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taskset sets or reads a Linux process’s CPU affinity—the logical CPUs on which it is allowed to run—and can start a command with that restriction already applied. Use a hexadecimal mask for compact bit-level control, or -c for readable CPU numbers and ranges. A successful change means the kernel accepted the new allowed set; it does not promise that the thread has migrated immediately.
What taskset does
The taskset(1) command, supplied by util-linux, manages CPU affinity for processes. Linux schedules a thread only on CPUs included in its affinity mask, subject also to cpuset and other kernel restrictions. The command can:
- launch a new command with a selected affinity;
- display the affinity of an existing process;
- change the affinity of an existing process; and
- operate on every thread in a process when requested.
Affinity is a scheduling constraint, not a performance guarantee. Pinning can reduce cache disruption caused by movement between CPUs, but contention, CPU topology, workload behavior and kernel policy determine whether an application actually improves.
Command syntax
taskset [options] mask command [argument...]
taskset [options] -p [mask] pidThe first form starts a command. The second reads or changes the affinity of an existing process identified by its PID.
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Start a command on selected CPUs
Use a hexadecimal mask
taskset 0x3 mycommand --input file.datBits in the mask correspond to logical CPU numbers, with bit 0 representing CPU 0. Therefore, 0x3 (binary 11) allows CPUs 0 and 1. A mask of 0x1 allows only CPU 0.
| Mask | Allowed logical CPUs | Reason |
|---|---|---|
0x1 |
CPU 0 | Only bit 0 is set |
0x3 |
CPUs 0 and 1 | Bits 0 and 1 are set |
0x32 |
CPUs 1, 4 and 5 | Bits 1, 4 and 5 are set |
Use a mask that contains at least one valid CPU. A mask with no valid CPU produces an error.
Use readable CPU lists
taskset --cpu-list 0-2,6 mycommand
# Equivalent short option
taskset -c 0-2,6 mycommandCPU-list syntax accepts individual numbers, comma-separated lists and ranges. A stride suffix selects regular intervals:
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taskset -c 0-10:2 mycommandThis allows CPUs 0, 2, 4, 6, 8 and 10. CPU numbers are logical CPU IDs exposed by Linux, not necessarily physical cores; simultaneous multithreading can give multiple logical IDs per physical core.
Read or change an existing process
Read the current affinity
taskset -p PIDReplace PID with the process ID. For example:
taskset -p 4242In query mode, taskset reports the process’s current mask. Reading another user’s process is generally permitted under the command’s documented rules.
Set a hexadecimal mask
taskset -p 0x3 4242This requests CPUs 0 and 1 for PID 4242. The -p option tells taskset to operate on an existing PID instead of launching a command.
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Set a CPU list
taskset -pc 0-3 4242Here -p selects PID mode and -c interprets 0-3 as a CPU list. Combining short options in this form is supported by the taskset manual.
Processes, threads and the -a option
Linux stores affinity per thread. A process ID normally identifies one task, commonly the thread-group leader; other threads can have independent masks.
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A child created with fork() inherits its parent’s affinity, and the setting is preserved across execve(). Thus a command launched by taskset starts with the selected mask, and an executed program keeps it unless changed later.
Options at a glance
| Option | Purpose |
|---|---|
-a, --all-tasks |
Read or set affinity for all threads belonging to a PID. |
-c, --cpu-list |
Interpret the selection as CPU numbers, lists, ranges and strides. |
-p, --pid |
Operate on an existing PID rather than launch a command. PID 0 refers to the taskset process. |
-h, --help |
Show command help. |
-V, --version |
Show the installed taskset version. |
Permissions and errors
You can normally change affinity for a process you own. Changing another user’s process requires the CAP_SYS_NICE capability. Without the required identity or capability, the underlying sched_setaffinity(2) call reports EPERM, and taskset fails.
- Invalid or empty selection: the mask contains no valid CPU, so the command reports an error.
- Permission denied: you lack ownership or
CAP_SYS_NICEfor the target. - Kernel thread limitation: some per-CPU kernel threads cannot have their affinity changed.
- Nonexistent PID: the target exited or the PID is incorrect.
Check the command’s exit status in a shell when scripting. In setting mode, success means the kernel accepted the requested mask; an illegal mask results in status 1.
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Why a process may not move immediately
A successful set does not mean the thread instantly appears on a different CPU. The scheduler may leave it on its current CPU when that CPU remains in the newly allowed set, and even a narrower mask can take effect without an observable immediate migration. The taskset manual specifically demonstrates that a kernel thread can retain its current CPU after a successful affinity change.
The effective run set can be narrower than the mask you request. Linux intersects the requested affinity with CPUs that are physically present and with cpuset restrictions. Containers, service managers and cgroup configuration can therefore limit the CPUs available to a process even when taskset accepts a broader mask. A task cannot run outside that effective intersection.
Choosing between a mask and -c
| Need | Best form | Example |
|---|---|---|
| Compact value for scripts or tooling | Hexadecimal mask | taskset 0x3 command |
| Human-readable CPU selection | CPU list | taskset -c 0-2,6 command |
| Change one task identified by PID | -p plus mask or -c |
taskset -pc 2-5 PID |
| Ensure every thread receives the setting | Add -a |
taskset -a -pc 2-5 PID |
A safe workflow for pinning a running application
- Find and verify the target PID using your normal process-inspection tool.
- Inspect its current setting with
taskset -p PID. - Choose logical CPU IDs that are valid in the target’s environment.
- Apply a readable list, for example
taskset -pc 2-3 PID, or use a mask when required by automation. - Add
-aif the application uses multiple threads and every thread must be restricted. - Read the affinity again and interpret success as acceptance of the mask, not proof of immediate migration or improved performance.
How taskset relates to the kernel API
taskset is a command-line interface to Linux scheduling affinity operations, principally sched_setaffinity(2) and its corresponding retrieval operation. The kernel definition is per-thread: a thread’s affinity mask determines the CPUs on which it is eligible to run. taskset provides convenient process and all-thread handling, while cpusets and other policy layers can impose additional limits.
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