1. Exploiting Sharing for Data Center Consolidation
Timothy Wood, Jim Cipar, Gabriel Tarasuk-Levin, Peter Desnoyers, Emery Berger, Mark Corner, Prashant Shenoy University of Massachusetts, Amherst
Motivation and Challenges
Using the potential for memory sharing as a guide for placing VMs can lead to substantial memory savings.
Memory requirements and the potential for sharing fluctuate over time, thus the system must monitor memory utilization to prevent hotspots.
Memory is an expensive resource and can be the limiting factor when consolidating virtual machines with low CPU utilization.
ESX Server supports page sharing – allowing virtual machines to reduce memory consumption by sharing identical pages.
Sharing Memory
If two VM’s have an identical page in memory, only store a single copy until one makes a write.
Matches are found by comparing hashes generated for each page in a VM’s memory.
Currently, ESX only monitors sharing within a single host.
A VMware experiment reports memory savings of over 30% for a group of Windows servers.
Table from Memory Resource Management in VMware ESX Server, Carl A. Waldspurger, OSDI 2002
Finding Similar Virtual Machines
We must efficiently calculate the sharing potential between VMs across large data centers.
Brute force comparison of page hashes is costly in both computation and memory.
We propose an efficient Bloom filter based “fingerprinting” technique.
Bloom filters still maintain a high degree of accuracy, and allow for a tradeoff between storage requirements and prediction accuracy.
Bloom Filter Accuracy
Sharing Estimation Time
Relative Error
Time (sec)
Number of VMs
Sharing Rate
Placement Algorithms VM VM VM VM VM VM
Using sharing potential can help optimize placement of virtual machines.
Sharing reduces memory requirements, increasing consolidation possibilities.
Additional memory tracing techniques can help detect and prevent memory hotspots.
Host
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Host
Taking advantage of memory sharing allows more virtual machines to run on a smaller number of hosts VM VM VM VM VM VM
Host
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