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Top Forums UNIX for Dummies Questions & Answers Size of swap partition during installation Post 302354052 by Scrutinizer on Thursday 17th of September 2009 02:34:30 AM
Old 09-17-2009
Quote:
Originally Posted by Corona688
Let me get this straight... It wastes cycles in advance, to waste swap in advance, so processes can allocate vastly more memory than they need and just not instantiate it, to avoid overcommit problems. In what sense is this conservative?

Overcommit may be dangerous in some circumstances, but there isn't enough disk space in the world to swap-back a 64-bit address space.
Hi Corona688,

It does not waste cycles, it just reserves the appropriate amount of paging space in one go when a process starts, so it does not have to do it in small bits for every page it needs in strained memory conditions when there may also be a lot of CPU usage. In this sense it is conservative. It does not actually use the paging space. it just reserves it. These are just design choices, each with their own pros and cons.

Also, this is not about inefficient apps, but about reaping the benefits of demand paging and efficient memory use, or in other words, not needlessly wasting RAM.

---------- Post updated at 10:34 PM ---------- Previous update was at 09:25 PM ----------

Quote:
Originally Posted by jlliagre
HP-UX has indeed this bizarre approach by default and is the only current Unix lacking virtual swap. This design limitation can be partially overcome by enabling pseudo-swap.

Overcomitting memory is a different and dubious beast.
Hi jlliagre,

Actually pseudo-swap is the default on HP-UX systems. I think virtual swap or pseudo-swap is a terrible idea, especially if you make it the default. It leads admins into thinking that they do not need to allocate swap space, while there are penalties involved in doing so.

It effectively means you are using memory for paging space, well basically you are marking pages as non-pageable. This part of memory will then fill up much more quickly than usual, so in fact you are throwing away the benefit of demand-paging while keeping its downside.

Also what happens under load is that only a small part of memory actually gets paged out and if paging then occurs, then instead of a smooth transition it will be much more abruptly with the most recent processes taking the brunt.

In other words the system will waste copious amounts of memory and becomes less stable under load. The only time it is useful is when you have much more internal memory then you are ever going to need or you do not have room for swap space.

S.
 

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SWAPON(8)						     Linux Programmer's Manual							 SWAPON(8)

NAME
swapon, swapoff - enable/disable devices and files for paging and swapping SYNOPSIS
/sbin/swapon [-h -V] /sbin/swapon -a [-v] [-e] /sbin/swapon [-v] [-p priority] specialfile ... /sbin/swapon [-s] /sbin/swapoff [-h -V] /sbin/swapoff -a /sbin/swapoff specialfile ... DESCRIPTION
Swapon is used to specify devices on which paging and swapping are to take place. Calls to swapon normally occur in the system multi-user initialization file /etc/rc making all swap devices available, so that the paging and swapping activity is interleaved across several devices and files. Normally, the first form is used: -h Provide help -V Display version -s Display swap usage summary by device. Equivalent to "cat /proc/swaps". Not available before Linux 2.1.25. -a All devices marked as ``swap'' swap devices in /etc/fstab are made available. Devices that are already running as swap are silently skipped. -e When -a is used with swapon, -e makes swapon silently skip devices that do not exist. -p priority Specify priority for swapon. This option is only available if swapon was compiled under and is used under a 1.3.2 or later kernel. priority is a value between 0 and 32767. See swapon(2) for a full description of swap priorities. Add pri=value to the option field of /etc/fstab for use with swapon -a. Swapoff disables swapping on the specified devices and files. When the -a flag is given, swapping is disabled on all known swap devices and files (as found in /proc/swaps or /etc/fstab). NOTE
You should not use swapon on a file with holes. Swap over NFS may not work. SEE ALSO
swapon(2), swapoff(2), fstab(5), init(8), mkswap(8), rc(8), mount(8) FILES
/dev/hd?? standard paging devices /dev/sd?? standard (SCSI) paging devices /etc/fstab ascii filesystem description table HISTORY
The swapon command appeared in 4.0BSD. Linux 1.x 25 September 1995 SWAPON(8)
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