How does swap in Linux work?

Question

I thought I knew, but I am not so sure anymore.
I have 4GB ram and 2GB swap (I know it works because I even have hibernation), fast SATA HDD and Core 2 Duo.
On occasions I run opera (~1GB), and two virtual machines. As soon as RAM utilization goes in the vicinity of 3.8GB, my system becomes totally unresponsive and to recover it, I try to close opera window, but the first challenge is to actually move a mouse, and the second to wait about a minute before the click takes any effect.
I was thinking of putting swap on a USB flash drive to make things fast, but judging from this, it does not look like swap is being used at all.

Answer 1

meph wrote:This is odd. I'm starting to doubt that it's caused by swapping itself. Just think about it, how does running out of memory present itself? It could hang, it could give you all sorts of error messages, but being unresponsive without any errors is weird. It means that the system is actually running but is being slowed down by something. Misbehaving process, whatever.
I don't know. Windows behaves pretty much as my Arch when it runs out of memory.
meph wrote:
1) Try using swap file instead. Using it will mean that different physical location on your hdd will be used, and also that it will use your root partition, which is known to work well (i guess?). If it still misbehaves after you start using it, you will rule out faulty hdd (however unlikely that already is).
2) Did you ever have this problem without using your VM's? Try and use all your memory without running those, and see what happens. For example opening a dvd image in a text editor could work? Just fill your memory with random garbage and see what happens, and while doing that you may want to watch htop again. This should rule out a problem in whatever virtualization software you're using.
3) Try and turn swap off before trying to induce the problem. Again, this is about knowing how exactly will your system behave when it actually runs out of memory. Expected outcome is flooding you with error messages about killing processess, but the system should remain somewhat responsive. Cluttered and obviously not working well, but responsive. This step could help identify a memory defect, obviously the next step would be running a memtest.
4) This one is rather obvious and I suppose you already did it, but look carefully into your log files, especially kernel.log. Or better yet, induce the problem, take a precise note about when exactly things started to go bad, and then look for errors in your logs around that time.
1. That's quite a bother. I gonna have to do it a little bit later, if you think it is absolutely necessary.
2. I opened two ISO files (2GB each) with geany, and the result was pretty much the same, except this time swap usage went up to 700MB, but that could be because htop managed to refres after 2-3 minutes, while previously it did not. But the system was freezing up pretty much the same way.
3. Again, the result is similar. Maybe the system is a bit more responsive, especially in the first minute of running on full RAM, but afterwards everything else locks up, too. Still, the mouse is movable for most of the time, but it could be a coincidence.
silentsnake,
I usually have +20 tabs opened in my browser. I think opera is doing a fairly good job. Certainly better than firefox.
The easiest solution would be to buy more RAM. RAM is very cheap - you can get 4x4 GB (16 GB total) for around 80 €.
Yeah, I wish I was running DDR3. The cheapest 4GB SODIMM DDR2 module on ebay.co.uk is almost £60