Agile Live Migration of Virtual Machines

Abstract

A key attraction of virtual machines (VMs) is live migration - the ability to move their execution state across physical machines even as the VMs continue to run. Unfortunately, the traditional pre-copy and post-copy techniques are not agile in the face of resource pressures at the source host, since it takes a long time to transfer the memory state of a VM. Consequently, the performance suffers for all VMs - those being migrated as well as those being left behind. Prior works have attempted to optimize indirect measures of migration effectiveness such as downtime, total migration time, and network overhead. However, none have treated the performance of VMs impacted by migration as the primary metric of migration effectiveness. We propose an Agile live migration technique that quickly recovers the performance of all VMs under resource pressure by eliminating resource pressure faster than traditional live migration. The working set of a VM is typically much smaller than its full memory footprint. Our approach works by transparently tracking the working set of each VM and offloading the non-working set (cold pages) in advance to portable per-VM swap devices. We present a new hybrid pre/post-copy technique that reduces the performance impact on the VM's workload by transferring only the working set of the VM while enabling destination to remotely access cold pages from the per-VM swap device. We describe the challenges in the design and implementation of Agile live migration in the KVM/QEMU platform without modifying the guest OS in the VM. When live migrating under memory pressure, we demonstrate a reduction in the performance impact on VMs by a up to factor of 2, reduction in migration time by up to factor of 4 besides reduction in memory pressure on both the source and destination hosts.