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Some hardware components like ethernet cards,disk controllers,etc. produces
interrupts
when needs to get attention from cpu. For example, when ethernet card
receives packet from network. You can examine your machine's interrupts
usage on cpu's by looking in /proc/interrupts proc entry.
# cat /proc/interrupts
This
information includes which devices are working on which irq and how
many interrupts processed by each cpu for this device.In normal cases,
you will not face with a problem and no need to change irq handling
process for any cpu.But in some cases, for example if you are running a
linux box as firewall which has high incoming or outgoing traffic, this
can cause problems. Suppose that you have 2 ethernet cards on your
firewall and both ethernet cards handling many packets. In some cases
you can see high cpu usage on one of your cpus.This can be caused by
many interrupts produced by your network cards. You can check this by
looking in /proc/interrupts and see if that cpu is handling interrupts
of both cards. If this is the case, what you can do is looking for most
idle cpus on your system and specify those ethernet card irqs to be
served by each cpu seperately. But beware that you can only do this in a
system with IO-APIC enabled device drivers. You can check if your
device supports IO-APIC by looking /proc/interrupts.
In ethernet card case, there is an implementation called
NAPI which can reduce interrupt usage on incoming network traffic.
You
can see which irq is served by which cpu or cpus by looking in
/proc/irq directory. Directory layout is very simple. For every used irq
in system it is presented by a directory by it's irq number.Every
directory contains a file called smp_affinity where you can set cpu
settings. File content shows currently which cpu is serving this irq.
The value is in hex format. Calculation is shown below.As you can see in
example figure, eth0 is on irq 25 and eth1 is in irq 26. Let's say we
want to set irq25 to be served only by cpu3. First of all, we have to
calculate the value for cpu3 in hex value. Calculation is shown below.
Calculation
Binary Hex
CPU 0 0001 1
CPU 1 0010 2
CPU 2 0100 4
+ CPU 3 1000 8
-----------------------
both 1111 f
Calculation
is shown for 4 cpu system for simplicity. normally the value for all
cpus on a system is represented by 8 digit hex value.As you can see in
binary format every bit represents a cpu. We see that binary
representation of cpu3 is 8 in hex. Then we write it into smp_affinity
file for irq 25 as show below.
# echo 8 > /proc/irq/25/smp_affinity
You can check the setting by looking in to file content.
# cat /proc/irq/25/smp_affinity
00000008
Another example, let's say we want irq25 to handled by cpu0 and cpu1.
CPU 0 0001 1
+ CPU 1 0010 2
--------------------------
0011 3
Setting
bit for cpu0 and cpu1 is giving us the value 3.For example if we need
all cpus to handle our device's irq, we set every bit in our calculation
and write it into smp_affinity as hex value which is F. There is an
implementation in Linux called
irqbalance
, where a daemon distributes interrupts automatically for every cpu in
the system. But in some cases this is giving bad performance where you
need to stop the service and do it manually by yourself as I described
above for higher performance.Also, irqbalance configuration let's you to
configure where it will not balance given irqs or use specified cpus.
In this case you can configure it to not touch your manually configured
irqs and your preferred cpus and let it run to automatically load
balance rest of the irqs and cpus for you.
