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Non-Uniform Memory Access (NUMA) refers to multiprocessor systems whose
memory is divided into multiple memory nodes. The access time of a
memory node depends on the relative locations of the accessing CPU and
the accessed node. (This contrasts with a symmetric multiprocessor
system, where the access time for all of the memory is the same for all
CPUs.) Normally, each CPU on a NUMA system has a local memory node
whose contents can be accessed faster than the memory in the node local
to another CPU or the memory on a bus shared by all CPUs.
NUMA system calls
The Linux kernel implements the following NUMA-related system calls:
get_mempolicy(2), mbind(2), migrate_pages(2), move_pages(2), and
set_mempolicy(2). However, applications should normally use the inter-
face provided by libnuma; see "Library Support" below.
/proc/[number]/numa_maps (since Linux 2.6.14)
This file displays information about a process's NUMA memory policy and
Each line contains information about a memory range used by the pro-
cess, displaying--among other information--the effective memory policy
for that memory range and on which nodes the pages have been allocated.
numa_maps is a read-only file. When /proc/<pid>/numa_maps is read, the
kernel will scan the virtual address space of the process and report
how memory is used. One line is displayed for each unique memory range
of the process.
The first field of each line shows the starting address of the memory
range. This field allows a correlation with the contents of the
/proc/<pid>/maps file, which contains the end address of the range and
other information, such as the access permissions and sharing.
The second field shows the memory policy currently in effect for the
memory range. Note that the effective policy is not necessarily the
policy installed by the process for that memory range. Specifically,
if the process installed a "default" policy for that range, the effec-
tive policy for that range will be the process policy, which may or may
not be "default".
The rest of the line contains information about the pages allocated in
the memory range, as follows:
The number of pages allocated on <node>. <nr_pages> includes
only pages currently mapped by the process. Page migration and
memory reclaim may have temporarily unmapped pages associated
with this memory range. These pages may show up again only
after the process has attempted to reference them. If the mem-
ory range represents a shared memory area or file mapping, other
processes may currently have additional pages mapped in a corre-
The number of anonymous page in the range.
Number of dirty pages.
Total number of mapped pages, if different from dirty and anon
Maximum mapcount (number of processes mapping a single page)
encountered during the scan. This may be used as an indicator
of the degree of sharing occurring in a given memory range.
Number of pages that have an associated entry on a swap device.
The number of pages on the active list. This field is shown
only if different from the number of pages in this range. This
means that some inactive pages exist in the memory range that
may be removed from memory by the swapper soon.
Number of pages that are currently being written out to disk.
No standards govern NUMA interfaces.
The Linux NUMA system calls and /proc interface are available only if
the kernel was configured and built with the CONFIG_NUMA option.
Link with -lnuma to get the system call definitions. libnuma and the
required <numaif.h> header are available in the numactl package.
However, applications should not use these system calls directly.
Instead, the higher level interface provided by the numa(3) functions
in the numactl package is recommended. The numactl package is avail-
able at The package is also included in some Linux distributions. Some
distributions include the development library and header in the sepa-
rate numactl-devel package.
get_mempolicy(2), mbind(2), move_pages(2), set_mempolicy(2), numa(3),