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SMP IRQ Affinity Background: Whenever a piece of hardware, such as disk controller or ethernet card, needs attention from the CPU, it throws an interrupt. The interrupt tells the CPU that something has happened and that the CPU should drop what it's doing to handle the event. In order to prevent mutliple devices from sending the same interrupts, the IRQ system was established where each device in a computer system is assigned its own special IRQ so that its interrupts are unique. Starting with the 2.4 kernel, Linux has gained the ability to assign certain IRQs to specific processors (or groups of processors). This is known as SMP IRQ affinity, and it allows you control how your system will respond to various hardware events. It allows you to restrict or repartition the work load that you server must do so that it can more efficiently do it's job. Obviously, in order for this to work, you will need a system that has more than one processor (SMP). You will also need to be running a 2.4 or higher kernel. Some brief and very bare information on SMP IRQ affinity is provided in the kernel source tree of the 2.4 kernel in the file: /usr/src/linux-2.4/Documentation/IRQ-affinity.txt

How to use it: SMP affinity is controlled by manipulating files in the /proc/irq/ directory. In /proc/irq/ are directories that correspond to the IRQs present on your system (not all IRQs may be available). In each of these directories is the "smp_affinity" file, and this is where we will work our magic. The first order of business is to figure out what IRQ a device is using. This information is available in the /proc/interrupts file. Here's a sample: [root@archimedes /proc]# cat /proc/interrupts CPU0 CPU1 CPU2 CPU3 0: 4865302 5084964 4917705 5017077 1: 132 108 159 113 2: 0 0 0 0


timer keyboard cascade

8: 10: 14: 24: 31: NMI: LOC: ERR: MIS:

0 0 0 87298 93707 0 19883500 0 0

1 0 0 86066 106211 0 19883555

0 0 1 86012 107988 0 19883441

0 0 1 86626 93329 0 19883424

IO-APIC-edge IO-APIC-level IO-APIC-edge IO-APIC-level IO-APIC-level

rtc usb-ohci ide0 aic7xxx eth0

As you can see, this is a 4 processor machine. The first column (unlabelled) lists the IRQs used on the system. The rows with letters (ie, "NMI", "LOC") are parts of other drivers used on the system and aren't really accessible to us, so we'll just ignore them. The the For for second through fifth columns (labelled CPU0-CPU3) show the number of times corresponding process has handled an interrupt from that particular IRQ. example, all of the CPUs have handled roughly the same number of interrupts IRQ 24 (around 86,000 with CPU0 handling a little over 87,000).

The sixth column lists whether or not the device driver associated with the interrupt supports IO-APIC (see /usr/src/linux/Documentation/i386/IO-APIC.txt for more information). The only reason to look at this value is that SMP affinity will only work for IO-APIC enabled device drivers. For example, we will not be able to change the affinity for the "cascade" driver (IRQ 2) because it doesn't support IO-APIC. Finally, the seventh and last column lists the driver or device that is associated with the interrupt. In the above example, our ethernet card (eth0) is using IRQ 31, and our SCSI controller (aic7xxx) is using IRQ 24. The first and last columns are really the only ones we're interested in here. For the rest of this example, I'm going to assume that we want to adjust the SMP affinity for th SCSI controller (IRQ 24). Now that we've got the IRQ, we can change the processor affinity. To do this, we'll go into the /proc/irq/24/ directory, and see what the affinity is currently set to: [root@archimedes Documentation]# cat /proc/irq/24/smp_affinity ffffffff This is a bitmask that represents which processors any interrupts on IRQ

24 should be routed to. Each field in the bit mask corresponds to a processor. The number held in the "smp_affinity" file is presented in hexadecimal format, so in order to manipulate it properly we will need to convert our bit patterns from binary to hex before setting them in the proc file. Each of the "f"s above represents a group of 4 CPUs, with the rightmost group being the least significant. For the purposes of our discussion, we're going to limit ourselves to only the first 4 CPUs (although we can address up to 32). In short, this means you only have to worry about the rightmost "f" and you can assume everything else is a "0" (ie, our bitmask is "0000000f"). "f" is the hexadecimal represenatation for the decimal number 15 (fifteen) and the binary pattern of "1111". Each of the places in the binary pattern corresponds to a CPU in the server, which means we can use the following chart to represent the CPU bit patterns: Binary 0001 0010 0100 1000 Hex 1 2 4 8


0 1 2 3

By combining these bit patterns (basically, just adding the Hex values), we can address more than one processor at a time. For example, if I wanted to talk to both CPU0 and CPU2 at the same time, the result is: Binary Hex CPU 0 0001 1 + CPU 2 0100 4 ----------------------both 0101 5 If I want to address all four of the processors at once, then the result is: Binary Hex CPU 0 0001 1 CPU 1 0010 2 CPU 2 0100 4 + CPU 3 1000 8 ----------------------both 1111 f

(Remember that we use the letters "a" through "f" to represent the numbers "10" to "15" in hex notation). Given that, we now know that if we have a four processor system, we can assign any of 15 different CPU combinations to an IRQ (it would be 16, but it isn't legal to assign an IRQ affinity of "0" to any IRQ... if you try, Linux will just ignore your attempt). So. Now we get to the fun part. Remember in our /proc/interrupts listing above that all four of our CPUs had handled the close to the same amount of interrupts for our SCSI card? We now have the tools needed to limit managing the SCSI card to just one processor and leave the other three free to concentrate on doing other tasks. Let's assume that we want to dedicate our first CPU (CPU0) to handling the SCSI controller interrupts. To do this, we would simply run the following command: [root@archimedes /proc]# echo 1 > /proc/irq/24/smp_affinity [root@archimedes /proc]# cat /proc/irq/24/smp_affinity 00000001 Now, let's test it out and see what happens: [root@archimedes /proc]# cd /tmp/ [root@archimedes /tmp]# tar -zcf test.tgz /usr/src/linux-2.4.2 tar: Removing leading `/' from member names [root@archimedes /tmp]# tar -zxf test.tgz && rm -rf usr/ [root@archimedes /tmp]# tar -zxf test.tgz && rm -rf usr/ [root@archimedes /tmp]# tar -zxf test.tgz && rm -rf usr/ [root@archimedes /tmp]# tar -zxf test.tgz && rm -rf usr/ [root@archimedes /tmp]# tar -zxf test.tgz && rm -rf usr/ [root@archimedes /tmp]# cat /proc/interrupts | grep 24: 24: 99719 86067 86012 86627 IO-APIC-level


Compare that to the previous run without having the IRQ bound to CPU0: 24: 87298 86066 86012 86626 IO-APIC-level aic7xxx

All of the interrupts from the disk controller are now handled exclusively by the first CPU (CPU0), which means that our other 3 proccessors are free to do other stuff now. Finally, it should be pointed out that if you decide you no longer want SMP affinity and would rather have the system revert back to the old set up, then you can simply do:

[root@archimedes /tmp]# cat /proc/irq/prof_cpu_mask >/proc/irq/24/smp_affinity This will reset the "smp_affinity" file to use all "f"s, and will return to the load sharing arrangement that we saw earlier.

What can I use it for? - "balance" out multiple NICs in a multi-processor machine. By tying a single NIC to a single CPU, you should be able to scale the amount of traffic your server can handle nicely. - database servers (or servers with lots of disk storage) that also have heavy network loads can dedicate a CPU to their disk controller and assign another to deal with the NIC to help improve response times.

Can I do this with processes? At this time, no.

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