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Hyper-V Dynamic Memory

Dynamic memory. The Hyper-V hypervisor can adjust machine memory grants to guest machines up or down dynamically, a feature that is called dynamic memory. Dynamic memory refers to adjustments in the size of the Guest Physical address space that the hypervisor grants to a guest machine running inside a child partition. When dynamic memory is configured for a guest machine, Hyper-V can give a partition more physical memory to use or remove guest physical memory from a guest machine that doesn’t require it, ignoring for a moment the relative memory priority of the guests. With the dynamic memory feature of Hyper-V, you can pack significantly more virtual machines into the memory footprint of the VM host machine, although you still must be careful not to pack in too many guest machines and create a memory bottleneck that can impact all the guest machines that are resident on the Hyper-V Host. 
When dynamic memory is enabled for a child partition, you set minimum and maximum Guest Physica…
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Hyper-V Memory Management: Introduction

Hyper-V Memory Management Introduction The hypervisor also contains a Memory Manager component for managing access to the machine’s physical memory, i.e., RAM. For the sake of clarity, when discussing memory management in the Hyper-V environment, I will call RAM machine memory, the Hyper-V host machine’s actual physical memory, to distinguish it from the view of virtualized physical memory granted to each partition. Guest machines never access machine memory directly. Each guest machine is presented with a range of Guest Physical memory addresses (GPA), based on its configuration definitions, that the hypervisor maps to machine memory with a set of page tables that the hypervisor maintains.
Machine memory cannot be shared in the same way that other computer resources like CPUs and disks can be shared. Once memory is in use, it remains 100% occupied until the owner of those memory locations frees it. The hypervisor’s Memory Manager is responsible for distributing machine memory among …

Hyper-V Architecture: Intercepts, interrupts and Hypercalls

Intercepts, interrupts and Hypercalls Three interfaces exist that allow for interaction and communication between the hypervisor, the Root partition and the guest partitions: intercepts, interrupts, and the direct Hypercall interface. These interfaces are necessary for the virtualization scheme to function properly, and their usage accounts for much of the overhead virtualization adds to the system. Hyper-V measures and reports on the rate these different interfaces are used, which is, of course, workload dependent. Frankly, the measurements that show the rate that the hypervisor processes interrupts and Hypercalls is seldom of interest outside the Microsoft developers working on Hyper-V performance itself. But these measurements do provide insight into the Hyper-V architecture and can help us understand how the performance of the applications running on guest machines is impacted due to virtualization. Figure 3 is a graph showing these three major sources of virtualization overhead …

Hyper-V Performance expectations

Hyper-V Performance expectations
Before drilling deeper into the Hyper-V architecture and discussing the performance impacts of virtualization in some detail, it will help to put the performance implications of virtualization technology in perspective. Let’s begin with some very basic performance expectations.

Of the many factors that persuade IT organizations to configure Windows to run as a virtual machine guest under either VMware or Microsoft’s Hyper-V, very few pertain to performance. To be sure, many of the activities virtualization system administrators perform are associated with the scalability of an application, specifically, provisioning a clustered application tier so that a sufficient number of guest machine images are active concurrently. Of course, the Hyper-V Host machine and the shared storage and networking infrastructure that is used must be adequately provisioned to support the planned number of guests. Provisioning the Hyper-V Host machine also extends to ensurin…

Hyper-V Performance: Virtual Processor Scheduling Priority

Virtual Processor Scheduling Priority The Hyper-V hypervisor, which is responsible for guest machine scheduling, by default implements a simple, round-robin policy in which any virtual processor in the execution state is equally likely to be dispatched. If the CPU capacity of the hardware is adequate, defining and running more virtual processors than logical processors leads to the possibility of dispatching queuing delays, but normally these dispatching delays are minimal because many virtual processors are in the idle state, and so over-commitment of the physical processors often does not impact performance greatly. On the other hand, once these dispatching delays start to become significant, the priority scheduling options that Hyper-V provides can become very useful.

In addition to the basic round-robin scheme, the hypervisor also factors processor affinity and the NUMA topology into scheduling decisions, considerations that add asymmetric constraints to processor scheduling. Note…

Hyper-V Processor performance monitoring

Virtual Processor performance monitoring When Hyper-V is running and controlling the use of the machine CPU resources, to monitor processor utilization by the hypervisor, the Root partition, and guest machines running in child partitions, you must access the counters associated with the Hyper-V Hypervisor. These can be gathered by running a performance monitoring application on the Root partition. The Hyper-V processor usage statistics need to be used instead of the usual Processor and Processor Information performance counters available at the OS level.
There are three sets of Hyper-V Hypervisor processor utilization counters and the key to using them properly is to understand what entity each instance of the counter set represents. The following three Hyper-V processor performance counter sets are provided:
·Hyper-V Hypervisor Logical Processor
There is one instance of HVH Logical Processor counters available for each hardware logical processor that is present on the machine. The ins…