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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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dynamic_pager(8)					    BSD System Manager's Manual 					  dynamic_pager(8)

NAME
dynamic_pager -- external storage manager for dynamic pager SYNOPSIS
dynamic_pager [-F filename] [-S filesize] [-H high-water-trigger] [-L low-water-trigger] [-P priority] DESCRIPTION
The dynamic_pager daemon manages a pool of external swap files which the kernel uses to support demand paging. This pool is expanded with new swap files as load on the system increases. It is contracted when the swapping resources are no longer needed. The dynamic_pager daemon also provides a notification service for those applications which wish to receive notices when the external paging pool expands or contracts. OPTIONS
-F The base name of the filename to use for the external paging files. By default this is /private/var/vm/swapfile. -H If there are less than high-water-trigger bytes free in the external paging files, the kernel will signal dynamic_pager to add a new external paging file. -L If there are more than low-water-trigger bytes free in the external paging files, the kernel will coalese in-use pages and signal dynamic_pager to discard an external paging file. Low-water-trigger must be greater than high-water-trigger + filesize. -P This option is currently unimplemented. -S The fixed filesize [in bytes] to use for the paging files. By default dynamic_pager uses variable sized paging files, using larger sized files as paging demands increase. The -S, -H and -L options disable that default and cause dynamic_pager to use a series of fixed sized external paging files. FILES
/private/var/vm/swapfile* Default external paging files. /Library/Preferences/com.apple.virtualMemory.plist Configuration file. XML PROPERTY LIST KEYS
The following keys can be specified in the configuration file. Please see plist(5) for more information about property list files. UseEncryptedSwap <boolean> This optional key activates encrypted swap (aka Secure VM), so that all data is encrypted before being written to a swap file. The default is on for portable computers and off for other computers. Mac OS X July 8, 2003 Mac OS X
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