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ctinnist
Asked: 2009-06-20 07:17:06 +0800 CST

How many hardware (PCI-MSI) interrupts can a PC have?

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On my workstation I got only two PCI-MSI interrupts in /proc/interrupts:

  0:   11726919          0    IO-APIC-edge  timer
  1:          3          0    IO-APIC-edge  i8042
  7:          0          0    IO-APIC-edge  parport0
  8:          1          0    IO-APIC-edge  rtc
  9:          0          0   IO-APIC-level  acpi
 12:          4          0    IO-APIC-edge  i8042
 58:         13      94016         PCI-MSI  eth0
169:     843271   71766622   IO-APIC-level  uhci_hcd:usb3, eth1, HDA Intel
201:          1          0   IO-APIC-level  uhci_hcd:usb4, uhci_hcd:usb6
209:          0          0   IO-APIC-level  ehci_hcd:usb1
217:          2          0   IO-APIC-level  ehci_hcd:usb2, uhci_hcd:usb5
225:         27   72309141   IO-APIC-level  uhci_hcd:usb7, eth2
233:       6683     103839         PCI-MSI  ahci
NMI:       2236       4056
LOC:   11696665   11696593
ERR:          0
MIS:          0

On other, much stronger RACK Dell machine (8 cores), there are seven such interrupts.

Does this depend on number of processors? Is IO-APIC-level a software interrupt?

I want to assign PCI-MSI interrupts to my network cards to improve performance.

hardware linux networking ethernet

3 Answers

  1. Best Answer

    IIRC PC can have any number of MSI interrupts. But each device is limited to 32 (for PCI 2.2) or 2048 (for PCI 3.0) MSI interrupts.

    See also wiki about Message Signaled Interrupts

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  2. From some basic research it looks like different hardware will have different numbers.

    "Each receive DMA channel of a NIC can generate one interrupt, and each interrupt will target one CPU. Sun Multi-threaded 10GbE has 8 receive DMA channels per port, and Quad GbE has 4, so their interrupts can target at most 8 and 4 different CPU, respectively. To avoid interrupt CPU becoming the performance bottleneck, it is recommended to start with a value of the number of receive DMA channels per port or (# of CPU), whichever is lower, so that interrupt loads are distributed to enough CPU."

    From http://www.solarisinternals.com/wiki/index.php/Networks

    So we'd need to know your motherboard/proc/NIC to find out what was the best setup.

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  3. "edge" and "level" refer to edge vs level triggering, which is a difference very much in the hardware domain. The distinction only makes sense when talking about the 4 physical interrupt lines of a parallel PCI bus, unless it is a software emulation of such behaviour or on the parallel bus segment behind a PCIe-PCI bridge (which is sometimes used WITHIN extension boards).

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