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Using Linux KSM in uWSGI

Kernel Samepage Merging is a feature of Linux kernels >= 2.6.32 which allows processes to share pages of memory with the same content. This is accomplished by a kernel daemon that periodically performs scans, comparisons, and, if possible, merges of specific memory areas. Born as an enhancement for KVM it can be used for processes that use common data (such as uWSGI processes with language interpreters and standard libraries).

If you are lucky, using KSM may exponentially reduce the memory usage of your uWSGI instances. Especially in massive :doc:`Emperor<Emperor>` deployments: enabling KSM for each vassal may result in massive memory savings. KSM in uWSGI was the idea of Giacomo Bagnoli of Asidev s.r.l.. Many thanks to him.

Enabling the KSM daemon

To enable the KSM daemon (ksmd), simply set /sys/kernel/mm/ksm/run to 1, like so:

echo 1 > /sys/kernel/mm/ksm/run

Note

Remember to do this on machine startup, as the KSM daemon does not run by default.

Note

KSM is an opt-in feature that has to be explicitly requested by processes, so just enabling KSM will not be a savior for everything on your machine.

Enabling KSM support in uWSGI

If you have compiled uWSGI on a kernel with KSM support, you will be able to use the ksm option. This option will instruct uWSGI to register process memory mappings (via madvise syscall) after each request or master cycle. If no page mapping has changed from the last scan, no expensive syscalls are used.

Performance impact

Checking for process mappings requires parsing the /proc/self/maps file after each request. In some setups this may hurt performance. You can tune the frequency of the uWSGI page scanner by passing an argument to the ksm option.

# Scan for process mappings every 10 requests (or 10 master cycles)
./uwsgi -s :3031 -M -p 8 -w myapp --ksm=10

Check if KSM is working well

The /sys/kernel/mm/ksm/pages_shared and /sys/kernel/mm/ksm/pages_sharing files contain statistics regarding KSM's efficiency. The higher values, the less memory consumption for your uWSGI instances.

KSM statistics with collectd

A simple Bash script like this is useful for keeping an eye on KSM's efficiency:

#!/bin/bash

export LC_ALL=C

if [ -e /sys/kernel/mm/ksm/pages_sharing ]; then
    pages_sharing=`cat /sys/kernel/mm/ksm/pages_sharing`;
    page_size=`getconf PAGESIZE`;
    saved=$(echo "scale=0;$pages_sharing * $page_size"|bc);
    echo "PUTVAL <%= cn %>/ksm/gauge-saved interval=60 N:$saved"
fi

In your collectd configuration, add something like this:

LoadPlugin exec
<Plugin exec>
   Exec "nobody" "/usr/local/bin/ksm_stats.sh"
</Plugin>