Converged Enhanced Ethernet (CEE) is a crucial step in embracing storage, cluster, and high-performance computing fabrics under a common network. However, the adoption of lossless CEE in virtualized clusters is hindered by the lack of network hypervisor software that addresses the major issues of losslessness, i.e., head-of-line blocking and saturation trees. Our objective is to design a hypervisor that prevents miscon-figured or malicious virtual machines from filling the lossless network with stalled packets, thus compromising tenant isolation. Furthermore, we observe that current hypervisors perform compulsory isolation, management, and mobility functions, but introduce new bottlenecks on the data-path. By taking advantage of the lossless fabric, we deconstruct the existing virtualized networking stack into its core functions and consolidate them into zFabric, an efficient hypervisor that meets our aforementioned goals. To demonstrate zFabric's benefits, we evaluate a prototype implementation on a datacenter testbed. Besides resolving head-of-line blocking, zFabric improves throughputs for long flows by up to 56%, lowers CPU utilization by up to 63%, and shortens completion times by up to 7x for partition-aggregate queries when compared with current virtualized TCP stacks.

Datacenter-based Cloud computing has induced new disruptive trends in networking, key among which is network virtualization. Software-Defined Networking overlays aim to improve the efficiency of the next generation multitenant datacenters. While early overlay prototypes are already available, they focus mainly on core functionality, with little being known yet about their impact on the system level performance. Using query completion time as our primary performance metric, we evaluate the overlay network impact on two representative datacenter workloads, Partition/Aggregate and 3-Tier. We measure how much performance is traded for overlay's benefits in manageability, security and policing. Finally, we aim to assist the datacenter architects by providing a detailed evaluation of the key overlay choices, all made possible by our accurate cross-layer hybrid/mesoscale simulation platform.

Datacenter networking is currently dominated by two major trends. One aims toward lossless, flat layer-2 fabrics based on Converged Enhanced Ethernet or InfiniBand, with benefits in efficiency and performance. The other targets flexibility based on Software Defined Networking, which enables Overlay Virtual Networking. Although clearly complementary, these trends also exhibit some conflicts: In contrast to physical fabrics, which avoid packet drops by means of flow control, practically all current virtual networks are lossy. We quantify these losses for several common combinations of hypervisors and virtual switches, and show their detrimental effect on application performance. Moreover, we propose a zero-loss Overlay Virtual Network (zOVN) designed to reduce the query and flow completion time of latency-sensitive datacenter applications. We describe its architecture and detail the design of its key component, the zVALE lossless virtual switch. As proof of concept, we implemented a zOVN prototype and benchmark it with Partition-Aggregate in two testbeds, achieving an up to 15-fold reduction of the mean completion time with three widespread TCP versions. For larger-scale validation and deeper introspection into zOVN, we developed an OMNeT++ model for accurate cross-layer simulations of a virtualized datacenter, which confirm the validity of our results.

One of the prevalent trends in emerging large scale multi-tenant datacenters is network virtualization using overlays. Here we investigate application performance degradation in such an overlay applied to commodity 10 Gigabit Ethernet networks. We have adopted partition/aggregate as a representative commercial workload that today is deployed on bare metal servers and is notoriously sensitive to latency and TCP incast congestion. Using query completion time as the primary metric, we evaluate the degree to which a software-defined network (SDN) overlay impacts this application's behavior, the performance bounds of partition/aggregate with an SDN overlay, and whether active queue management (AQM) such as random early detection (RED) can benefit this environment. We introduce a generic SDN overlay framework, which we measure in hardware and simulate using a real TCP stack extracted from FreeBSD v9, running over a detailed Layer 2 commodity 10G Ethernet fabric network simulator. To further alleviate TCP incast congestion and support legacy congestion control, we propose an AQM translation scheme called v-RED. Finally, we report results concerning SDN's benefits in addressing TCP incast. Contrary to our expectations, we found that latency-sensitive applications do not necessarily suffer from performance degradation when deployed over SDN overlays.

On-chip coherence directories of today's multi-core systems are not energy efficient. Coherence directories dissipate a significant fraction of their power on unnecessary lookups when running commercial server and scientific workloads. These workloads have large working sets that are beyond the reach of on-chip caches of modern processors. Limited to capturing a small part of the working set, private caches retain cache blocks only for a short period of time before replacing them with new blocks. Moreover, coherence enforcement is a known performance bottleneck of multi-threaded software, hence data-sharing in optimized high performance software is minimal. Consequently, the majority of the accesses to the coherence directory find no sharers in the directory because the data are not available in the on-chip private caches, effectively wasting power on the coherence checks. To improve energy-efficiency for future many-core systems, we propose TurboTag, a filtering mechanism to eliminate needless directory lookups. We analyze full-system traces of server and scientific workloads and find that over 69% of accesses to the directory find no sharers and can be entirely avoided. Taking advantage of this behavior, TurboTag achieves a 58% reduction in the directory's dynamic power consumption.