PIO disk writes using 100% system time and performing poorly with VIA vt82c686b on kernels 2.2 & 2.4

[Thomas Ford]

Heavy disc writes (eg. unzipping linux kernel source) cause the system
processor usage (as reported by top/xosview) to jump to 100%, making
the X mouse/audio freeze etc.

Such problems occur with the drives connected to VIA vt82c686b south
bridge: the same drives on a mvp3 show no such problems.

The behaviour is the same on kernels 2.2.17 & 2.4.3 (both hand
compiled & RedHat's 2.4.2-2 & 2.2.17-14 in case I was doing something
wrong).

The problem is easily demonstrated by hdparm -t. The CPU use jumps to
system 100% as above and all my drives report ~1.9 MB/sec in PIO mode
which is far lower than PIO on the mvp3 (~10MB/s).

DMA mode appears to work fine but I am not using it due to publicised
potential problems.

Regards,

Tom Ford