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kvm ksm tuning

Context

I am running a no-brand machine, 

  • CentOS 6.3
  • 2 cores
  • 8GB RAM
  • 1 SATA disk

On this I fired 6 virtual machines

  • 2 Centos 6, 1GB RAM each
  • 4 Windows XP, 2GB RAM each

The reader probably noticed I overbooked RAM: If all the VM will use the available RAM it will fill up to 10GB.

I wanted to test the KSM effect on this. The default KSM settings on this OS is:

 KSM_MONITOR_INTERVAL=60
KSM_SLEEP_MSEC=100
KSM_NPAGES_MAX=1500

The result I got is:

  • Memory usage fits in RAM (very low swap usage
  • Too high CPU usage for scanning "same pages" (constantly using the equivalent of 70% of 1 core)
  • Disk IO is reasonnable, but on the other hand load average is high (10)
  • The Windows XP VM are desperately slow (not a surprise)

Tuning

Before tuning, I collected what ressource I could use more

  • This machine can bear more disk IO: I can use swap and RAM a bit more
  • This weak CPU should be a bit more used by VMs instead of KSM

My fisrt idea is to make KSM check less often. This is my new KSM setting:

KSM_MONITOR_INTERVAL=600
KSM_SLEEP_MSEC=1000
KSM_NPAGES_MAX=5000

The result is:

  • Less load average (now 6)
  • Less CPU used (now about 40% of 1 core)
  • More swap used, but IO statistics are still OK (IO Wait, IO throughput)

I'll still wait for users feedback before sticking to this setting (or not).

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