IRQs, SATA, and APICs in the 865/875 chipset

Here's a rambling question. Basically I want to understand the relationship
between IRQs, SATA, and APICs, on mainboards that use the Intel 865/875
chipset.

I have an Abit IS7, running Linux 2.4.21 and Win2K. My focus right now is
on Linux. (a.c.p.m.asus readers, please don't flame me-- my question
applies equally well to any 865/875 mobo, and I figure there are quite a few
owners of those reading your NG.)

I'm considering adding a SATA drive to my rig. I've seen reports in the
newsgroups that some people are having trouble getting good performance out
of these drives, especially in Linux, using mobos that have the 865/875
chipset. The problem seems to be related to IRQ sharing-- the SATA
channels are multiplexed with other devices on a single IRQ (e.g.
http://lists.insecure.org/lists/linu.../Jul/0429.html). Here's
the situation on my box:

$ cat /proc/interrupts
CPU0
0: 153043 IO-APIC-edge timer
1: 2693 IO-APIC-edge keyboard
2: 0 XT-PIC cascade
8: 3 IO-APIC-edge rtc
9: 0 XT-PIC acpi
12: 68748 IO-APIC-edge PS/2 Mouse
14: 18451 IO-APIC-edge ide0
15: 6 IO-APIC-edge ide1
16: 90301 IO-APIC-level usb-uhci, usb-uhci, nvidia
18: 16555 IO-APIC-level usb-uhci, SysKonnect SK-98xx
19: 0 IO-APIC-level usb-uhci
20: 21 IO-APIC-level DC395x_TRM
23: 0 IO-APIC-level ehci-hcd
NMI: 0
LOC: 152994
ERR: 0
MIS: 0

My boot log tells me that both SATA channels will go onto IRQ 18, sharing
that one IRQ with a USB 1.1 device and my ethernet controller. So it seems
that I may be headed for the sort of IRQ logjam that others have
reported.

Several questions arise:

- Is it likely to be a problem to share one IRQ between two SATA drives, a
USB 1.1 device, and my ethernet controller? The USB is a slow device, but
the Ethernet could run at 100 or even 1000 Mb/s if I use it on my LAN.

- Do I really have 24 IRQs now? Or do I still really have 16, which are
logically presented as 24? In the old days, there were only 16 IRQs (15
really), and it was sometimes a problem that there weren't enough to go
around. But the FOLDOC definition of an APIC
(http://foldoc.doc.ic.ac.uk/foldoc/fo...&action=Search)
says that it "usually has more available interrupt lines that a typical
PIC."

- Can any of you report success or problems with throughput of SATA drives,
using the 865/875 chipset, especially in Linux? What does your IRQ
arrangement look like?

- Has anyone successfully rearranged their IRQ usage to avoid logjams? Did
you do it in the BIOS, or in the OS (which OS?)? Unfortunately, the IS7's
BIOS doesn't offer any explicit control over IRQs that I can find.

Thanks in advance for your time.
Andrew.

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