|---|
+| SSH accepts the connection then closes it instantly, before any banner | `sshd` forks a session per connection; the fork fails |
+| The web UI still responds quickly | Already running, serves from existing threads, forks nothing |
+| Ping is perfect, 0% loss | Handled entirely in the kernel |
+| The panel is dark | The display process was killed and cannot be respawned |
+| The clock is wrong after the next boot | `fake-hwclock`'s periodic save is a scheduled job, and it cannot fork either |
+
+The board looks healthy from the outside and cannot be logged into. Only a
+power cycle clears it. If you are here because SSH stopped working, also see
+[SSH_UNAVAILABLE_AFTER_INSTALL.md](SSH_UNAVAILABLE_AFTER_INSTALL.md), which
+covers the more common cause (AP mode).
+
+## Check your headroom
+
+```bash
+free -m
+ps -eo rss,comm --sort=-rss | head -5
+```
+
+If `MemAvailable` is under ~150 MB while the display is running, you are close
+to the edge. To watch it over time:
+
+```bash
+watch -n 30 'free -m | head -2'
+```
+
+Available memory that falls steadily rather than holding flat means you will
+reach the wall; it is a question of when.
+
+## What to do
+
+**1. Enable the memory cgroup controller.** Without it, the `MemoryMax=85%` in
+`systemd/ledmatrix.service` is accepted by systemd and silently ignored, so the
+service has no ceiling and a runaway takes the whole board down instead of just
+restarting. Raspberry Pi firmware disables this controller by default.
+
+`first_time_install.sh` does this for you. To check it took effect:
+
+```bash
+grep memory /sys/fs/cgroup/cgroup.controllers
+```
+
+If that prints nothing, add `cgroup_enable=memory cgroup_memory=1` to the
+single line in `/boot/firmware/cmdline.txt` and reboot.
+
+This changes the failure mode from "the board becomes unreachable" to "the
+display service restarts". It is a safety net, not a fix.
+
+**2. Run fewer plugins.** This is the actual remedy. Every enabled plugin costs
+memory permanently — its module, its parsed config, and its cached API
+responses. On a 512 MB or 1 GB board, keep the enabled set small and prefer
+plugins that poll infrequently.
+
+**3. Lower the cache ceiling.** The in-memory cache is sized from total RAM
+(150 entries at 1 GB and below, up to 1500 at 8 GB). To go lower still:
+
+```ini
+# /etc/systemd/system/ledmatrix.service.d/override.conf
+[Service]
+Environment=LEDMATRIX_CACHE_MAX_ENTRIES=75
+```
+
+Fewer entries means more API calls, so lower this only while you are actually
+short of memory.
+
+**4. Consider `MemoryHigh`.** `MemoryMax` kills and restarts. `MemoryHigh`
+throttles and reclaims instead, which is gentler — but on a board where the
+process genuinely wants more than the limit, sustained reclaim can stall the
+render loop and show as visible stutter on the panel. Add it only if you prefer
+degraded output to a restart:
+
+```ini
+[Service]
+MemoryHigh=70%
+```
+
+## Keep your logs
+
+These images default to volatile journald storage, so every reboot destroys the
+logs — including the ones explaining why the board rebooted. `first_time_install.sh`
+enables persistent storage capped at 64 MB. To confirm:
+
+```bash
+journalctl --list-boots
+```
+
+More than one boot listed means logs are surviving reboots. If only one is
+listed, journald is still writing to `/run` (tmpfs).

docs/README.md

@@ -14,6 +14,7 @@ the one-shot installer. The pages here go deeper.
 5. [TROUBLESHOOTING.md](TROUBLESHOOTING.md) — common issues and fixes
 6. [SSH_UNAVAILABLE_AFTER_INSTALL.md](SSH_UNAVAILABLE_AFTER_INSTALL.md) — recovering SSH after install
 7. [CONFIG_DEBUGGING.md](CONFIG_DEBUGGING.md) — diagnosing config problems
+8. [LOW_MEMORY_BOARDS.md](LOW_MEMORY_BOARDS.md) — Pi Zero 2 W / 3B+ / 1GB Pi 4 memory limits
 
 ## I want to write a plugin
 

docs/SSH_UNAVAILABLE_AFTER_INSTALL.md

@@ -20,7 +20,22 @@ The installation script:
 - Installs and configures `dnsmasq` (DHCP server for AP mode)
 - These services can interfere with normal WiFi client mode
 
-### 3. Reboot After Installation
+### 3. The Board Ran Out of Memory
+
+On a 512MB or 1GB board, memory exhaustion stops `sshd` being able to fork a
+session process. The connection is accepted and then closed immediately, before
+any banner:
+
+```
+kex_exchange_identification: Connection closed by remote host
+```
+
+The giveaway is that the board is otherwise healthy — ping is clean and the web
+UI still responds — but nothing that needs to start a new process works, and
+the panel is usually dark. Only a power cycle clears it. See
+[LOW_MEMORY_BOARDS.md](LOW_MEMORY_BOARDS.md).
+
+### 4. Reboot After Installation
 
 If the script reboots the Pi (which it recommends), network services may restart in a different state, potentially triggering AP mode.
 

first_time_install.sh

@@ -1688,6 +1688,43 @@ else
     echo "✗ $CMDLINE_FILE not found; skipping isolcpus optimization"
 fi
 
+# Enable the memory cgroup controller (idempotent).
+# The Pi firmware boots with cgroup_disable=memory, so systemd's MemoryMax= is
+# accepted and silently ignored — the display service then has no ceiling, and
+# a runaway takes the whole board down (sshd can no longer fork, the panel goes
+# dark) rather than just restarting the one service.
+if [ "$SKIP_PERF" != "1" ] && [ -f "$CMDLINE_FILE" ]; then
+    if grep -q 'cgroup_enable=memory' "$CMDLINE_FILE"; then
+        echo "cgroup_enable=memory already present in $CMDLINE_FILE"
+    else
+        echo "Adding cgroup_enable=memory to $CMDLINE_FILE..."
+        cp "$CMDLINE_FILE" "$CMDLINE_FILE.bak" 2>/dev/null || true
+        sed -i '1 s/$/ cgroup_enable=memory cgroup_memory=1/' "$CMDLINE_FILE"
+        echo "  Takes effect after reboot. Verify with:"
+        echo "    grep memory /sys/fs/cgroup/cgroup.controllers"
+    fi
+fi
+
+# Persist the journal (idempotent).
+# These images default to volatile storage: journald keeps everything in /run
+# (tmpfs), so every reboot destroys the logs — including the ones that would
+# explain why the board rebooted. Capped so an SD card is not worn out by logs.
+if [ -d /var/log/journal ] && [ -n "$(ls -A /var/log/journal 2>/dev/null)" ]; then
+    echo "Persistent journald storage already enabled"
+else
+    echo "Enabling persistent journald storage..."
+    mkdir -p /etc/systemd/journald.conf.d
+    cat > /etc/systemd/journald.conf.d/ledmatrix-persistent.conf <<'JOURNALD'
+# Installed by LEDMatrix first_time_install.sh
+[Journal]
+Storage=persistent
+SystemMaxUse=64M
+JOURNALD
+    mkdir -p /var/log/journal
+    systemd-tmpfiles --create --prefix /var/log/journal >/dev/null 2>&1 || true
+    systemctl restart systemd-journald >/dev/null 2>&1 || true
+fi
+
 # Ensure dtparam=audio=off in config.txt (idempotent)
 if [ "$SKIP_PERF" = "1" ]; then
     : # skipped

src/cache/memory_cache.py

@@ -4,11 +4,58 @@ Memory Cache
 Handles in-memory caching with TTL support, size limits, and automatic cleanup.
 """
 
+import os
 import time
 import threading
 import logging
 from typing import Dict, Any, Optional
 
+# Historical fixed ceiling, kept as the fallback when RAM cannot be read.
+DEFAULT_MAX_SIZE = 1000
+
+
+def _total_memory_mb() -> Optional[float]:
+    """Physical RAM in MB, or None where /proc/meminfo is unavailable."""
+    try:
+        with open('/proc/meminfo', 'r', encoding='utf-8') as fh:
+            for line in fh:
+                if line.startswith('MemTotal:'):
+                    return int(line.split()[1]) / 1024
+    except (OSError, ValueError, IndexError):
+        return None
+    return None
+
+
+def default_max_size() -> int:
+    """Entry ceiling scaled to this machine's RAM.
+
+    One fixed ceiling cannot serve both a 512 MB Pi Zero 2 W and an 8 GB Pi 5.
+    Entries here are parsed API payloads that routinely run tens of kilobytes
+    each, so a thousand of them is a comfortable cache on a large board and a
+    substantial fraction of total RAM on a small one — where the process
+    competing for that RAM is also driving the panel. Set
+    LEDMATRIX_CACHE_MAX_ENTRIES to override.
+    """
+    override = os.environ.get('LEDMATRIX_CACHE_MAX_ENTRIES')
+    if override:
+        try:
+            value = int(override)
+            if value > 0:
+                return value
+        except ValueError:
+            pass
+
+    total_mb = _total_memory_mb()
+    if total_mb is None:
+        return DEFAULT_MAX_SIZE
+    if total_mb < 1536:      # 512 MB and 1 GB boards
+        return 150
+    if total_mb < 3072:      # 2 GB
+        return 400
+    if total_mb < 6144:      # 4 GB
+        return 800
+    return 1500              # 8 GB and up
+
 
 class MemoryCache:
     """Manages in-memory cache with TTL and size limits."""

src/cache_manager.py

@@ -33,7 +33,7 @@ import logging
 import threading
 import tempfile
 from src.exceptions import CacheError
-from src.cache.memory_cache import MemoryCache
+from src.cache.memory_cache import MemoryCache, default_max_size
 from src.cache.disk_cache import DiskCache
 from src.cache.cache_strategy import CacheStrategy
 from src.cache.cache_metrics import CacheMetrics
@@ -84,7 +84,9 @@ class CacheManager:
             self.logger.warning("ConfigManager not available, using default cache intervals")
         
         # Initialize cache components using composition
-        self._memory_cache_component = MemoryCache(max_size=1000, cleanup_interval=300.0)
+        self._memory_cache_component = MemoryCache(
+            max_size=default_max_size(), cleanup_interval=300.0
+        )
         self._disk_cache_component = DiskCache(cache_dir=self.cache_dir, logger=self.logger)
         self._strategy_component = CacheStrategy(config_manager=self.config_manager, logger=self.logger)
         self._metrics_component = CacheMetrics(logger=self.logger)

src/plugin_system/plugin_loader.py

@@ -14,7 +14,7 @@ import sys
 import subprocess
 import threading
 from pathlib import Path
-from typing import Dict, Any, Optional, Tuple, Type
+from typing import Dict, Any, List, Optional, Tuple, Type
 import logging
 
 from packaging.requirements import InvalidRequirement, Requirement
@@ -45,6 +45,58 @@ def requirements_has_real_deps(requirements_file: str) -> bool:
     return False
 
 
+def _extra_dependencies(dist_name: str, extras) -> Optional[List[Requirement]]:
+    """Dependencies a distribution declares *only* behind the given extras.
+
+    Returns None when the installed metadata cannot be read or parsed, so the
+    caller can fall back to running pip rather than assuming anything.
+    """
+    try:
+        meta = importlib.metadata.metadata(dist_name)
+    except importlib.metadata.PackageNotFoundError:
+        return None
+
+    gated: List[Requirement] = []
+    for raw in meta.get_all('Requires-Dist') or []:
+        try:
+            dep = Requirement(raw)
+        except InvalidRequirement:
+            return None
+        if dep.marker is None:
+            continue
+        # Keep only what the distribution gates behind an extra we asked for:
+        # satisfied when `extra` is that name, but not when no extra is
+        # requested. A marker that holds either way (python_version, sys_platform)
+        # belongs to the base install and is already covered by the version check.
+        if dep.marker.evaluate({'extra': ''}):
+            continue
+        if any(dep.marker.evaluate({'extra': extra}) for extra in extras):
+            gated.append(dep)
+    return gated
+
+
+def _extras_are_satisfied(req: Requirement) -> bool:
+    """Check the dependencies pulled in by req's extras are installed.
+
+    One level deep, not transitive: enough to tell "the extra was installed"
+    from "the extra was never installed", which is all the caller needs to
+    decide whether pip has work to do. Anything unreadable returns False, so
+    the caller still falls through to pip.
+    """
+    gated = _extra_dependencies(req.name, req.extras)
+    if gated is None:
+        return False
+
+    for dep in gated:
+        try:
+            dep_version = importlib.metadata.version(dep.name)
+        except importlib.metadata.PackageNotFoundError:
+            return False
+        if dep.specifier and not dep.specifier.contains(dep_version, prereleases=True):
+            return False
+    return True
+
+
 def requirements_are_satisfied(requirements_file: str) -> bool:
     """
     Check whether every real requirement line in requirements.txt is already
@@ -76,9 +128,6 @@ def requirements_are_satisfied(requirements_file: str) -> bool:
         except InvalidRequirement:
             return False
 
-        if req.extras:
-            return False  # verifying extras' sub-dependencies isn't worth it here
-
         if req.marker is not None and not req.marker.evaluate():
             continue  # not applicable on this platform/interpreter
 
@@ -90,6 +139,9 @@ def requirements_are_satisfied(requirements_file: str) -> bool:
         if req.specifier and not req.specifier.contains(installed_version, prereleases=True):
             return False
 
+        if req.extras and not _extras_are_satisfied(req):
+            return False
+
     return True
 
 

systemd/ledmatrix.service

@@ -15,6 +15,19 @@ ExecStart=/usr/bin/python3 __PROJECT_ROOT_DIR__/run.py
 # that a successful outcome and never bringing it back.
 Restart=always
 RestartSec=10
+# Memory ceiling as a share of physical RAM, so one unit file suits a 512 MB
+# Pi Zero 2 W and an 8 GB Pi 5 alike. This is a backstop, not a tuning knob: it
+# turns "the board runs out of memory, stops being able to fork, and takes sshd
+# and the panel down together until someone pulls the plug" into "this one
+# service restarts".
+#
+# NOTE: Raspberry Pi firmware boots the kernel with cgroup_disable=memory, and
+# systemd accepts this setting and then silently ignores it. Verify with:
+#     grep memory /sys/fs/cgroup/cgroup.controllers
+# If that prints nothing, add "cgroup_enable=memory cgroup_memory=1" to
+# /boot/firmware/cmdline.txt (all on line 1) and reboot. first_time_install.sh
+# does this for you.
+MemoryMax=85%
 StandardOutput=journal
 StandardError=journal
 SyslogIdentifier=ledmatrix