When memory is overcommitted, ESX Server must employ some mechanism to reclaim space from one or more virtual machines. The standard approach used by earlier virtual machine systems is to introduce another level of paging [9,20], moving some VM ``physical'' pages to a swap area on disk. Unfortunately, an extra level of paging requires a meta-level page replacement policy: the virtual machine system must choose not only the VM from which to revoke memory, but also which of its particular pages to reclaim.
In general, a meta-level page replacement policy must make relatively uninformed resource management decisions. The best information about which pages are least valuable is known only by the guest operating system within each VM. Although there is no shortage of clever page replacement algorithms , this is actually the crux of the problem. A sophisticated meta-level policy is likely to introduce performance anomalies due to unintended interactions with native memory management policies in guest operating systems. This situation is exacerbated by diverse and often undocumented guest OS policies , which may vary across OS versions and may even depend on performance hints from applications .
The fact that paging is transparent to the guest OS can also result in a double paging problem, even when the meta-level policy is able to select the same page that the native guest OS policy would choose [9,20]. Suppose the meta-level policy selects a page to reclaim and pages it out. If the guest OS is under memory pressure, it may choose the very same page to write to its own virtual paging device. This will cause the page contents to be faulted in from the system paging device, only to be immediately written out to the virtual paging device.