How to completely disable swap?

Question

I am using Debian sid, hard drive formatted with ext4, running on linux 3.1

I remember on previous linux versions (maybe before 3.0), if I run out of memory, and swap is not enabled, programs will usually crash. This is perfect for my environment: simple web browsing with no critical operations. That is, if I accidentally run across a bad website which uses up too much memory, it just crashes without rendering my terminal unusable.

But in my current setup, the computer hangs with violent I/O throughput in the background. iotop reveals kswapd0 to be the culprit, which means it is due to swapping. After using swapon -s to determine any swaps that were enabled, I used swapoff -a to disable all swaps and swapon -s again to confirm that all swaps were disabled.

Then I tried maximizing my memory usage again. Alas, the behavior I expected didn't happen. Instead, kswapd0 tries over and over to swap out the RAM and fails as there is no swap space. Because it never gives up, my computer is locked in eternal I/O heavy freeze, bad for my disk's health.

Am I doing something wrong in trying to swapoff -a? Why is the behavior different than what it used to be (probably pre-3.0 times)?

Answer 1

Disabling swap won't do what you want. You will still get violent I/O throughput, but it will be of clean pages rather than dirty ones.

With no swap, the system will compress the cache of clean (unmodified) pages to near zero, because those are the only pages it can evict from physical memory. It can only evict dirty (modified) pages from memory by writing them to swap, with no swap, it has no way to evict dirty pages.

As you run low on physical memory, each process will have to load its code pages from disk as it evicts the previous process code pages. The result will be violent thrashing and excessive work done by the swap subsystem.

This is a special case of a very important principle: For a well-designed system, you can't make it run better by reducing its choices. Linux is a well-designed system. Removing swap just gives it fewer choices, so it's not surprising that it behaves worse.

Answer 2

A better solution than turning off swap, which will at best cause random processes to be killed when memory runs low, is to set the per process data segment limit for processes that pull stuff off the net. This way a runaway browser will hit the limit and die, rather than cause the whole system to become unusable. Example, from the shell

(ulimit -d 400000; firefox) &

The number after -d is in kilobytes. You should experiment with this on your system to choose the best value for your browsing habits. The parentheses cause a subshell to be created; the ulimit command only affects that shell and its children, isolating its effects from the parent shell.

Answer 3

To make sure that swap is not used, you'd be better off preventing any swap being added at boot. This can be done, depending on the system, by disabling the swap boot service or just commenting out the swap entry in /etc/fstab.

As far as your hangup is concerned, the stop() function in /etc/init.d/swap might give a clue:

stop()
{
       ebegin "Deactivating swap devices"

       # Try to unmount all tmpfs filesystems not in use, else a deadlock may
       # occure. As $RC_SVCDIR may also be tmpfs we cd to it to lock it
       cd "$RC_SVCDIR"
       umount -a -t tmpfs 2>/dev/null

       case "$RC_UNAME" in
               NetBSD|OpenBSD) swapctl -U -t noblk >/dev/null;;
               *)              swapoff -a >/dev/null;;
       esac
       eend 0
}

Notice the part about deadlock. You can try doing umount -a -t tmpfs yourself before turning swap off.

---

Edit:

Probably, you might also achieve your goal by modifying sysctl settings (see this question).

Answer 4

It is better to comment out swap partition entry in /etc/fstab than running swapoff -a after each boot.

I have the same issue with kswapd0 on my hardware.

Tuning vm.swappiness system parameter does not help for me.

sysctl -w vm.swappiness=0

I googled and read a lot of posts, mailing lists, and now I think that this is kernel bug.

When there is no active swap partition and free memory becomes less then some threshold (about 300MB in my case) the system becomes unresponsive due to kswapd0 madness.

Probably it is reproduced with special configuration and conditions.

For somebody it is solved by system re-installation with re-partitioning for others by building custom kernel with kswapd0 disabled.

Answer 5

On my system (debian sid 2016-11-15), I did this:

1. disable the swap now:

   swapoff -a
   

2. comment the line with swap partition in /etc/fstab (you may not need this, maybe only step 3 without step 2 will work for you)

   #### #UUID=c6ddbc95-3bb5-49e1-ab25-b1c505e5360c none            swap    sw              0       0
   

3. disable the mounting of swap in systemd (Note, wrap the unit name in quotes in case the unit name has backslash characters):

   systemctl --type swap
   systemctl stop "dev-\x2da821.swap"
   systemctl mask "dev-\x2da821.swap"
   

That seems to do the trick.

Answer 6

the computer hangs with violent I/O throughput in the background. iotop reveals kswapd0 to be the culprit

I've found one way (so far) to avoid that. If you want to test it and see how it does on your system, see the kernel patch inside this question. Basically, it doesn't evict Active(file) pages (at least) when under memory pressure, thus the disk thrashing (constant reading) is reduced to almost nothing and OOM-killer is allowed to trigger within 1 second, instead of freezing the OS for what seems like permanently(or at least for many minutes). I am hoping that actual programmers(of which I'm not) would improve the patch and make it into an actual solution, now that they see that what it does is working for these situations.