If you’re running low of memory on a server, there may be scope to free up memory through use of ‘ksm‘.
(Note, that the kernel will start looking for memory to de-duplicate if KSM is turned on AND there are some processes running which have notified the kernel that they’re candidates; ‘kvm’ does the latter.)
Assuming a recent enough kernel (that’s anything since about v2.6.32), turning it on is a simple :
echo 1 > /sys/kernel/mm/ksm/run
to enable, and
echo 0 > /sys/kernel/mm/ksm/run
to disable.
There are a few tunable settings – sleep_millisecs and pages_to_scan – which you may wish to adjust if you feel KSM is gobbling up too much CPU time.We’ve arbitrarily set them to 5000 and 100 respectively. Increasing sleep_millisecs and decreasing pages_to_scan will make the algorithm less aggressive and reduce CPU usage.
Once running, memory should drop – in our case we’re saving about 1gb of memory on a host with 16gb of memory and 6 kvm instances — pages_sharing lists the number of 4kb pages shared between processes as a result of ksm —
/sys/kernel/mm/ksm/pages_sharing 285603
(about 1115mb).