![]() ![]() Nothing fancy, right? Looks familiar, doesn’t it? Let’s kick this off by looking at how CPUs are used in regular Windows. Understanding Operating System Processor Scheduling So, does this mean that you just can’t meet that vendor request to dedicate a core or two? Well, not exactly. You cannot assign a physical core to a VM at all. Assigning 2 vCPUs to a system does not mean that Hyper-V plucks two cores out of the physical pool and permanently marries them to your virtual machine. Make sure that you understand this section before moving on. ![]() Hyper-V Never Assigns Physical Processors to Specific Virtual Machines The hypervisor controls access to the real CPUs just as it controls access to all other hardware. The hypervisor uses the physical host’s real CPUs to create virtual constructs to present to virtual machines. Like all other virtual machine “hardware”, virtual CPUs do not exist. It has been fully updated to be relevant as of November 2019. Note: This article was originally published in February 2014. If the device frequently requires more than 50 microseconds to update state, consider setting up a device-dedicated thread in the driver.Did your software vendor indicate that you can virtualize their application, but only if you dedicate one or more CPU cores to it? Not clear on what happens when you assign CPUs to a virtual machine? You are far from alone. The granularity of a KeStallExecutionProcessor interval is one microsecond. The driver should minimize the interval it stalls and should, in general, specify a stall interval no longer than 50 microseconds. Note Follow this implementation guideline if the driver must stall while the device hardware updates state:Ī driver can call KeStallExecutionProcessor before it reads device registers. Furthermore, if the driver is compiled with a good optimizing compiler, the compiler might remove the driver's counter variable and the loop(s) where it is incremented. The loop counter maximum would require customization for each platform. Such devices might require polling or some other means of waiting for the device to update its registers.Īlthough it might seem logical to solve a slow-device problem by coding a simple loop that increments a counter, thereby "wasting" a minimum interval while the device updates registers, such a driver is unlikely to be portable across Windows platforms. Nevertheless, some devices still in use were designed to work with old processors, which had narrow data buses, slow clock rates, and single-user, single-tasking operating systems that did synchronous I/O. Recently developed devices, which are as technologically advanced as the processors on which Windows is designed to run, seldom require a driver to poll its device, either to ensure that the device is ready to start an I/O operation or that an operation is complete. A device driver that does much polling interferes with I/O operations on other devices and can make the system slow and unresponsive to users. Polling a device is an expensive operation that makes any operating system compute-bound within the polling driver. A device driver should avoid polling its device unless absolutely necessary, and should never use a whole time-slice for polling. ![]()
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