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Improve Article Save Article View Discussion Improve Article Save Article Virtual Machine abstracts the hardware of our personal computer such as CPU, disk drives, memory, NIC (Network Interface Card) etc, into many different execution environments as per our requirements, hence giving us a feel that each execution environment is a single computer. For example, VirtualBox. When we run different processes on an operating system, it creates an illusion that each process is running on a different processor having its own virtual memory, with the help of CPU scheduling and virtual-memory techniques. There are additional features of a process that cannot be provided by the hardware alone like system calls and a file system. The virtual machine approach does not provide these additional functionalities but it only provides an interface that is same as basic hardware. Each process is provided with a virtual copy of the underlying computer system. We can create a virtual machine for several reasons, all of which are fundamentally related to the ability to share the same basic hardware yet can also support different execution environments, i.e., different operating systems simultaneously. The main drawback with the virtual-machine approach involves disk systems. Let us suppose that the physical machine has only three disk drives but wants to support seven virtual machines. Obviously, it cannot allocate a disk drive to each virtual machine, because virtual-machine software itself will need substantial disk space to provide virtual memory and spooling. The solution is to provide virtual disks. Users are thus given their own virtual machines. After which they can run any of the operating systems or software packages that are available on the underlying machine. The virtual-machine software is concerned with multi-programming multiple virtual machines onto a physical machine, but it does not need to consider any user-support software. This arrangement can provide a useful way to divide the problem of designing a multi-user interactive system, into two smaller pieces. Advantages:
Disadvantages:
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This article describes new modes of virtual processor scheduling logic first introduced in Windows Server 2016. These modes, or scheduler types, determine how the Hyper-V hypervisor allocates and manages work across guest virtual processors. A Hyper-V host administrator can select hypervisor scheduler types that are best suited for the guest virtual machines (VMs) and configure the VMs to take advantage of the scheduling logic. Note Updates are required to use the hypervisor scheduler features described in this document. For details, see Required updates. BackgroundBefore discussing the logic and controls behind Hyper-V virtual processor scheduling, it's helpful to review the basic concepts covered in this article. Understanding SMTSimultaneous multithreading, or SMT, is a technique utilized in modern processor designs that allows the processor's resources to be shared by separate, independent execution threads. SMT generally offers a modest performance boost to most workloads by parallelizing computations when possible, increasing instruction throughput, though no performance gain or even a slight loss in performance may occur when contention between threads for shared processor resources occurs. Processors supporting SMT are available from both Intel and AMD. Intel refers to their SMT offerings as Intel Hyper Threading Technology, or Intel HT. For the purposes of this article, the descriptions of SMT and how it is utilized by Hyper-V apply equally to both Intel and AMD systems.
Understanding how Hyper-V virtualizes processorsBefore considering hypervisor scheduler types, it's also helpful to understand the Hyper-V architecture. You can find a general summary in Hyper-V Technology Overview. These are important concepts for this article:
Hypervisor scheduler typesStarting with Windows Server 2016, the Hyper-V hypervisor supports several modes of scheduler logic, which determine how the hypervisor schedules virtual processors on the underlying logical processors. These scheduler types are: The classic schedulerThe classic scheduler has been the default for all versions of the Windows Hyper-V hypervisor since its inception, including Windows Server 2016 Hyper-V. The classic scheduler provides a fair share, preemptive round- robin scheduling model for guest virtual processors. The classic scheduler type is the most appropriate for the vast majority of traditional Hyper-V uses – for private clouds, hosting providers, and so on. The performance characteristics are well understood and are best optimized to support a wide range of virtualization scenarios, such as over-subscription of VPs to LPs, running many heterogeneous VMs and workloads simultaneously, running larger scale high performance VMs, supporting the full feature set of Hyper-V without restrictions, and more. The core schedulerThe hypervisor core scheduler is a new alternative to the classic scheduler logic, introduced in Windows Server 2016 and Windows 10 version 1607. The core scheduler offers a strong security boundary for guest workload isolation, and reduced performance variability for workloads inside of VMs that are running on an SMT-enabled virtualization host. The core scheduler allows running both SMT and non-SMT virtual machines simultaneously on the same SMT-enabled virtualization host. The core scheduler utilizes the virtualization host's SMT topology, and optionally exposes SMT pairs to guest virtual machines, and schedules groups of guest virtual processors from the same virtual machine onto groups of SMT logical processors. This is done symmetrically so that if LPs are in groups of two, VPs are scheduled in groups of two, and a core is never shared between VMs. When the VP is scheduled for a virtual machine without SMT enabled, that VP consumes the entire core when it runs. The overall result of the core scheduler is that:
The core scheduler is used by default starting in Windows Server 2019. On Windows Server 2016, the core scheduler is optional and must be explicitly enabled by the Hyper-V host administrator, and the classic scheduler is the default. Core scheduler behavior with host SMT disabledIf the hypervisor is configured to use the core scheduler type but the SMT capability is disabled or not present on the virtualization host, then the hypervisor uses the classic scheduler behavior, regardless of the hypervisor scheduler type setting. The root schedulerThe root scheduler was introduced with Windows 10 version 1803. When the root scheduler type is enabled, the hypervisor cedes control of work scheduling to the root partition. The NT scheduler in the root partition's OS instance manages all aspects of scheduling work to system LPs. The root scheduler addresses the unique requirements inherent with supporting a utility partition to provide strong workload isolation, as used with Windows Defender Application Guard (WDAG). In this scenario, leaving scheduling responsibilities to the root OS offers several advantages. For example, CPU resource controls applicable to container scenarios may be used with the utility partition, simplifying management and deployment. In addition, the root OS scheduler can readily gather metrics about workload CPU utilization inside the container and use this data as input to the same scheduling policy applicable to all other workloads in the system. These same metrics also help clearly attribute work done in an application container to the host system. Tracking these metrics is more difficult with traditional virtual machine workloads, where some work on all running VM's behalf takes place in the root partition. Root scheduler use on client systemsStarting with Windows 10 version 1803, the root scheduler is used by default on client systems only, where the hypervisor may be enabled in support of virtualization-based security and WDAG workload isolation, and for proper operation of future systems with heterogeneous core architectures. This is the only supported hypervisor scheduler configuration for client systems. Administrators should not attempt to override the default hypervisor scheduler type on Windows 10 client systems. Virtual Machine CPU resource controls and the root schedulerThe virtual machine processor resource controls provided by Hyper-V are not supported when the hypervisor root scheduler is enabled as the root operating system's scheduler logic is managing host resources on a global basis and does not have knowledge of a VM's specific configuration settings. The Hyper-V per-VM processor resource controls, such as caps, weights, and reserves, are only applicable where the hypervisor directly controls VP scheduling, such as with the classic and core scheduler types. Root scheduler use on server systemsThe root scheduler is not recommended for use with Hyper-V on servers at this time, as its performance characteristics have not yet been fully characterized and tuned to accommodate the wide range of workloads typical of many server virtualization deployments. Enabling SMT in guest virtual machinesOnce the virtualization host's hypervisor is configured to use the core scheduler type, guest virtual machines may be configured to utilize SMT if desired. Exposing the fact that VPs are hyperthreaded to a guest virtual machine allows the scheduler in the guest operating system and workloads running in the VM to detect and utilize the SMT topology in their own work scheduling. On Windows Server 2016, guest SMT is not configured by default and must be explicitly enabled by the Hyper-V host administrator. Starting with Windows Server 2019, new VMs created on the host inherits the host's SMT topology by default. That is, a version 9.0 VM created on a host with 2 SMT threads per core would also see 2 SMT threads per core. PowerShell must be used to enable SMT in a guest virtual machine; there is no user interface provided in Hyper-V Manager. To enable SMT in a guest virtual machine, open a PowerShell window with sufficient permissions, and type: Set-VMProcessor -VMName <VMName> -HwThreadCountPerCore <n>Where <n> is the number of SMT threads per core the guest VM sees. Note that <n> = 0 sets the HwThreadCountPerCore value to match the host's SMT thread count per core value. Note Setting HwThreadCountPerCore = 0 is supported beginning with Windows Server 2019. Below is an example of System Information taken from the guest operating system running in a virtual machine with 2 virtual processors and SMT enabled. The guest operating system is detecting 2 logical processors belonging to the same core.  Configuring the hypervisor scheduler type on Windows Server 2016 Hyper-VWindows Server 2016 Hyper-V uses the classic hypervisor scheduler model by default. The hypervisor can be optionally configured to use the core scheduler, to increase security by restricting guest VPs to run on corresponding physical SMT pairs, and to support the use of virtual machines with SMT scheduling for their guest VPs. Note Microsoft recommends that all customers running Windows Server 2016 Hyper-V select the core scheduler to ensure their virtualization hosts are optimally protected against potentially malicious guest VMs. Windows Server 2019 Hyper-V defaults to using the core schedulerTo help ensure Hyper-V hosts are deployed in the optimal security configuration, Windows Server 2019 Hyper-V now uses the core hypervisor scheduler model by default. The host administrator may optionally configure the host to use the legacy classic scheduler. Administrators should carefully read, understand and consider the impacts each scheduler type has on the security and performance of virtualization hosts prior to overriding the scheduler type default settings. See About Hyper-V hypervisor scheduler type selection for more information. Required updatesNote The following updates are required to use the hypervisor scheduler features described in this document. These updates include changes to support the new hypervisorschedulertype BCD option, which is necessary for host configuration.
Selecting the hypervisor scheduler type on Windows ServerThe hypervisor scheduler configuration is controlled via the hypervisorschedulertype BCD entry. To select a scheduler type, open a command prompt with administrator privileges: bcdedit /set hypervisorschedulertype typeWhere type is one of:
The system must be rebooted for any changes to the hypervisor scheduler type to take effect. Note The hypervisor root scheduler is not supported on Windows Server Hyper-V at this time. Hyper-V administrators should not attempt to configure the root scheduler for use with server virtualization scenarios. Determining the current scheduler typeYou can determine the current hypervisor scheduler type in use by examining the System log in Event Viewer for the most recent hypervisor launch event ID 2, which reports the hypervisor scheduler type configured at hypervisor launch. Hypervisor launch events can be obtained from the Windows Event Viewer, or via PowerShell. Hypervisor launch event ID 2 denotes the hypervisor scheduler type, where:
Querying the Hyper-V hypervisor scheduler type launch event using PowerShellTo query for hypervisor event ID 2 using PowerShell, enter the following commands from a PowerShell prompt. Get-WinEvent -FilterHashTable @{ProviderName="Microsoft-Windows-Hyper-V-Hypervisor"; ID=2} -MaxEvents 1FeedbackSubmit and view feedback for What is virtual machine scheduling?VM scheduling is a combination of distributed and centralized. Cloud computing provides the users with the relative concentrated resources in the form of data center, so the resource pool is a whole for users. But in the process of the internal VM scheduling, the distributed processing method is adopted.
What is virtual machine explain?A Virtual Machine (VM) is a compute resource that uses software instead of a physical computer to run programs and deploy apps. One or more virtual “guest” machines run on a physical “host” machine.
What is CPU scheduling explain?CPU Scheduling is a process that allows one process to use the CPU while another process is delayed (in standby) due to unavailability of any resources such as I / O etc, thus making full use of the CPU. The purpose of CPU Scheduling is to make the system more efficient, faster, and fairer.
What is a virtual machine in a CPU?A virtual machine is a computer file, typically called an image, that behaves like an actual computer. It can run in a window as a separate computing environment, often to run a different operating system—or even to function as the user's entire computer experience—as is common on many people's work computers.
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