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Comparing Linux huge memory pages & Kernel Samepage Merging for KVM virtualization

A quick test (so take it with a grain of salt):

  1. Huge pages is slightly faster than not using huge pages (~10% with 4 winxp virtual machines copying 512MB from one memory location to another).
  2. KSM is slightly slower then not using KSM (~5% with 4 winxp virtual machines copying 512MB from one memory location to another).

So, at first glance it would appear that we can use a loose rule of thumb:

  1. to consolidate the maximum number of machines, use KSM as it will allow you to over commit the amount of memory on your box.
    1. Risk: if the memory pages are significantly different, you may start swapping in a very bad way. This is where monitoring comes in
  2. to give the best performance to a number of machines, use huge memory pages.. does not allow you to over commit
    1. Risk: if you don’t leave enough memory for the host os, you can crash your machine. This is also where monitoring comes in
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