What is a multisocket motherboard?
Most desktops and laptops have one CPU socket. A multisocket (multiprocessor) motherboard has two or more, each holding a separate physical processor package with its own cores. Dual-socket boards, often written 2P, are the most common; four- and eight-socket systems exist for very large database and virtualization hosts. The operating system sees all the cores from every socket as one pool of processors.
Do not confuse multisocket with multicore. Multicore means several cores inside one CPU package, which Core 1 covers as core configurations under CPU architecture. Simultaneous multithreading such as Hyper-Threading adds a second hardware thread per core. A dual-socket server with two 16-core CPUs has 32 physical cores, and 64 threads if SMT is enabled.
Requirements and compatibility
- Multi-socket capable CPUs. The processor must be designed for multiprocessor systems. Server lines such as Intel Xeon and AMD EPYC include multi-socket models, while mainstream desktop processors such as Intel Core and AMD Ryzen are single-socket only. Even within server lines, some models are single-socket only, so check the specification sheet.
- Matching processors. Install identical CPUs: the same model, and the same stepping where the board or CPU vendor requires it. Mixing models is not supported.
- Socket type. As with any board, the CPU must match the socket and appear on the board's supported CPU list.
- Interconnect. The processors talk to each other over a high-speed link, Ultra Path Interconnect (UPI) on current Intel Xeon platforms and Infinity Fabric on AMD EPYC.
- Power and cooling. Each socket typically has its own 8-pin 12 V CPU power connector and its own heat sink. Rack servers usually use passive heat sinks cooled by high-airflow chassis fans.
- Size. The extra sockets and memory slots need larger boards, such as E-ATX or proprietary server boards, so they do not fit standard ATX cases.
Memory and NUMA
Each CPU has its own memory controller and its own set of DIMM slots, so multisocket boards hold far more RAM than desktop boards. Servers normally use ECC registered memory (RDIMMs). Many boards can run with only the first socket filled, but the DIMM slots and some PCIe slots wired to an empty socket do not work until a second CPU is installed.
Because memory is attached to a specific CPU, a core reaching memory on the other CPU must go across the interconnect, which is slower than using its own local memory. This design is called NUMA (non-uniform memory access). Server operating systems and hypervisors are NUMA-aware and try to keep a process or virtual machine's threads and memory on the same socket.
Where multisocket boards are used
- Virtualization hosts running many virtual machines on a Type 1 hypervisor
- Database, application, and file servers that need many cores and large amounts of RAM
- Engineering, rendering, and scientific workstations
Some server software is licensed per processor or per core, so adding a second CPU can raise licensing costs as well as hardware costs.
Upgrading a multisocket system
- Confirm the board supports the CPU model and update the firmware first if the support list requires a newer version.
- Buy a CPU identical to the one already installed.
- Install it with its own heat sink and thermal paste, and connect its CPU power cable.
- Add memory to the new CPU's slots, following the board manual's population order so each processor's channels are balanced.
- Check in firmware or the OS that both processors and all memory are detected.
Multisocket motherboard on the CompTIA A+ exam
Core 1 objective 3.5 lists multisocket under motherboard compatibility, next to CPU socket types for AMD and Intel. Know that multisocket means more than one physical CPU, that it is found in servers and high-end workstations, that the CPUs must be identical and multi-socket capable, and that each CPU brings its own memory slots. If a question describes a server with an empty second CPU socket that needs more processing power, the answer is a matching CPU of the same model, not a different or faster one.