The problem I think is somewhat similar to this thread.
It doesn't matter if I have swap enabled or disabled, whenever the real used RAM amount starts going close to maximum and there is almost no space left for disk cache, system becomes totally unresponsive.
Disk is spinnig wildly, and sometimes after long waits 10-30 minutes it will unfreeze, and sometimes not (or I run out of patiance). Sometimes if I act quickly I can manage to slowly open console and kill some of ram eating applications like browser, and the system unfreezes almost instantly.
Because of this problem I almost never see anything in the swap, only sometimes there are some few MB there, and then soon after this problem appears.
My not so educated guess would be that it is connected somehow to the disk cache being too greedy, or memory management too lenient, so when the memory is needed it is not freed quickly enough and starves the system.
Problem can be achieved really fast if working with lagrge files (500MB+) which are loaded in disk cache and aparantly afterwards system is unable to unload them fast enough.
Any help or Ideas will be greatly appreciated.
For now I have to live in constant fear, when doing something computer can just freeze and I usually have to restart It, if it is really running out of ram I would much more like it to just kill some of userspace applications, like broser (preferably if I could somehow mark which to kill first)
Although the mistery is why doesnt swap save me in this situation.
UPDATE:
It didnt hang for some time, but now I got several occurances again. I am now keeping ram monitor on my screen at all times and when the hang happened it still showed ~30% free (Used by disk cache probably).
Additional symptoms: If at the time I am watching video (VLC player) the sound stops first, after a few seconds the image stops. While the sound has stopped I still have some control over PC, but when the image stops I cannot even move the mouse anymore, so I restarted it after some waiting. Btw, this didnt happen when I started to watch the video but some time in (20min) and I didn't actively do anything else at the time, even though browser and oowrite were open on the second screen the whole time. Basically something just decides to happen at one point and hangs the system.
As per request in the comments I ran dmesg right after the hang. I didnt notice anything weird, but didnt know for what to look, so here it is:
https://docs.google.com/document/d/1iQih0Ee2DwsGd3VuQZu0bPbg0JGjSOCRZhu0B05CMYs/edit?hl=en_US&authkey=CPzF7bcC
Best Answer
To fix this problem I have found that you need to set the following setting to something around 5%-6% of your total physical RAM, divided by the number of cores in the computer:
Keep in mind that this is a per-core setting, so if I have 2GB RAM and two Cores, then I calculated 6% of only 1 GB and added a little extra just to be safe.
This forces the computer to try to keep this amount of RAM free, and in doing so limits the ability to cache disk files. Of course it still tries to cache them and immediately swap them out, so you should probably limit your swapping as well:
(100 = swap as often as possible, 0= swap only on total necessity)
The result is that linux no longer randomly decides to load a whole movie file of approx 1GB in ram while watching it, and killing the machine in doing so.
Now there is enough reserved space to avoid memory starvation, which aparrently was the problem (seeing as there are no more freezes like before).
After testing for a day - lockups are gone, sometimes there are minor slowdowns, because stuff gets cached more often, but I can live with that if I dont have to restart computer every few hours.
The lesson here is - default memory management is just one of use cases and is not allways the best, even though some people try to suggest otherwise - home entertainment ubuntu should be configured differently than server.
You probably want to make these settings permanent by adding them to your
/etc/sysctl.conflike this: