Non-Uniform Memory Access

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Non-Uniform Memory Access or Non-Uniform Memory Architecture (NUMA) is a computer memory design used in multiprocessors, where the memory access time depends on the memory location relative to a processor. Under NUMA, a processor can access its own local memory faster than non-local memory, that is, memory local to another processor or memory shared between processors.

NUMA architectures logically follow in scaling from symmetric multiprocessing (SMP) architectures. Their commercial development came in work by Burroughs (later Unisys), Convex Computer (later Hewlett-Packard), Silicon Graphics, Sequent Computer Systems, Data General and Digital during the 1990s. Techniques developed by these companies later featured in a variety of Unix-like operating systems, and somewhat in Windows NT.


Basic concept

Modern CPUs operate considerably faster than the main memory to which they are attached. In the early days of computing and data processing the CPU generally ran slower than its memory. The performance lines crossed in the 1960s with the advent of the first supercomputers and high-speed computing. Since then, CPUs, increasingly starved for data, have had to stall while they wait for memory accesses to complete. Many supercomputer designs of the 1980s and 90s focused on providing high-speed memory access as opposed to faster processors, allowing them to work on large data sets at speeds other systems could not approach.

Limiting the number of memory accesses provided the key to extracting high performance from a modern computer. For commodity processors, this means installing an ever-increasing amount of high-speed cache memory and using increasingly sophisticated algorithms to avoid "cache misses". But the dramatic increase in size of the operating systems and of the applications run on them has generally overwhelmed these cache-processing improvements. Multi-processor systems make the problem considerably worse. Now a system can starve several processors at the same time, notably because only one processor can access memory at a time.

NUMA attempts to address this problem by providing separate memory for each processor, avoiding the performance hit when several processors attempt to address the same memory. For problems involving spread data (common for servers and similar applications), NUMA can improve the performance over a single shared memory by a factor of roughly the number of processors (or separate memory banks).

Of course, not all data ends up confined to a single task, which means that more than one processor may require the same data. To handle these cases, NUMA systems include additional hardware or software to move data between banks. This operation has the effect of slowing down the processors attached to those banks, so the overall speed increase due to NUMA will depend heavily on the exact nature of the tasks run on the system at any given time.

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