backing up ext4 fs, system unresponsive, thrashing like crazy eventhough swap is unused

From: Dimitrios Apostolou
Date: Sun Nov 25 2012 - 10:03:36 EST

Hello list,

on an old PIII-500MHz laptop, 128MB RAM, kernel 3.6.6, I started a
backup process (tar|xz -4, nice'd and ionice'd -c3) from ext4 on local
ATA disk to ext3 on external USB disk (USB-2.0 port on PCMCIA card).
Even though earlier system load was minimal, free memory was plenty, the
system now is unresponsive and is thrashing the disk, but the swapfile
is rarely touched.

I managed to get some information by renicing a root console to -20, but
even then each keypress showed with a minimum 10s delay! In the attached
file you can see the dmesg output, SysRq+{W,T}, ps, vmstat, slabs,

I think I'm seeing paging of executable pages because ext4_inode_cache
is aggressively using all memory, evicting other pages. However under no
condition should realtime processes be unresponsive. What do you think?
Please note that I've set vm.swappiness to 0 and gradually increased
vm.vfs_cache_pressure to 1000000, but see no difference.

Thanks in advance,

P.S. Please CC me in all replies

Attachment: HANG.log.gz
Description: GNU Zip compressed data