System unusable when RAM is full

Question

There was always this thing that annoyed me: When the RAM is full and the swapping starts the System is getting slow as hell. And by system I mean the GUI: X + DE. I sometimes had the problem that there was an application that ate all the RAM and therefore Linux swapped any other program to hard disk. When I was quick enaugh to kill the application in time before the system got unusable there was the problem that then all application caches were in Swap and doing aything with previously open programs was slow as hell.
Therefore I reduced the swappiness. I thought I rather throw old data out of Ram instead of caching everything to disk would make it better. But it made it worse.
At the moment I compile wine 1.2 with warcraft patches. At the same time I decided to try lightspark - so I started to compile that too - while playing a youtube video. VERY BAD IDEA.
After some time the youtube video stopped playing. There was some stuttering over the next 10 seconds but then the system was completely unusable:
Input from mouse and keyboard lagged with a latency over a minute, login over ssh needs like two minutes to even display the login.
For about ten minutes I could do nothing with the system. It had heavy IO all the time.
That's what it looks like afterwards:
chris@chrispc ~ % free -m
total used free shared buffers cached
Mem: 1004 572 431 0 0 47
-/+ buffers/cache: 524 480
Swap: 2314 430 1883
If you have less then 2 gb ram and want to try it yourself I can recommend this:
https://bugzilla.gnome.org/show_bug.cgi?id=621025
There is just one question that I have:
Is it possible to not make the Swapping process eat up all ressources?
Can you tell Linux to not swap "important" stuff like X, a Desktop Environment, zsh, sshd AND reserve some ressources to keep this responding?
After so many years of active development of Linux I wonder if somebody has got a solution for this.
Last edited by Cdh (2010-07-20 21:20:41)

Answer 1

korpenkraxar wrote:
I can relate to this issue. Desktop responsiveness has been a problem for years now. It did not use to be like this. Basically, it seems like applications gets swapped to disk as response to the write cache getting big. In my experience, this happens even when you should have enough RAM to deal with it, although those parameters can be tweaked. In addition, the developers have recently acknowledged that the implementation for how processes wait while others are doing I/O is indeed not optimal. KDE usually responds horribly when doing big disk I/O on my system and setting swappiness=0 to stop apps being swapped to disk helps a lot in my case.
Check out these links and follow the development:
https://bugzilla.kernel.org/show_bug.cgi?id=12309
http://www.phoronix.com/forums/showthread.php?t=25065
your links are about different issues. Anyway:
from your link:
https://bugzilla.kernel.org/show_bug.cgi?id=12309
read
post #502 (almost the last one and most current), older post #268 refers to Morton email. Read this
this is basically a proper and working way.
Someone suggested to test VM by loading as many apps as possible in short time. This is most ridiculous test I heard of.
To test VM and I/O scheduler performance one need to keep system with high I/O running for long time, not few seconds, If starting many apps at a time kills your system, then obviously you need to work out more basic problems (once I loaded OOorg with 1MB file, amarok, firefox, dolphin, thunderbird, digikam, gimp, k3b and some smaller stuff. This would not get me even to 1GB of used RAM. not to mention that it has nothing to do with the problem which is VM and I/O scheduler).
If you want your system to be responsive, you need to optimize fs with I/O scheduler and specific system calls.
if swappiness=0 works for you, then you have badly configured system and once you get to real I/O use you will see that swapp is used. By definition swappiness will keep apps out of disk only as long as VM thinks that it can handle apps without swapping, it will swap anyway if VM thinks that this is enough. In consequence with swappinness=0 and still free RAM, you may see disk swapping. Not to mention that this will not help with (fast) swap recovery (if needed).
If swappiness setting keeps you happy then by all means use it of course.