Minimizing the total amount of physical memory consumption of the virtual machines (VM) concurrently running on a physical server is a key to improving a hyper visor's consolidation ratio, defined as the maximum number of VMs that can run on a physical server without performance degradation. This paper describes the design, implementation and evaluation of a memory resource manager called Gatun that constantly measures the working set size of each running VM, and exploits this information to make the best of the available physical memory on a physical server. The per-VM working set size information allows Gatun to reclaim only unused physical memory from each running VM without negatively affecting its performance, and to compress the right subset of memory pages for each VM to reduce its memory pressure to the maximal extent possible. Compared with a state-of-the-art commercial hyper visor, Gatun is able to make noticeably more efficient utilization of a virtualized server's physical memory because of its ability to capture and leverage the per-VM working set size information.

The effectiveness of a modern hyper visor on a virtualized server is measured by its consolidation ratio, which is defined as the maximum number of virtual machines (VM) that can run on a physical server without performance degradation. A key factor in determining a hyper visor's consolidation ratio is its effectiveness in fairly distributing a virtualized server's physical memory resource among the VMs running on it when the total memory demand of these VMs exceeds the server's physical memory size. This paper describes the design, implementation and evaluation of a memory pressure balancer called Gatto that constantly measures the working set size of each VM running on a virtualized server, and exploits this information to distribute the server's physical memory resource so as to balance the memory pressures experienced by these VMs. Gatto's memory pressure balancing mechanism could be generalized and applied to achieve memory quality of service (QoS) guarantee onmultitenant virtualized servers.

Memory virtualization abstracts a physical machine's memory resource and presents to the virtual machines running on it a piece of physical memory that could be shared, compressed and moved. To optimize the memory resource utilization by fully leveraging the flexibility afforded by memory virtualization, it is essential that the hypervisor have some sense of how the guest VMs use their allocated physical memory. One way to do this is virtual machine introspection (VMI), which interprets byte values in a guest memory space into semantically meaningful data structures. However, identifying a guest VM's memory usage information such as free memory pool is non-trivial. This paper describes a bootstrapping VM introspection technique that could accurately extract free memory pool information from multiple versions of Windows and Linux without kernel version-specific hard-coding, how to apply this technique to improve the efficiency of memory de-duplication and memory state migration, and the resulting improvement in memory de-duplication speed, gain in additional memory pages de-duplicated, and reduction in traffic loads associated with memory state migration.

A powerful functionality enabled by modern virtualization technologies is the ability to move a virtual machine (VM) from one physical machine to another, which enables unprecedented flexibility for system fault tolerance and load balancing. However, no similar capability exists for physical machines. This paper describes the first known successful implementation of migrating a physical machine's state from one physical Linux machine to another. This physical machine state migration (PMSM) capability greatly decreases the amount of disruption due to scheduled shut-down for non-virtualized physical machines, and is more challenging than VM migration because it cannot rely on a separate piece of software to perform the state transfer, e.g., the hyper visor in the case of VM migration. The PMSM prototype described in this paper is adapted from Linux's hibernation facility. The current PMSM prototype can migrate a physical machine running the MySQL DBMS server under 7 seconds.

Memory de-duplication removes the memory state redundancy among virtual machines that run on the same physical machine by identifying common memory pages shared by these virtual machines and storing only one copy for each of common memory pages. A standard approach to identifying common memory pages is to hash the content of each memory page, and compare the resulting hash values. In a virtualized server, only the hyper visor is in a position to compute the hash value of every physical memory page on the server, but the memory de-duplication engine is best implemented outside the hyper visor for flexibility and simplicity reasons. A key design issue in the memory de-duplication engine is to minimize the performance impact of these hashing computations on the running VMs. To reduce this impact, memory page hashing should be performed with low overhead and when the CPU is idle. This paper describes why existing hyper visors do not provide adequate support for our memory de-duplication engine, how a new primitive called deferrable aggregate hyper call (DAH) fills the need, and what the resulting performance improvement is.

Clock synchronization is an essential building block for many control mechanisms used in wireless networks, including frequency hopping, power management, and packet scheduling. Although the IEEE 802.11 standard provides a clock synchronization mechanism for multi-hop wireless networks, it has well-documented accuracy problems, because the beacon frames transmitted by nodes with faster clock are not prioritized at the MAC layer. This paper describes the design and implementation of the first known software-only solution that successfully eliminates the deficiency of IEEE 802.11's clock synchronization mechanism when operating on multihop wireless networks without requiring any hardware/protocol modifications. The key idea of this solution is to periodically arrange for a node to become the fastest node in the network by jumping its clock with a substantial amount, and force other nodes to synchronize their clocks with this leading node by virtue of IEEE 802.11's native clock synchronization mechanism. Empirical measurements of a working implementation of the proposed algorithm on a multi-hop wireless network testbed conclusively demonstrates its effectiveness in removing IEEE 802.11's clock synchronization accuracy problem.

A voice-over-IP (VoIP) gateway bridges IP-based packet-switched networks (i.e. Internet) with public circuitswitched telephone networks (i.e. PSTN). The key building block of a VoIP gateway is a telephony card that interfaces with the PSTN and converts signals from the PSTN to bits that can be manip