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.\"
.\" This file and its contents are supplied under the terms of the
.\" Common Development and Distribution License ("CDDL"), version 1.0.
.\" You may only use this file in accordance with the terms of version
.\" 1.0 of the CDDL.
.\"
.\" A full copy of the text of the CDDL should have accompanied this
.\" source. A copy of the CDDL is also available via the Internet at
.\" http://www.illumos.org/license/CDDL.
.\"
.\"
.\" Copyright 2017, Joyent, Inc.
.\"
.Dd Aug 14, 2017
.Dt SWAP 4
.Os
.Sh NAME
.Nm swap
.Nd swap space
.Sh DESCRIPTION
The operating system uses demand paging as the primary mechanism to implement
virtual memory.
The system can also use traditional swapping, wherein an entire process state
is moved between physical memory and swap space on disk, but it is very rare
for the system to actually swap an entire process out to disk.
A system which is swapping does not have enough enough physical memory to
support the workload on the machine.
In this case, work should be reduced or more memory added.
.Pp
Given that the system rarely, if ever, swaps, but still has swap space
configured, the question arises as to what this space is for?
.Pp
In a demand paged virtual memory system, every page mapped by a process is
backed by some object.
For the actual program code used by the process, the backing objects are the
underlying files in the filesystem for the program's binary, and any
dynamically linked libraries.
The portions of a process that are not backed by a named file, including its
stack and its heap, are called anonymous memory.
The system uses the swap space as the backing store for these pages.
When the system determines that it needs to page out one of the anonymous pages,
the swap space is where that page is written.
.Pp
Unlike some other operating systems, illumos reserves the backing storage
space for anonymous memory at the time of allocation.
For example, if a process asks for more heap space, the total size of the
swap allocation that may be required to store the new pages if they need to
be paged out is reserved at that time.
This does not mean that anything is written to the swap space, but simply that
the space is reserved for the entire allocation.
Thus, a process will always get a correct error from the
.Xr sbrk 2
system call if swap space is unavailable.
Some other operating systems don't allocate backing store for anonymous memory
until it is used, so the error handling when space is not available can be
complex or problematic on those systems.
.Pp
The
.Xr vmstat 8
command can be used to monitor swap and paging activity.
The
.Xr pmap 1
command can be used to inspect all of the mappings in a process address space,
and their backing objects.
The
.Xr swap 8
command can be used to monitor, add and remove swap space.
.Pp
The operating system provides the
.Ql zone.max-swap
resource control to limit the amount of
anonymous memory used by all of the processes within a zone.
This resource control can also be configured under the
.Xr capped-memory 2
setting for a zone.
See the
.Xr prctl 1
and
.Xr zonecfg 8
man pages for information on setting this limit.
The zone's usage against this resource control can be seen using the
.Ql swapresv_zone_{zoneid}
kstat.
.Sh SEE ALSO
.Xr pmap 1 ,
.Xr prctl 1 ,
.Xr sbrk 2 ,
.Xr resource_controls 7 ,
.Xr kstat 8 ,
.Xr swap 8 ,
.Xr vmstat 8 ,
.Xr zonecfg 8
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