Locality Group Affinity

The kernel assigns a thread to a locality group when the lightweight process (LWP) for that thread is created. That lgroup is called the thread's home lgroup. The kernel runs the thread on the CPUs in the thread's home lgroup and allocates memory from that lgroup whenever possible. If resources from the home lgroup are unavailable, the kernel allocates resources from other lgroups. When a thread has affinity for more than one lgroup, the operating system allocates resources from lgroups chosen in order of affinity strength. Lgroups can have one of three distinct affinity levels:

1. LGRP_AFF_STRONG – Indicates strong affinity. If this lgroup is the thread's home lgroup, the operating system avoids rehoming the thread to another lgroup if possible. Events such as dynamic reconfiguration, processor, offlining, processor binding, and processor set binding and manipulation might still result in thread rehoming.

2. LGRP_AFF_WEAK – Indicates weak affinity. If this lgroup is the thread's home lgroup, the operating system rehomes the thread if necessary for load balancing purposes.

3. LGRP_AFF_NONE– Indicates no affinity. If a thread has no affinity to any lgroup, the operating system assigns a home lgroup to the thread .

The operating system uses lgroup affinities as advice when allocating resources for a given thread. The advice is factored in with the other system constraints. Processor binding and processor sets do not change lgroup affinities, but might restrict the lgroups on which a thread can run.