Page Cache Management in Virtual Environments

i Declaration I, Prateek Sharma, declare that this written submission represents my ideas in my own words and where others' ideas or words have been included, I have adequately cited and referenced the original sources. I also declare that I have adhered to all principles of academic honesty and integrity and have not misrepresented or fabricated or falsified any idea/data/fact/source in my submission. I understand that any violation of the above will be cause for disciplinary action by the Institute and can also evoke penal action from the sources which have thus not been properly cited or from whom proper permission has not been taken when needed. Signature Name of Student Roll number Date ii We investigate memory-management in hypervisors and propose Singleton, a KVM-based system-wide page deduplication solution to increase memory usage efficiency. We address the problem of double-caching that occurs in KVM—the same disk blocks are cached at both the host(hypervisor) and the guest(VM) page caches. Singleton's main components are identical-page sharing across guest virtual machines and an implementation of an exclusive-cache for the host and guest page cache hierarchy. We use and improve KSM–Kernel SamePage Merging to identify and share pages across guest virtual machines. We utilize guest memory-snapshots to scrub the host page cache and maintain a single copy of a page across the host and the guests. Singleton operates on a completely black-box assumption—we do not modify the guest or assume anything about its behaviour. We show that conventional operating system cache management techniques are sub-optimal for virtual environments, and how Singleton supplements and improves the existing Linux kernel memory-management mechanisms. Singleton is able to improve the utilization of the host cache by reducing its size(by upto an order of magnitude), and increasing the cache-hit ratio(by factor of 2x). This translates into better VM perfor-mance(40% faster I/O). Singleton's unified page deduplication and host cache scrubbing is able to reclaim large amounts of memory and facilitates higher levels of memory over-commitment. The optimizations to page deduplication we have implemented keep the overhead down to less than 20% CPU utilization. we argue that second-level page-cache allocation to Virtual Machines plays an important role in determining the performance, isolation, and Quality-of-Serice of Virtual Machines. The second-level cache is a contended resource, and also competes with the memory allocated to the Virtual Machines themselves. We show that the phenomenon of swapping-while-caching is particularly detrimental to VM performance. Our solution …