zram swap script

zram-swap.sh

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+#!/bin/bash
+
+set -e
+trap 'echo "[ERROR] Error in line $LINENO when executing: $BASH_COMMAND"' ERR
+
+NAME=zram0
+DEV=/dev/$NAME
+
+if { mount; cat /proc/swaps; } | grep -qs "$DEV"; then
+    echo "zram-swap.sh: $DEV is already mounted or used as swap, no actions performed."
+    exit 0
+fi
+
+modprobe zram
+
+# reset the device to ensure we can set the parameters
+echo 1 > "/sys/block/$NAME/reset"
+
+# https://github.com/lz4/lz4?tab=readme-ov-file#benchmarks
+# Core i7-9700K single thread
+# Compressor            Ratio   Compression Decompression
+# LZ4 default (v1.9.0)  2.101   780 MB/s    4970 MB/s
+# Zstandard 1.4.0 -1    2.883   515 MB/s    1380 MB/s
+
+echo zstd > "/sys/block/$NAME/comp_algorithm"
+# lz4 is quicker but less compression
+#echo lz4 > "/sys/block/$NAME/comp_algorithm"
+
+# use 1/4 of memory
+use=$(( $(grep -e MemTotal /proc/meminfo | tr -s ' ' | cut -d' ' -f2) / 4 ))
+
+# use at least 256M for systems with small memory
+min=$(( 256 * 1024 ))
+
+if (( use < min )); then
+    size=$min
+else
+    size=$use
+fi
+
+# zram maximum memory usage
+echo $(( size ))K > "/sys/block/$NAME/mem_limit"
+
+# disk size
+# let's make this 3x the memory limit in case we get good compression
+echo $(( 4 * size ))K > "/sys/block/$NAME/disksize"
+
+mkswap "$DEV"
+swapon -p 100 "$DEV"
+
+# turn off file based swap
+TURNOFF=$(grep </proc/swaps -F -v -e zram -e Filename | cut -d' ' -f1)
+if [[ -n $TURNOFF ]]; then
+    swapoff $TURNOFF || true
+fi
+
+if ! grep </proc/swaps -qs -F -v -e zram -e Filename; then
+
+    # for zram swap, most guides reommend the following:
+    # swappiness 200, watermark_scale_factor 125, watermark_boost_factor 0
+    #
+    # swappiness
+    # if zram is the only swap, increase swappiness (default swappiness 60)
+    # zram guides propose 200 but that seems excessive
+    echo 100 > /proc/sys/vm/swappiness
+
+    # watermark_scale_factor
+    # This factor controls the aggressiveness of kswapd. It defines the amount
+    # of memory left in a node/system before kswapd is woken up and how much
+    # memory needs to be free before kswapd goes back to sleep.
+    # 60: 0.6 percent free memory (default 10 / 0.1%)
+    echo 60 > /proc/sys/vm/watermark_scale_factor
+
+    # watermark_boost_factor
+    # this has to do with reclaiming on fragmentation, swap on zram guides seem to disable this but don't give a reason
+    echo 0 > /proc/sys/vm/watermark_boost_factor
+
+    # disable readahead for reading from swap (default is 3 which means 2^3 = 8 pages)
+    echo 0 > /proc/sys/vm/page-cluster
+fi