Adjust host zram percentage of total memory

To alleviate memory pressure, increase zram to 50% or a given
value 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: minor
Signed-off-by: Kyle Harding <kyle@balena.io>

Author: klutchell

docker-compose.yml

@@ -209,6 +209,61 @@ services:
     labels:
       io.balena.features.dbus: '1'
 
+  zram:
+    image: alpine:3.22
+    restart: no
+    privileged: true
+    labels:
+      io.balena.features.procfs: '1'
+    entrypoint:
+      - /bin/sh
+      - -c
+    environment:
+      ENABLED: true
+      ZRAM_PCT: 50
+      ZRAM_ALGO: lz4
+    command:
+      - |
+        set -ex
+
+        case ${ENABLED} in
+          true|1|y|yes|on)
+            ;;
+          *)
+            exit 0
+            ;;
+        esac
+
+        apk add --no-cache util-linux-misc
+
+        echo "Disabling existing swap devices..."
+        swaps=$(awk 'NR>1 { print $1 }' /proc/swaps)
+
+        if [ -n "${swaps}" ]; then
+          for swap in ${swaps}; do
+            # Swap devices reported by /proc in the container are missing the /dev/ prefix
+            echo "Disabling swap device /dev${swap}..."
+            swapoff /dev${swap}
+          done
+        fi
+
+        echo "Removing existing zram devices..."
+        zrams=$(ls /dev/zram* 2>/dev/null || true)
+        if [ -n "${zrams}" ]; then
+          for d in ${zrams}; do
+            echo "Removing zram device ${d}..."
+            zramctl -r ${d}
+          done
+        fi
+
+        echo "Creating a new zram device with ${ZRAM_PCT}% of total memory..."
+        memtotal=$(grep MemTotal /proc/meminfo | awk '{ print $2 }')
+        zram_size=$((memtotal * ${ZRAM_PCT} / 100))
+        zram_dev=$(zramctl --find --size ${zram_size}K --algorithm ${ZRAM_ALGO})
+
+        echo "Creating a swap device on the zram block device and enabling it..."
+        mkswap ${zram_dev} && swapon ${zram_dev}
+
   # create DNS record
   upsert-dns:
     image: bash:alpine3.14