Red Hat Enterprise Linux 5 Administration Unleashed

Setting SMP IRQ Affinity 443Setting SMP IRQ AffinityIf you have ever explored your system’s BIOS or reviewed your system’s configuration,you have probably noticed that hardware such as the Ethernet card or the sound card isassigned an IRQ. This IRQ allows the hardware to send event requests to the processor orprocessors in the system. When the hardware sends a request, it is called an interrupt.21For multi-processor systems, the Linux kernel balances interrupts across processors accordingto the type of requests. It is possible to configure the kernel to send interrupts from aspecific IRQ to a designated processor or group of processors. This concept is known bythe Linux kernel as SMP IRQ affinity. First, determine which IRQs are being used and bywhat hardware from the /proc/interrupts file.CAUTIONBe extremely careful when using SMP IRQ affinity. Assigning too many interrupts to asingle processor can cause a performance degradation. In most cases, the IRQ balancingdone by the kernel is the most optimal solution.In our example /proc/interrupts file as shown in Listing 21.6, the first column is a listof used IRQs, and the next two columns request the number of interrupts sent to eachprocessor. SMP IRQ affinity is only possible with interrupts on the IO-APIC controller,which is displayed in the second to last column. The last column is the kernel module ordevice associated with the IRQ.LISTING 21.6Example /proc/interruptsCPU0 CPU10: 10293911 10289264 IO-APIC-edge timer1: 3958 4135 IO-APIC-edge i80428: 3 0 IO-APIC-edge rtc9: 0 0 IO-APIC-level acpi14: 148702 149207 IO-APIC-edge ide050: 6967 6924 IO-APIC-level uhci_hcd:usb1, ehci_hcd:usb558: 171 0 IO-APIC-level HDA Intel66: 6222841 0 PCI-MSI eth0169: 0 0 IO-APIC-level uhci_hcd:usb4177: 0 0 IO-APIC-level libata225: 3 0 IO-APIC-level uhci_hcd:usb3, ohci1394233: 95803 90174 IO-APIC-level libata, uhci_hcd:usb2NMI: 0 0LOC: 20654428 20654427ERR: 0MIS: 0