  Intel Page Modification Logging (PML) is a novel hardware feature for
tracking virtual machine (VM) accessed memory pages. This task is essential in
today's data centers since it allows, among others, checkpointing, live
migration and working set size (WSS) estimation. Relying on the Xen hypervisor,
this paper studies PML from three angles: power consumption, efficiency, and
performance impact on user applications. Our findings are as follows. First,
PML does not incur any power consumption overhead. Second, PML reduces by up to
10.18% both VM live migration and checkpointing time. Third, PML slightly
reduces by up to 0.95% the performance degradation on applications incurred by
live migration and checkpointing. Fourth, PML however does not allow accurate
WSS estimation because read accesses are not tracked and hot pages cannot be
identified. A naive extension of PML for addressing these limitations could
lead to severe performance degradation (up to 34.8%) for the VM whose WSS is
computed.
  This paper presents Page Reference Logging (PRL), a smart extension of PML
for allowing both read and write accesses to be tracked. It does this without
impacting user VMs. The paper also presents a WSS estimation system which
leverages PRL and shows how this algorithm can be integrated into a data center
which implements memory overcommitment. We implement PRL and the WSS estimation
system under Gem5, a very popular hardware simulator. The evaluation results
validate the accuracy of PRL in the estimation of WSS. They also show that PRL
incurs no performance degradation for user VMs.
