Proactively Breaking Large Pages to Improve Memory Overcommitment Performance in VMware ESXi

VMware ESXi leverages hardware support for MMU virtualization available in modern Intel/AMD CPUs. To optimize address translation performance when running on such CPUs, ESXi preferably uses host large pages (2MB in x86-64 systems) to back VM's guest memory. While using host large pages provides best performance when host has sufficient free memory, it increases host memory pressure and effectively defeats page sharing. Hence, the host is more likely to hit the point where ESXi has to reclaim VM memory through much more expensive techniques such as ballooning or host swapping. As a result, using host large pages may significantly hurt consolidation ratio. To deal with this problem, we propose a new host large page management policy that allows to: a) identify 'cold' large pages and break them even when host has plenty of free memory; b) break all large pages proactively when host free memory becomes scarce, but before the host starts ballooning or swapping; c) reclaim the small pages within the broken large pages through page sharing. With the new policy, the shareable small pages can be shared much earlier and the amount of memory that needs to be ballooned or swapped can be largely reduced when host memory pressure is high. We also propose an algorithm to dynamically adjust the page sharing rate when proactively breaking large pages using a VM large page shareability estimator for higher efficiency. Experimental results show that the proposed large page management policy can improve the performance of various workloads up to 2.1x by significantly reducing the amount of ballooned or swapped memory when host memory pressure is high. Applications still fully benefit from host large pages when memory pressure is low.