|Started Nov 1999 by Kanoj Sarcar <firstname.lastname@example.org>
|What is NUMA?
|This question can be answered from a couple of perspectives: the
|hardware view and the Linux software view.
|From the hardware perspective, a NUMA system is a computer platform that
|comprises multiple components or assemblies each of which may contain 0
|or more CPUs, local memory, and/or IO buses. For brevity and to
|disambiguate the hardware view of these physical components/assemblies
|from the software abstraction thereof, we'll call the components/assemblies
|'cells' in this document.
|Each of the 'cells' may be viewed as an SMP [symmetric multi-processor] subset
|of the system--although some components necessary for a stand-alone SMP system
|may not be populated on any given cell. The cells of the NUMA system are
|connected together with some sort of system interconnect--e.g., a crossbar or
|point-to-point link are common types of NUMA system interconnects. Both of
|these types of interconnects can be aggregated to create NUMA platforms with
|cells at multiple distances from other cells.
|For Linux, the NUMA platforms of interest are primarily what is known as Cache
|Coherent NUMA or ccNUMA systems. With ccNUMA systems, all memory is visible
|to and accessible from any CPU attached to any cell and cache coherency
|is handled in hardware by the processor caches and/or the system interconnect.
|Memory access time and effective memory bandwidth varies depending on how far
|away the cell containing the CPU or IO bus making the memory access is from the
|cell containing the target memory. For example, access to memory by CPUs
|attached to the same cell will experience faster access times and higher
|bandwidths than accesses to memory on other, remote cells. NUMA platforms
|can have cells at multiple remote distances from any given cell.
|Platform vendors don't build NUMA systems just to make software developers'
|lives interesting. Rather, this architecture is a means to provide scalable
|memory bandwidth. However, to achieve scalable memory bandwidth, system and
|application software must arrange for a large majority of the memory references
|[cache misses] to be to "local" memory--memory on the same cell, if any--or
|to the closest cell with memory.
|This leads to the Linux software view of a NUMA system:
|Linux divides the system's hardware resources into multiple software
|abstractions called "nodes". Linux maps the nodes onto the physical cells
|of the hardware platform, abstracting away some of the details for some
|architectures. As with physical cells, software nodes may contain 0 or more
|CPUs, memory and/or IO buses. And, again, memory accesses to memory on
|"closer" nodes--nodes that map to closer cells--will generally experience
|faster access times and higher effective bandwidth than accesses to more
|For some architectures, such as x86, Linux will "hide" any node representing a
|physical cell that has no memory attached, and reassign any CPUs attached to
|that cell to a node representing a cell that does have memory. Thus, on
|these architectures, one cannot assume that all CPUs that Linux associates with
|a given node will see the same local memory access times and bandwidth.
|In addition, for some architectures, again x86 is an example, Linux supports
|the emulation of additional nodes. For NUMA emulation, linux will carve up
|the existing nodes--or the system memory for non-NUMA platforms--into multiple
|nodes. Each emulated node will manage a fraction of the underlying cells'
|physical memory. NUMA emluation is useful for testing NUMA kernel and
|application features on non-NUMA platforms, and as a sort of memory resource
|management mechanism when used together with cpusets.
|For each node with memory, Linux constructs an independent memory management
|subsystem, complete with its own free page lists, in-use page lists, usage
|statistics and locks to mediate access. In addition, Linux constructs for
|each memory zone [one or more of DMA, DMA32, NORMAL, HIGH_MEMORY, MOVABLE],
|an ordered "zonelist". A zonelist specifies the zones/nodes to visit when a
|selected zone/node cannot satisfy the allocation request. This situation,
|when a zone has no available memory to satisfy a request, is called
|"overflow" or "fallback".
|Because some nodes contain multiple zones containing different types of
|memory, Linux must decide whether to order the zonelists such that allocations
|fall back to the same zone type on a different node, or to a different zone
|type on the same node. This is an important consideration because some zones,
|such as DMA or DMA32, represent relatively scarce resources. Linux chooses
|a default zonelist order based on the sizes of the various zone types relative
|to the total memory of the node and the total memory of the system. The
|default zonelist order may be overridden using the numa_zonelist_order kernel
|boot parameter or sysctl. [see Documentation/kernel-parameters.txt and
|By default, Linux will attempt to satisfy memory allocation requests from the
|node to which the CPU that executes the request is assigned. Specifically,
|Linux will attempt to allocate from the first node in the appropriate zonelist
|for the node where the request originates. This is called "local allocation."
|If the "local" node cannot satisfy the request, the kernel will examine other
|nodes' zones in the selected zonelist looking for the first zone in the list
|that can satisfy the request.
|Local allocation will tend to keep subsequent access to the allocated memory
|"local" to the underlying physical resources and off the system interconnect--
|as long as the task on whose behalf the kernel allocated some memory does not
|later migrate away from that memory. The Linux scheduler is aware of the
|NUMA topology of the platform--embodied in the "scheduling domains" data
|structures [see Documentation/scheduler/sched-domains.txt]--and the scheduler
|attempts to minimize task migration to distant scheduling domains. However,
|the scheduler does not take a task's NUMA footprint into account directly.
|Thus, under sufficient imbalance, tasks can migrate between nodes, remote
|from their initial node and kernel data structures.
|System administrators and application designers can restrict a task's migration
|to improve NUMA locality using various CPU affinity command line interfaces,
|such as taskset(1) and numactl(1), and program interfaces such as
|sched_setaffinity(2). Further, one can modify the kernel's default local
|allocation behavior using Linux NUMA memory policy.
|System administrators can restrict the CPUs and nodes' memories that a non-
|privileged user can specify in the scheduling or NUMA commands and functions
|using control groups and CPUsets. [see Documentation/cgroups/cpusets.txt]
|On architectures that do not hide memoryless nodes, Linux will include only
|zones [nodes] with memory in the zonelists. This means that for a memoryless
|node the "local memory node"--the node of the first zone in CPU's node's
|zonelist--will not be the node itself. Rather, it will be the node that the
|kernel selected as the nearest node with memory when it built the zonelists.
|So, default, local allocations will succeed with the kernel supplying the
|closest available memory. This is a consequence of the same mechanism that
|allows such allocations to fallback to other nearby nodes when a node that
|does contain memory overflows.
|Some kernel allocations do not want or cannot tolerate this allocation fallback
|behavior. Rather they want to be sure they get memory from the specified node
|or get notified that the node has no free memory. This is usually the case when
|a subsystem allocates per CPU memory resources, for example.
|A typical model for making such an allocation is to obtain the node id of the
|node to which the "current CPU" is attached using one of the kernel's
|numa_node_id() or CPU_to_node() functions and then request memory from only
|the node id returned. When such an allocation fails, the requesting subsystem
|may revert to its own fallback path. The slab kernel memory allocator is an
|example of this. Or, the subsystem may choose to disable or not to enable
|itself on allocation failure. The kernel profiling subsystem is an example of
|If the architecture supports--does not hide--memoryless nodes, then CPUs
|attached to memoryless nodes would always incur the fallback path overhead
|or some subsystems would fail to initialize if they attempted to allocated
|memory exclusively from a node without memory. To support such
|architectures transparently, kernel subsystems can use the numa_mem_id()
|or cpu_to_mem() function to locate the "local memory node" for the calling or
|specified CPU. Again, this is the same node from which default, local page
|allocations will be attempted.