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, Convex Computer (later HP), SGI, Sequent and Data General during the 1990s. Techniques developed by these companies later featured in a variety of Unix-like operating systems, as well as to some degree in Windows NT and in later versions of Microsoft Windows.

Basic concept
Nearly all CPU architectures use a small amount of very fast non-shared memory known as cache to exploit locality of reference in memory accesses. With NUMA, maintaining cache coherence across shared memory has a significant overhead.
Although simpler to design and build, non-cache-coherent NUMA systems become prohibitively complex to program in the standard von Neumann architecture programming model. As a result, all NUMA computers sold to the market use special-purpose hardware to maintain cache coherence, and thus class as "cache-coherent NUMA", or ccNUMA.
Typically, this takes place by using inter-processor communication between cache controllers to keep a consistent memory image when more than one cache stores the same memory location. For this reason, ccNUMA performs poorly when multiple processors attempt to access the same memory area in rapid succession. Operating-system support for NUMA attempts to reduce the frequency of this kind of access by allocating processors and memory in NUMA-friendly ways and by avoiding scheduling and locking algorithms that make NUMA-unfriendly accesses necessary.
Current implementations of ccNUMA systems are multi processor systems based on the Intel Itanium and AMD Opteron processor. Earlier ccNUMA approaches were systems based on the MIPS processor in SGI systems and Alpha processor EV7 of Digital Equipment Corporation (DEC).