Effectively Mitigating I/O Inactivity in vCPU Scheduling

Abstract

In clouds where CPU cores are time-shared by virtual CPUs (vCPU), vCPUs are scheduled and descheduled by the virtual machine monitor (VMM) periodically. In each virtual machine (VM), when its vCPUs running I/O bound tasks are descheduled, no I/O requests can be made until the vCPUs are rescheduled. These inactivity periods of I/O tasks cause severe performance issues, one of them being the utilization of I/O resources in the guest OS tends to be low during I/O inactivity periods. Worse, the I/O scheduler in the host OS could suffer from low performance because the I/O scheduler assumes that I/O tasks make I/O requests constantly. Fairness among the VMs within a host can also be at stake. Existing works typically would adjust the time slices of vCPUs running I/O tasks, but vCPUs are still descheduled frequently and cause I/O inactivity. Our idea is that since each VM often has active vCPUs, we can migrate I/O tasks to active vCPUs, thus mitigating the I/O inactivity periods and maintaining the fairness. We present vMigrater, which runs in the user level of each VM. It incorporates new mechanisms to efficiently monitor active vCPUs and to accurately detect I/O bound tasks. Evaluation on diverse real world applications shows that vMigrater can improve I/O performance by up to 4.42X compared with default Linux KVM. vMigrater can also improve I/O performance by 1.84X to 3.64X compared with two related systems.