Processor affinity, also known as CPU affinity, is a feature of Windows that allows an application or service to be bound to a specific processor or processors out of all the available CPUs on a system. This enables the application or service to run only on the assigned processor or processors, and can be beneficial in certain scenarios, such as when an application or service needs to access data from a specific processor more quickly than other processors.
When it comes to processor affinity and hyperthreaded CPUs, the concept is the same, but the application of the feature can be slightly different. Hyperthreading is a technology that allows a single CPU to act as multiple virtual processors, which can improve performance in some scenarios. With hyperthreading enabled, the system will appear to have more processors than it actually does.
When setting processor affinity for an application on a system with hyperthreading enabled, it is important to understand how the system views the virtual processors. When setting processor affinity for an application, Windows will actually assign the application to two physical CPUs, rather than just one. This is because the hyperthreaded CPU acts as two separate logical processors, and so the application must be assigned to both of them.
For example, if the system has two physical CPUs, each with hyperthreading enabled, the system will appear to have four processors. In this case, if the application is assigned to processor number two, Windows will actually assign the application to both physical processor number one and physical processor number two, since the virtual processor number two is actually made up of both physical processors.
In some cases, this can be beneficial, since it allows the application to access data from both physical processors more quickly. However, it can also be detrimental to performance, since the application will use more resources, as it is spread across two physical processors.
Overall, when it comes to processor affinity and hyperthreaded CPUs, it is important to understand how the system views the virtual processors, and how assigning an application to one of them will actually assign the application to two physical processors. In some cases, this can be beneficial, but in others, it can cause a decrease in performance. As such, it is important to consider the impact of assigning processor affinity carefully, and to always be aware of how the system views the virtual processors.
