2.4. Interrupt and Process Binding
00000000000000000000000000000001as a bitmask,
1as a decimal, and
0x00000001as a hexadecimal. The CPU mask for both CPU 0 and 1 is
00000000000000000000000000000011as a bitmask,
3as a decimal, and
0x00000003as a hexadecimal.
Procedure 2.3. Disabling the
irqbalancedaemon is not required.
- Check the status of the
systemctl status irqbalanceirqbalance.service - irqbalance daemon Loaded: loaded (/usr/lib/systemd/system/irqbalance.service; enabled) Active: active (running) …
- If the
irqbalancedaemon is running, stop it.
systemctl stop irqbalance
- Ensure that
irqbalancedoes not restart on boot.
systemctl disable irqbalance
Procedure 2.4. Excluding CPUs from IRQ Balancing
/etc/sysconfig/irqbalanceconfiguration file contains a setting that allows CPUs to be excluded from consideration by the IRQ balacing service. This parameter is named
IRQBALANCE_BANNED_CPUSand is a 64-bit hexadecimal bit mask, where each bit of the mask represents a CPU core.
/etc/sysconfig/irqbalancein your preferred text editor and find the section of the file titled
# IRQBALANCE_BANNED_CPUS # 64 bit bitmask which allows you to indicate which cpu's should # be skipped when reblancing irqs. Cpu numbers which have their # corresponding bits set to one in this mask will not have any # irq's assigned to them on rebalance # #IRQBALANCE_BANNED_CPUS=
- Exclude CPUs 8 to 15 by uncommenting the variable
IRQBALANCE_BANNED_CPUSand setting its value this way:
- This will cause the
irqbalanceprocess to ignore the CPUs that have bits set in the bitmask; in this case, bits 8 through 15.
- If you are running a system with up to 64 CPU cores, separate each group of eight hexadecimal digits with a comma:
IRQBALANCE_BANNED_CPUS=00000001,0000ff00The above mask excludes CPUs 8 to 15 as well as CPU 33 from IRQ balancing.
irqbalancetool automatically avoids IRQs on CPU cores isolated via the
isolcpus=kernel parameter if
IRQBALANCE_BANNED_CPUSis not set in the
Procedure 2.5. Manually Assigning CPU Affinity to Individual IRQs
- Check which IRQ is in use by each device by viewing the
cat /proc/interruptsThis file contains a list of IRQs. Each line shows the IRQ number, the number of interrupts that happened in each CPU, followed by the IRQ type and a description:
CPU0 CPU1 0: 26575949 11 IO-APIC-edge timer 1: 14 7 IO-APIC-edge i8042 ...[output truncated]...
- To instruct an IRQ to run on only one processor, use the
echocommand to write the CPU mask, as a hexadecimal number, to the
smp_affinityentry of the specific IRQ. In this example, we are instructing the interrupt with IRQ number 142 to run on CPU 0 only:
echo 1 > /proc/irq/142/smp_affinity
- This change will only take effect once an interrupt has occurred. To test the settings, generate some disk activity, then check the
/proc/interruptsfile for changes. Assuming that you have caused an interrupt to occur, you will see that the number of interrupts on the chosen CPU have risen, while the numbers on the other CPUs have not changed.
Procedure 2.6. Binding Processes to CPUs Using the
tasksetutility uses the process ID (PID) of a task to view or set the affinity, or can be used to launch a command with a chosen CPU affinity. In order to set the affinity,
tasksetrequires the CPU mask expressed as a decimal or hexadecimal number. The mask argument is a bitmask that specifies which CPU cores are legal for the command or PID being modified.
- To set the affinity of a process that is not currently running, use
tasksetand specify the CPU mask and the process. In this example,
my_embedded_processis being instructed to use only CPU 3 (using the decimal version of the CPU mask).
taskset 8 /usr/local/bin/my_embedded_process
- It is also possible to specify more than one CPU in the bitmask. In this example,
my_embedded_processis being instructed to execute on processors 4, 5, 6, and 7 (using the hexadecimal version of the CPU mask).
taskset 0xF0 /usr/local/bin/my_embedded_process
- Additionally, you can set the CPU affinity for processes that are already running by using the
--pid) option with the CPU mask and the PID of the process you wish to change. In this example, the process with a PID of 7013 is being instructed to run only on CPU 0.
taskset -p 1 7013
- Lastly, using the
-cparameter, you can specify a CPU list instead of a CPU mask. For example, in order to use CPU 0, 4 and CPUs 7 to 11, the command line would contain
-c 0,4,7-11. This invocation is more convenient in most cases.
For more information, or for further reading, the following man pages are related to the information given in this section.
- sched_setscheduler(2) for a description of the Linux scheduling scheme.