Processor groups is one way around this, to stop threads jumping from CPU to CPU. This occurs when the CPU cores in the system might have different latencies to main memory, such as within a dual socket system: it can be quick to access the memory directly attached to its own core, but it can be a lot slower if a core needs to access memory attached to the other physical CPU. If this sounds somewhat familiar, then you may have heard of NUMA, or non-uniform memory architecture. This means that if a multi-threaded program can use 128 threads, if it isn’t built with processor groups in mind, then it might only spawn with access to 64. When the program is running inside the group, unless it is processor group aware, then it can only access other threads in the same group.
When a program is launched, it will be pushed into one of the processor groups based on load – if one group is busy, the program will be spawned in the other. When the system is in this mode, it becomes very tricky for most software to operate properly. This is split into two groups, as shown above. With our 64 core processor, when simultaneous multithreading is enabled, we get a system with 128 threads. This is most easily observed by going into task manager and trying to set the affinity of a particular program: The way this is done is very rudimentary: of the enumerated cores and threads, the first 64 go into the first group, the second 64 go into the next group, and so on. Whenever Windows experiences more than 64 threads in a system, it separates those threads into processor groups. Now to be clear, Microsoft never expected home (or even most workstations) systems to go above this amount, and to a certain extent they are correct. The problem that these operating systems have rears its ugly head when we go above 64 threads. Most home users that have Windows typically have versions of Windows 10 Home or Windows 10 Pro, which are both fairly ubiquitous even among workstation users. Unfortunately, not everything is just as straightforward as installing Windows 10 and going off on a 128 thread adventure.
The Windows and Multithreading Problem (A Must Read)
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