diff options
Diffstat (limited to 'usr/src/man/man7')
| -rw-r--r-- | usr/src/man/man7/Makefile | 4 | ||||
| -rw-r--r-- | usr/src/man/man7/swap.7 | 92 |
2 files changed, 95 insertions, 1 deletions
diff --git a/usr/src/man/man7/Makefile b/usr/src/man/man7/Makefile index 929693a927..c53357d29a 100644 --- a/usr/src/man/man7/Makefile +++ b/usr/src/man/man7/Makefile @@ -12,6 +12,7 @@ # # Copyright 2011, Richard Lowe # Copyright 2013 Nexenta Systems, Inc. All rights reserved. +# Copyright 2017 Joyent, Inc. # include $(SRC)/Makefile.master @@ -22,7 +23,8 @@ MANSECT= 7 MANFILES= FSS.7 \ Intro.7 \ cpr.7 \ - ibmf.7 + ibmf.7 \ + swap.7 MANLINKS= intro.7 diff --git a/usr/src/man/man7/swap.7 b/usr/src/man/man7/swap.7 new file mode 100644 index 0000000000..d060e993a9 --- /dev/null +++ b/usr/src/man/man7/swap.7 @@ -0,0 +1,92 @@ +.\" +.\" 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 7 +.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 1m +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 1m +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 1m +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 kstat 1m , +.Xr swap 1m , +.Xr vmstat 1m , +.Xr zonecfg 1m , +.Xr sbrk 2 , +.Xr resource_controls 5 |