piixide write speed very slow

[Stephen Borrill <netbsd%precedence.co.uk@localhost>]

I've seen a couple of boxes recently where writing to a piixide-attached 
disk was excruciatingly slow:

              -------Sequential Output-------- ---Sequential Input-- --Random--
              -Per Char- --Block--- -Rewrite-- -Per Char- --Block--- --Seeks---
Machine    MB K/sec %CPU K/sec %CPU K/sec %CPU K/sec %CPU K/sec %CPU  /sec %CPU
         4096  4268  3.1  4264  0.5  3994  0.6 92913 72.1 112090 11.4 1957.4  
3.7

This is with:

piixide0 at pci0 dev 31 function 2: Intel 3400 Serial ATA Controller (rev. 0x05)
piixide0: bus-master DMA support present
piixide0: primary channel configured to native-PCI mode
piixide0: using ioapic0 pin 19 for native-PCI interrupt
atabus0 at piixide0 channel 0
piixide0: secondary channel configured to native-PCI mode
atabus1 at piixide0 channel 1
piixide1 at pci0 dev 31 function 5: Intel 3400 Serial ATA Controller (rev. 0x05)
piixide1: bus-master DMA support present
piixide1: primary channel wired to native-PCI mode
piixide1: using ioapic0 pin 18 for native-PCI interrupt
atabus2 at piixide1 channel 0
piixide1: secondary channel wired to native-PCI mode
atabus3 at piixide1 channel 1
wd0 at atabus0 drive 0: <WD2502ABYS-23B7A0 39M4511 42C0463IBM>
wd0: drive supports 16-sector PIO transfers, LBA48 addressing
wd0: 232 GB, 484521 cyl, 16 head, 63 sec, 512 bytes/sect x 488397168 sectors
wd0: 32-bit data port
wd0: drive supports PIO mode 4, DMA mode 2, Ultra-DMA mode 6 (Ultra/133)
wd0(piixide0:0:0): using PIO mode 4, Ultra-DMA mode 6 (Ultra/133) (using DMA)
wd1 at atabus1 drive 0: <WD2502ABYS-23B7A0 39M4511 42C0463IBM>
wd1: drive supports 16-sector PIO transfers, LBA48 addressing
wd1: 232 GB, 484521 cyl, 16 head, 63 sec, 512 bytes/sect x 488397168 sectors

vmstat -i looks healthy (if anything, too healthy):

interrupt                                     total     rate
global TLB IPI                              3266518       42
ioapic0 pin 19                              1080143       13
ioapic0 pin 17                                  535        0
Total                                       4347196       56

Also seen the same on:
piixide0 at pci0 dev 31 function 1
piixide0: Intel 631xESB/632xESB IDE Controller (rev. 0x09)
piixide0: bus-master DMA support present
piixide0: primary channel configured to compatibility mode
piixide0: primary channel interrupting at ioapic0 pin 14
atabus0 at piixide0 channel 0

But this is OK:
piixide0 at pci0 dev 31 function 2
piixide0: Intel 82801I Serial ATA Controller (ICH9) (rev. 0x02)
piixide0: bus-master DMA support present
piixide0: primary channel configured to compatibility mode
piixide0: primary channel interrupting at ioapic0 pin 14

--
Stephen