Understanding Memory Management in VMware vSphere 5

vSphere 5.0 leverages the advantages of several memory reclamation techniques to allow users to over-commit host memory, With the support of memory overcommitment, vSphere 5.0 can achieve a remarkable VM consolidation ratio while maintaining reasonable performance. This paper presents how ESXi in vSphere 5.0 manages the host memory and how the memory reclamation techniques efficiently reclaim host memory without much impact on VM performance. A share-based memory management mechanism is used to enable both performance isolation and efficient memory utilization. The algorithm relies on a working set estimation technique which measures the idleness of a VM. Ballooning reclaims memory from a VM by implicitly causing the guest OS to invoke its own memory management technique. Transparent page sharing exploits sharing opportunities within and between VMs without any guest OS involvement. Memory compression reduces the amount of host swapped pages by storing the compressed format of the pages in a per-VM memory compression cache. Swap to SSD leverages SSD’s low read latency to alleviate the host swapping penalty. Finally, a high level dynamic memory reallocation policy coordinates all these techniques to efficiently support memory overcommitment.

http://www.vmware.com/files/pdf/mem_mgmt_perf_vsphere5.pdf