Server processors have been directly responsible for memory addressing since the introduction of Intel Xeon CPUs. Each CPU is assigned a group of memory slots.
This enables direct and fast memory access to the memory assigned to the respective processor. Additionally, if needed, indirect memory access to memory assigned to another CPU is also possible. The technology that enables this direct and indirect access is called NUMA (Non-Uniform Memory Access).
The indirect memory access, where access to the memory of another CPU occurs, is made possible through QuickPath Interconnect (QPI) connections between the processors.
To achieve high performance, it is highly beneficial to equip the different CPUs of a server with identical memory configurations in order to minimize indirect access via the QPI connections.