limit swap caching

timblech · Tux's lil' helper Joined: 28 Oct 2003 Posts: 89

hi all,

on my system, most of my memory is used by caching the swap space in physical memory:

chunderbunny · Posted: Sat Jan 21, 2006 6:39 pm Post subject:

I'm not sure exactly what you mean, but are you saying you want your system to use swap space less often? Swap is never cached in physical memory, the whole point of swap space is to cache the physical memeory, not the other way around. What is cached in physical memory is stuff that is on your hard disk, applications, libraries etc. The kernel can free this up instantly if it needs to, so it is not considered part of the used RAM. If an application suddenly needs an extra 100MB of RAM then the kernel will just dump 100MB of disk cache to make room.

Having said that, if you do want to alter the amount of swap space the kernel uses you can alter the /proc/sys/vm/swappiness file. It contains a number between 0 and 100, 0 means "never use swap ever" and 100 means "use swap as much as possible". The default value is 60. This file is recreated on reboot, so you will need to stick a command in /etc/conf.d/local.start or in your crontab if you want it to be set all the time.

timblech · Tux's lil' helper Joined: 28 Oct 2003 Posts: 89

yes, i'm familiar with the swappiness, but i'm concerned about the "cached 503848" of the free output ...

it has nothing to do with swapping, but with the caching of the swapping ...

cheers ... tim

SinoTech · Posted: Sat Jan 21, 2006 7:43 pm Post subject:

timblech · Tux's lil' helper Joined: 28 Oct 2003 Posts: 89

SinoTech · Posted: Sat Jan 21, 2006 9:01 pm Post subject:

Never used "free" and never tries to understand it. But think "cached" means the size of data currently stored in memory which is NOT used by an running process (Data which could also be swapped), but since you've got enough memory it is still kept in memory (For linux there's no reason to write that data to swap). Now when allocating hundred megabytes of ram, linux recognize it must swap unnneded data to your hard drive and then starts to do that. Sure, if swapping from the beginning you could allocate memory alot faster, but linux doesn't know that you will run an application which needs hundred memgabytes of ram at once, and so it starts swapping only if needed. If you want to tell inux to start swapping earlier, use the swappiness method mentioned above.
So here's my summary:
Swapping early
- Kept your memory free and so you can allocate much memory very fast
- Can slow down applications since linux needs often to reload data from swap

Don't swapping (or less as possible)
- Don't decrease the speed of your applications since reloading of data from swap isn't needed
- Slow down applications if they allocate a huge amount of memory which isn't currently available (So linux must write alot of data to swap first)

Anyway, it could also be possible that I am wrong with all my statements here (Not sure if they are all correct), so tell me if I am wrong.
Regards,

Sino

Gentoo_freak · n00b Joined: 07 Oct 2005 Posts: 10

[quote="timblech"]