Windows® Internals, Sixth Edition, Part 1

Figure 3-40. Dynamic memory configuration dialog

The associated settings include the amount of memory that will be assigned to the VM when it starts (Startup RAM), the maximum amount that it can be assigned (Maximum RAM), the percentage of the VM’s memory that should be available for immediate use by the operating system if its memory demand increases, and finally, the weight of the VM with respect to other VMs. In addition to serving as weighting for the distribution of physical memory among virtual machines that have dynamic memory enabled, the hypervisor also uses it as a guide for the startup order of virtual machines configured to start when the system boots. Finally, the available memory percentage is a reference to memory within the VM that the VM’s operating system has not assigned to a process, device drivers, or itself, and that can be assigned without incurring a page fault. Chapter 10 in Part 2 describes available memory in more detail.

When the DM VSC starts in a child partition that has dynamic memory enabled in its memory configuration, it first checks to see if the operating system supports dynamic memory capabilities. It performs this check by simply calling the memory manager’s hot-add memory function, specifying a block of child physical memory already assigned to the virtual machine. If the memory manager supports hot add, it returns an error indicating that the address range is already in use, and if it doesn’t, it reports that the function is not supported. If dynamic memory is supported, the DM VSC establishes a connection to the DM VSP via VMBus. Because the system’s memory usage fluctuates during the boot process, after all autostart Windows services have finished initializing, the VSC begins reporting memory statistics once per second that indicate the current system commit level in the virtual machine. (See Chapter 10 in Part 2 for more information on system commit.)

The DM VSP in the parent partition calculates a memory pressure value for its corresponding VM using the following calculation based on the VM’s memory report:

Memory Pressure = Committed Memory / Physical Memory

Physical Memory refers to the amount of memory currently assigned to the VM’s partition. It also keeps a running exponential average pressure that represents the previous 20 seconds of pressure reports, adjusting the average pressure only when the current pressure deviates from the average by at least a standard deviation.

A component called the balancer executes in the VMMS service. Once per second, it analyzes the memory pressures reported by the DM VSPs, considers VM policy configuration, and determines if and how much memory should be redistributed. If a global Hyper-V setting called NUMA spanning is enabled, the balancer uses two balancing engines: one engine is the global balancer, and it is responsible for assigning new VMs to NUMA nodes. It does so based on the memory usage and VM pressures of the nodes at the time of the assignment. Each NUMA node has its own local balancer that manages the distribution of the node’s memory across the VMs assigned to the node. If the NUMA spanning option is off, the global balancer has no role other than to invoke the only local balancer for the system.

The benefit of assigning VMs to NUMA nodes is that VMs will be guaranteed the fastest memory accesses possible. The tradeoff, however, is that it might not be possible to start or add memory to a VM in the case where the sum of unassigned memory is sufficient but no one node has enough available memory to accommodate the amount of memory requested.

A local balancer increases or decreases a global target memory pressure to use all available memory under its management or to use it until a minimum pressure level is reached that indicates all VMs have ample memory. The balancer then loops over the VMs, determining how much memory to add or remove from each VM to reach the target pressure. During the calculations, the balancer reserves a minimum amount of memory for the host. The host’s reservation is a base amount of approximately 400 MB plus 30 MB for each 1 GB of RAM on the system. Factors that can affect the amount of memory reserved include whether or not the system is using SLAT or software paging, and whether multimedia redirection is enabled. Every five minutes, the balancer also removes memory from VMs that have so much memory that their pressure is essentially zero.

Note that if the child partition’s operating system is running a 32-bit version of Windows, the dynamic memory engine will not assign the partition more than 4 GB of memory.