Power Mac G4 hangs under heavy IO load

[Mihkel Ader <nodscytale%gmail.com@localhost>]

5.99.59 ran fine, 6.0 BETA and 6.99.7 (at least) have the following problem:

The machine becomes unresponsive after a few minutes when stressing the disks
(try to build NetBSD, copy files from one disk to another, cat disk
image to disk device).

Unresponsive means that it can be pinged, but no keyboard input is
possible on console,
can't SSH in, OTOH some programs like top in an existing SSH session
keep working
(quitting top would leave an unresponsive terminal window though). The
machine hangs
forever - it doesn't come back when the disk activity should have finished.

An anomaly in top output - the gzcat process would show CPU usage, but
the summary
line says 100% idle.

No messages in system logs or dmesg.

May be unimportant or unrelated, but when the machine hasn't crashed
yet, top shows
abnormally high "interrupt" and "system" usage.

The hardware is fine, no problems with 5.1.2 that I just installed and
it used to hang no matter which one of the three disks I stressed for testing.

Dmesg (from 5.1.2) attached.
wd2 no disklabel message is normal - it's a disk for another OS shared
over iSCSI.
MYGENERIC kernel means that it has cgd and filemon enabled (why is
there no cgd in
macppc GENERIC?)

Copyright (c) 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005,
    2006, 2007, 2008, 2009, 2010, 2011, 2012
    The NetBSD Foundation, Inc.  All rights reserved.
Copyright (c) 1982, 1986, 1989, 1991, 1993
    The Regents of the University of California.  All rights reserved.

NetBSD 5.1.2 (MYGENERIC) #0: Thu May 31 21:32:09 EEST 2012
        root@ubin:/usr/src/sys/arch/macppc/compile/MYGENERIC
total memory = 768 MB
avail memory = 733 MB
timecounter: Timecounters tick every 10.000 msec
found openpic PIC at 80040000
OpenPIC Version 1.2: Supports 4 CPUs and 64 interrupt sources.
bootpath: /pci@f2000000/mac-io@17/ata-4@1f000/disk@0:3,/netbsd
mainbus0 (root)
cpu0 at mainbus0: 7410 (Revision 1.3), ID 0 (primary)
cpu0: HID0 8094c0a4<EMCP,DOZE,DPM,EIEC,ICE,DCE,SGE,BTIC,BHT>, powersave: 1
cpu0: 466.67 MHz, no-parity 1MB WB L2 cache (PB SRAM) at 2:1 ratio
memory0 at mainbus0: len=512
spdmem0 at memory0
spdmem0: SDRAM memory, no parity or ECC, 256MB, 133MHz (PC-1100)
spdmem0: 12 rows, 10 cols, 2 banks, 4 banks/chip, 7.5ns cycle time
spdmem0: tAA-tRCD-tRP-tRAS: 3-20-20-45
spdmem0: voltage LvTTL (not 5V tolerant), refresh time 15.625us 
(self-refreshing)
spdmem1 at memory0
spdmem1: SDRAM memory, no parity or ECC, 512MB, 133MHz (PC-1100)
spdmem1: 13 rows, 10 cols, 2 banks, 4 banks/chip, 7.5ns cycle time
spdmem1: tAA-tRCD-tRP-tRAS: 3-20-20-45
spdmem1: voltage LvTTL (not 5V tolerant), refresh time 7.8us (self-refreshing)
spdmem2 at memory0
spdmem2: SDRAM memory, no parity or ECC, 128MB, 100MHz (PC-800)
spdmem2: 12 rows, 9 cols, 2 banks, 4 banks/chip, 10.0ns cycle time
spdmem2: tAA-tRCD-tRP-tRAS: 3-20-20-50
spdmem2: voltage LvTTL (not 5V tolerant), refresh time 15.625us 
(self-refreshing)
uni_n0 at mainbus0 address 0xf8000000
ki2c0 at uni_n0 address 0xf8001000
iic1 at ki2c0: I2C bus
uninorth0 at mainbus0
pci0 at uninorth0 bus 0
pci0: i/o space, memory space enabled
pchb0 at pci0 dev 11 function 0
pchb0: Apple Computer UniNorth AGP Interface (rev. 0x00)
r128fb0 at pci0 dev 16 function 0: ATI Technologies Rage Fury MAXX AGP 4x (TMDS)
r128fb0: 64 MB aperture at 0x94000000
wsdisplay0 at r128fb0 kbdmux 1: console (default, vt100 emulation)
wsmux1: connecting to wsdisplay0
uninorth1 at mainbus0
pci1 at uninorth1 bus 0
pci1: i/o space, memory space enabled
pchb1 at pci1 dev 11 function 0
pchb1: Apple Computer UniNorth Host-PCI Bridge (rev. 0x00)
obio0 at pci1 dev 23 function 0: addr 0x80000000
obio0: enabling KeyLargo internal modem
zsc0 at obio0 irq 22,50
zstty0 at zsc0 channel 0
zstty1 at zsc0 channel 1
snapper0 at obio0 offset 0x10000: irq 30,1,2
pmu0 at obio0 offset 0x16000 irq 47:  rev. 12
pmu0: initializing RTC
pmu0: power-mgt not configured
ki2c1 at obio0
iic2 at ki2c1: I2C bus
deq0 at ki2c1 Apple Digital Equalizer, addr 0x68
wdc0 at obio0 offset 0x1f000 irq 19, level triggered: DMA transfer
atabus0 at wdc0 channel 0
wdc1 at obio0 offset 0x20000 irq 20, level triggered: DMA transfer
atabus1 at wdc1 channel 0
wdc2 at obio0 offset 0x21000 irq 21, level triggered: DMA transfer
atabus2 at wdc2 channel 0
snapper0: codec: TAS3001
audio0 at snapper0: full duplex, playback, capture
ohci0 at pci1 dev 24 function 0: Apple Computer KeyLargo USB Controller (rev. 
0x00)
ohci0: interrupting at irq 27
ohci0: OHCI version 1.0
usb0 at ohci0: USB revision 1.0
ohci1 at pci1 dev 25 function 0: Apple Computer KeyLargo USB Controller (rev. 
0x00)
ohci1: interrupting at irq 28
ohci1: OHCI version 1.0
usb1 at ohci1: USB revision 1.0
uninorth2 at mainbus0
pci2 at uninorth2 bus 0
pci2: i/o space, memory space enabled
pchb2 at pci2 dev 11 function 0
pchb2: Apple Computer UniNorth Host-PCI Bridge (rev. 0x00)
fwohci0 at pci2 dev 14 function 0: Lucent Technologies FW322/323 IEEE 1394 Host 
Controller (rev. 0x00)
fwohci0: interrupting at irq 40
fwohci0: OHCI version 1.0 (ROM=0)
fwohci0: No. of Isochronous channels is 8.
fwohci0: EUI64 00:30:65:ff:fe:c8:bb:56
fwohci0: Phy 1394a available S400, 2 ports.
fwohci0: Link S400, max_rec 2048 bytes.
ieee1394if0 at fwohci0: IEEE1394 bus
fwip0 at ieee1394if0: IP over IEEE1394
fwohci0: Initiate bus reset
gem0 at pci2 dev 15 function 0: Apple Computer GMAC Ethernet (rev. 0x01)
gem0: interrupting at irq 41
brgphy0 at gem0 phy 0: BCM5401 1000BASE-T media interface, rev. 3
brgphy0: 10baseT, 10baseT-FDX, 100baseTX, 100baseTX-FDX, 1000baseT, 
1000baseT-FDX, auto
gem0: Ethernet address 00:30:65:c8:bb:56, 10KB RX fifo, 4KB TX fifo
biomask 8001ffe netmask 8001ffe ttymask 8001ffe
timecounter: Timecounter "clockinterrupt" frequency 100 Hz quality 0
timecounter: Timecounter "mftb" frequency 33290001 Hz quality 100
fwohci0: BUS reset
fwohci0: node_id=0xc800ffc0, gen=2, CYCLEMASTER mode
ieee1394if0: 1 nodes, maxhop <= 0, cable IRM = 0 (me)
ieee1394if0: bus manager 0 (me)
uhub0 at usb0: Apple Computer OHCI root hub, class 9/0, rev 1.00/1.00, addr 1
uhub0: 2 ports with 2 removable, self powered
uhub1 at usb1: Apple Computer OHCI root hub, class 9/0, rev 1.00/1.00, addr 1
uhub1: 2 ports with 2 removable, self powered
wd0 at atabus0 drive 0: <ST38421A>
wd0: drive supports 16-sector PIO transfers, LBA addressing
wd0: 8056 MB, 16368 cyl, 16 head, 63 sec, 512 bytes/sect x 16498944 sectors
wd0: drive supports PIO mode 4, DMA mode 2, Ultra-DMA mode 2 (Ultra/33)
wd1 at atabus0 drive 1: <IBM-DTLA-307030>
wd1: drive supports 16-sector PIO transfers, LBA addressing
wd1: 29314 MB, 59560 cyl, 16 head, 63 sec, 512 bytes/sect x 60036480 sectors
wd1: drive supports PIO mode 4, DMA mode 2, Ultra-DMA mode 5 (Ultra/100)
wd0(wdc0:0:0): using PIO mode 4, DMA mode 2, Ultra-DMA mode 2 (Ultra/33) (using 
DMA)
wd1(wdc0:0:1): using PIO mode 4, DMA mode 2, Ultra-DMA mode 4 (Ultra/66) (using 
DMA)
uhidev0 at uhub0 port 1 configuration 1 interface 0
uhidev0: Holtek Semiconductor USB Keyboard, rev 1.10/1.01, addr 2, iclass 3/1
ukbd0 at uhidev0
wskbd0 at ukbd0 mux 1
wskbd0: connecting to wsdisplay0
uhidev1 at uhub0 port 1 configuration 1 interface 1
uhidev1: Holtek Semiconductor USB Keyboard, rev 1.10/1.01, addr 2, iclass 3/0
uhidev1: 2 report ids
uhid0 at uhidev1 reportid 1: input=1, output=0, feature=0
uhid1 at uhidev1 reportid 2: input=3, output=0, feature=0
wd2 at atabus1 drive 0: <IBM-DTLA-307015>
wd2: drive supports 16-sector PIO transfers, LBA addressing
wd2: 9536 MB, 19376 cyl, 16 head, 63 sec, 512 bytes/sect x 19531250 sectors
wd2: drive supports PIO mode 4, DMA mode 2, Ultra-DMA mode 5 (Ultra/100)
wd2(wdc1:0:0): using PIO mode 4, DMA mode 2 (using DMA)
Kernelized RAIDframe activated
wd2: no NetBSD disk label
boot device: wd0
root on wd0a dumps on wd0b
root file system type: ffs
wd2: no NetBSD disk label
wd2: no NetBSD disk label