[lug] Temporary system hanging

[Daniel Webb]

I have a strange situation where my system (I'll elaborate below) hangs for
2-10 seconds, then keeps going.  This can happen up to 5 times a minute, so
it's really getting on my nerves.

My "system" where this happens consists of:

  1) I actually type at old Thinkpad laptops running Debian stable and VNC4
     clients.  I've been using this setup for years with almost no changes,
     so I'm nearly certain the hang isn't on these systems.

  2) The VNC server is running on a Linux 2.6.15 Debian stable system with 1.5G memory, 
     two PATA drives setup as Linux soft RAID-1 with LVM over the top of that, an
     NFS server that gets regular use, an openvnc client, and a couple of USB2
     hard drives (that aren't NFS shared).

Other data:

  - When the hang is happening, if I keep typing, the letters I typed come out
    garbled when the hang ends.  This makes me thing it is NOT a network hang,
    since I've seen that plenty before and it just queues up your typing till
    the connection returns.

  - Rebooting doesn't help

  - I recently went from 512MB memory to 1.5G memory, and this problem started
    around the same time.  I can't be confident that it is the cause though,
    because I have also recently made numerous other changes.

My suspicions:

  - Back with Linux 2.4	I gave up using USB hard drives because it would hang
    the system intermittently like this, although that was much worse, with
    20-60 second hangs every minute or so.  This makes me wonder if the Linux
    USB code still stinks in 2.6?  I had heard it was much better.

  - The time and spacing of these hangs makes me suspect buffer flushing of
    some kind.  Any ideas?

  - Could the additional memory be exacerbating this problem?

I thought I would put this out on the list before I spend the time to start
methodically disconnecting subsystems like NFS and USB to save some time and
document the problem in case others are seeing the same thing.