Serious I/O problems (bad performance and live-lock)

[Daniel Eriksson]

Here are some further observations and speculations.

On a newly booted system, this is what happens:

1. Start a "dd if=/dev/zero of=/usr/test bs=128k".
2. While looking at 'top', "Inact" grows and "Free" shrinks.
3. Once "Free" has bottomed out, "Inact" stops growing (naturally).
4. 'dd' continues to put a load on the VM system, eventually forcing most
processes to be swapped out (illustrated by the "RES" column showing a very
low number for all but a few processes). This takes 30-60 seconds after
"Free" has bottomed out on my machine.
5. At this point the machine is mostly useless because it can take several
minutes to run a simple 'ls'.
6. After a little while in this useless state the machine becomes so
unresponsive that it no longer accepts any input, not even a CTRL-C to end
the 'dd' process. Breaking into DDB and killing 'dd' from there works
though.

I tested this again this morning on a system compiled from sources dated
2005.04.23.05.10.00 (just after the commit to kern_exit.c by David Xu), but
the behavior seems to be mostly the same. I did not manage to get it to
completely lock up though, but I only tested it once on my UP server.

/Daniel Eriksson