VMware memory management: Memory Tax

I'm looking into the specifics of how the ESX hypervisor handles memory, and how resource allocation is performed.

One of the things that has kept me busy was the so-called 'memory tax'. This concept is explained in esx3_memory.pdf but was not clear to me when reading this document. This is a quote from the document:

If a virtual machine is not actively using its currently allocated memory, ESX Server charges a memory tax — more for idle memory than for memory that is in use. That is, the idle memory counts more towards the share allocation than memory in use. The default tax rate is 75 percent, that is, an idle page of memory costs as much as four active pages. This rate can be changed by modifying a parameter setting.

Looking further on the web, I found that in 2002 at, a conference, Carl A. Waldspurger was awarded the best paper award for the following: Memory Resources Management in VMware ESX Server. The slides of the presentation can be found here. Maybe it is because I'm used to reading papers (or at least I used to), but I found the explanation much more clear in this document.

I encourage everyone interested in memory management to read the paper.