numa - overview of Non-Uniform Memory Architecture
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
NUMA system calls¶
The Linux kernel implements the following NUMA-related system calls:
, and set_mempolicy(2)
. However, applications
should normally use the interface provided by libnuma
"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 process,
displaying—among other information—the effective memory policy for
that memory range and on which nodes the pages have been allocated.
is a read-only file. When /proc/<pid>/numa_maps
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
The first field of each line shows the starting address of the memory range.
This field allows a correlation with the contents of the
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 effective 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 only show up
again after the process has attempted to reference them. If the memory
range represents a shared memory area or file mapping, other processes may
currently have additional pages mapped in a corresponding memory
- The file backing the memory range. If the file is mapped as
private, write accesses may have generated COW (Copy-On-Write) pages in
this memory range. These pages are displayed as anonymous pages.
- Memory range is used for the heap.
- Memory range is used for the stack.
- Huge memory range. The page counts shown are huge pages and
not regular sized pages.
- The number of anonymous page in the range.
- Number of dirty pages.
- Total number of mapped pages, if different from
dirty and anon pages.
- 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
- The number of pages on the active list. This field is only
shown 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
The Linux NUMA system calls and /proc
interface are only available if the
kernel was configured and built with the CONFIG_NUMA
Link with -lnuma
to get the system call definitions. libnuma
the required <numaif.h>
header are available in the
However, applications should not use these system calls directly. Instead, the
higher level interface provided by the numa(3)
functions in the
package is recommended. The numactl
package is available
The package is also included in some Linux distributions. Some distributions
include the development library and header in the separate
No standards govern NUMA interfaces.
This page is part of release 3.44 of the Linux man-pages
description of the project, and information about reporting bugs, can be found