Crazy constant disk I/O when swap is disabled

[Marius Gedminas]

I've got an Asus EeePC 900 plugged into a TV as an entertainment
console.  It's got 1 gig of RAM and two SSDs (4 GB and 16 GB).  It has
no swap file/partition because I don't want my SSDs to die again. (ext3
journal turned out to be a reliable way to exhaust the SSDs rewrite
limits in 6 months, probably due to crappy firmware write leveling.)

On a couple of occasions while watching a movie in Totem I noticed the
playback becoming jerky.  Looking at the GNOME system monitor applet I
saw that the system was spending a lot of time in I/O-wait state.  top
confirmed that: 1-10% user, >80% iowait.  vmstat showed that it was all
spent reading the disk, not writing.  top showed no usual suspects (no
cron jobs doing updatedb or popularity-contest).  iotop showed all kinds
of apps constantly reading small bits of data: compiz,
gnome-settings-daemon, gnome-panel, pidgin.

Its as if the kernel was feeling memory pressure and trying to free up
pages all the time, and then had to read them back into memory.  Only
there was plenty of free RAM (> 200 megs, and that's not counting ~280
megs in disk cache -- only about half of RAM was actually used).

When this happened the first time I couldn't figure a way to fix this
other than rebooting.  The second time I created a swap file on an
external USB hard disk and when I enabled it the system quieted down at
once.  free showed less than 500 kilobytes of swap being used.

There were no strange messages in dmesg.

Uptime: over 20 days.

OS: Ubuntu 9.04 (Jaunty)

Kernel: linux-image-2.6.28-14-generic, version 2.6.28-14.47
  (actually, /etc/motd says I need to reboot, so it may be that I'm
  still running an older version of the 2.6.28-14-generic kernel)

Is anyone else running a system without a swap file/partition?  Has
anyone else noticed constant disk I/O?  Is this a bug in the virtual
memory subsystem?

Marius Gedminas
-- 
Alan Turing thought about criteria to settle the question of whether
machines can think, a question of which we now know that it is about
as relevant as the question of whether submarines can swim.
                -- Dijkstra
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