ethernet smp_affinity vs /proc/interrupts vs /sys/class/net/ethX/device

My goal is to configure our CentOS ("free" RHEL) 5.x servers for custom low-latency network programs. I would like to experiment with binding ethernet NIC interrupt handling to the same CPU on which the program runs (to hopefully improve cache utilization). The first step in this process is to determine the NIC‘s IRQ.

Here is the contents of /proc/interrupts on one server (note that I deleted CPUs 2 through 14 for brevity):

    CPU0 CPU1 CPU15 

 0: 600299726 0 0 IO-APIC-edge timer 

 1: 3 0 0 IO-APIC-edge i8042 

 8: 1 0 0 IO-APIC-edge rtc 

 9: 0 0 0 IO-APIC-level acpi 

 12: 4 0 0 IO-APIC-edge i8042 

 50: 0 0 0 IO-APIC-level uhci_hcd:usb6, uhci_hcd:usb8 

 58: 6644 25103 0 IO-APIC-level ioc0 

 66: 0 0 0 IO-APIC-level ata_piix 

 74: 221 533830 0 IO-APIC-level ata_piix 

 98: 35 0 2902361 PCI-MSI-X eth1-0 

106: 61 11 3841 PCI-MSI-X eth1-1 

114: 28 0 61452 PCI-MSI-X eth1-2 

122: 24 1586 22 PCI-MSI-X eth1-3 

130: 2912 0 337 PCI-MSI-X eth1-4 

138: 21 0 28 PCI-MSI-X eth1-5 

146: 21 0 56 PCI-MSI-X eth1-6 

154: 34 1 1 PCI-MSI-X eth1-7 

209: 23 0 0 IO-APIC-level ehci_hcd:usb1 

217: 0 0 0 IO-APIC-level ehci_hcd:usb2, uhci_hcd:usb5, uhci_hcd:usb7 

225: 0 0 0 IO-APIC-level uhci_hcd:usb3 

233: 0 0 0 IO-APIC-level uhci_hcd:usb4 

NMI: 7615 2989 2931 LOC: 600328144 600328099 600327086 

ERR: 0 

MIS: 0

Why are there multiple entries for "eth1" in the form of "eth1-X"?

Furthermore, the contents of "/sys/class/net/eth1/device/irq" is "90". But there‘s no 90 in the interrupt list above.

So let‘s say I look at just "eth1-0", which is IRQ 98. The contents of /proc/irq/98/smp_affinity is:

00000000,00000000,00000000,00000000,00000000,00000000,00000000,00008000

That‘s a list of numbers, rather than just one number.

So how do I set eth1‘s smp_affinity?

None of the online examples and documentation I could find mentioned any cases like this; they always have exactly one "ethX" entry in /proc/interrupts; the indicated interrupt matches /sys/class/net/ethX/device/irq; and there is only one number in /proc/irq/N/smp_affinity.

FWIW, I‘ll add that this application is extremely latency sensitive. To the point where we disable C-states and processor frequency scaling (because those features induce too much latency). Micro seconds make a difference here.