compose: increase zram at runtime

To alleviate memory pressure, increase zram to 50% of total memory at
runtime. This allows the host to compress inactive pages to reduce
contention.

This service optionally allows for the maximum zram percentage and
compression algorithm to be modified at runtime using variables.

Change-type: patch
Signed-off-by: Joseph Kogut <joseph@balena.io>

Author: jakogut

docker-compose.yml

@@ -278,6 +278,48 @@ services:
       CLOUDFLARE_DNS_ZONE: product-os.io
       ROBOT_API: https://robot-ws.your-server.de
 
+  zram:
+    image: docker:27.3.1-cli-alpine3.20
+    restart: no
+    labels:
+      io.balena.features.balena-socket: '1'
+    entrypoint:
+      - /bin/sh
+      - -c
+    environment:
+      ENABLED: true
+      zram_pct: 50
+      zram_algo: lz4
+    command:
+      - |
+        set -e
+        [[ $ENABLED == 'true' ]] || exit
+
+        HOST_CONTAINER_NAME=host-chroot
+        stop_container() { docker stop "$(docker ps -qf "NAME=$HOST_CONTAINER_NAME")"; }
+        trap stop_container EXIT
+
+        host_cmd() {
+          if [ -z "$(docker ps -qf "NAME=$HOST_CONTAINER_NAME")" ]; then
+            docker run \
+              --interactive \
+              --tty=true \
+              --detach \
+              --rm \
+              --name="$HOST_CONTAINER_NAME" \
+              --volume=/:/host \
+              --privileged \
+              alpine:3.20.3
+          fi
+
+          HOST_CONTAINER_ID="$(docker ps -qf "NAME=$HOST_CONTAINER_NAME")"
+          docker exec "$HOST_CONTAINER_ID" chroot /host bash -c "$*"
+        }
+
+        host_cmd "swaps=\$(awk 'NR>1 { print \$1 }' /proc/swaps); if [ -n \"\$swaps\" ]; then swapoff \$swaps; fi"
+        host_cmd "zrams=\$(find /dev -name \"zram*\"); if [ -n \"\$zrams\" ]; then for d in \$zrams; do zramctl -r \$d; done; fi"
+        zram_dev=$(host_cmd "memtotal=\$(grep MemTotal /proc/meminfo | awk '{ print \$2 }'); zramctl --find --size \$((memtotal * $zram_pct / 100))K --algorithm $zram_algo")
+        host_cmd "mkswap $zram_dev && swapon $zram_dev"
   # https://github.com/balenablocks/cert-manager
   # https://certbot.eff.org/docs/using.html
   # https://certbot-dns-cloudflare.readthedocs.io/