Concept

Operating system

Summary
An operating system (OS) is system software that manages computer hardware and software resources, and provides common services for computer programs. Time-sharing operating systems schedule tasks for efficient use of the system and may also include accounting software for cost allocation of processor time, mass storage, peripherals, and other resources. For hardware functions such as input and output and memory allocation, the operating system acts as an intermediary between programs and the computer hardware, although the application code is usually executed directly by the hardware and frequently makes system calls to an OS function or is interrupted by it. Operating systems are found on many devices that contain a computer – from cellular phones and video game consoles to web servers and supercomputers. In the personal computer market, Microsoft Windows holds a dominant market share of around 75%. macOS by Apple Inc. is in second place (15%), and the varieties of Li
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Related publications (100)

Optimizing network performance in virtual machines

Aravind Menon

In recent years, there has been a rapid growth in the adoption of virtual machine technology in data centers and cluster environments. This trend towards server virtualization is driven by two main factors: the savings in hardware cost that can be achieved through the use of virtualization, and the increased flexibility in the management of hardware resources in a cluster environment. An important consequence of server virtualization is the negative impact it has on the networking performance of server applications running in virtual machines (VMs). In this thesis, we address the problem of efficiently virtualizing the network interface in Type-II virtual machine monitors. In the Type-II architecture, the VMM relies on a special, 'host' operating system to provide the device drivers to access I/O devices, and executes the drivers within the host operating system. Using the Xen VMM as an example of this architecture, we identify fundamental performance bottlenecks in the network virtualization architecture of Type-II VMMs. We show that locating the device drivers in a separate host VM is the primary reason for performance degradation in Type-II VMMs, because of two reasons: a) the switching between the guest and the host VM for device driver invocation, and, b) I/O virtualization operations required to transfer packets between the guest and the host address spaces. We present a detailed analysis of the virtualization overheads in the Type-II I/O architecture, and we present three solutions which explore the performance that can be achieved while performing network virtualization at three different levels: in the host OS, in the VMM, and in the NIC hardware. Our first solution consists of a set of packet aggregation optimizations that explores the performance achievable while retaining the Type-II I/O architecture in the Xen VMM. This solution retains the core functionality of I/O virtualization, including device driver execution, in the Xen 'driver domain'. With this set of optimizations, we achieve an improvement by a factor of two to four in the networking performance of Xen guest domains. In our second solution, we move the task of I/O virtualization and device driver execution from the host OS to the Xen hypervisor. We propose a new I/O virtualization architecture, called the TwinDrivers framework, which combines the performance advantages of Type-I VMMs with the safety and software engineering benefits of Type-II VMMs. (In a Type-I VMM, the device driver executes directly in the hypervisor, and gives much better performance than a Type-II VMM). The TwinDrivers architecture results in another factor of two improvements in networking performance for Xen guest domains. Finally, in our third solution, we describe a hardware based approach to network virtualization, in which we move the task of network virtualization into the network interface card (NIC). We develop a specialized network interface (CDNA) which allows guest operating systems running in VMs to directly access a private, virtual context on the NIC for network I/O, bypassing the host OS entirely. This approach also yields performance benefits similar to the TwinDrivers software-only approach. Overall, our solutions help significantly bridge the gap between the network performance in a virtualized environment and a native environment, eventually achieving network performance in a virtual machine within 70% of the native performance.
EPFL2009

Centralized and Decentralized Electricity Markets: Assessment of Operational and Economic Aspects

Farzaneh Abbaspourtorbati, Anneta Matenli

This paper presents the comparison of decentralized and centralized market settings in terms of economic efficiencies, particularly price signals, and operating aspects. The decentralized dispatch consists of an energy auction separated from a contingency analysis. The centralized setting is the conventional economic dispatch. This comparison includes deterministic and stochastic formulations (source of uncertainty is wind generation) applied on the Swiss power system.
Ieee2016

Advancing the State of Network Switch ASIC Offloading in the Linux Kernel

Andy Roulin

Modern data-center network operating systems rely on proprietary user-space daemons wrapping SDKs from switch vendors. Linux-based variants of these operating systems have benefited from increasing and simplified dataplane offloading support in recent years: kernel resources such as routes and next hops are offloaded to hardware and the kernel can, e.g., learn new MAC entries seen from the hardware forwarding plane. However, the Linux kernel in these operating systems has to be extended and customized as it still lacks constructs (constructs exposed to user-space or in-kernel constructs) needed for complete control plane processing and dataplane offloading. Managing limited hardware resources and keeping the kernel and hardware forwarding planes equivalent are two examples of challenges where the lack of appropriate constructs forces switch operating systems to use user-space solutions coupled with proprietary SDKs and custom kernel modifications. Filling the gaps, i.e., designing and adding the missing constructs, would enable faster control plane processing (less user-kernel context switches) and faster dataplane configura- tion. It would also encourage in-tree kernel drivers from hardware vendors instead of the current closed-source user-space SDKs which would also improve performance while estab- lishing Linux as the standardized API for network switches. This thesis explores the different designs, performance challenges and trade-offs of completing the in-kernel switching API, starting from switch port configuration, faster control path packet processing and ending with in-kernel ASIC resource management. As most hardware vendors are not yet ready to open their drivers in the kernel, workarounds to still provide support for vendors’ SDKs through the same in-kernel API are presented as well. A user-space switch port driver for Mellanox switches was ported to kernel space in order to accelerate control plane processing and bootstrap the in-kernel switch port configuration design. A prefetching scheme which hides PCI latency was designed to manage limited and shared ASIC resources with synchronous feedback to user-space daemons in case of exhausted resources.
2018
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