You need to use the sysctl command which is used to modify kernel parameters at runtime. The parameters available are those listed under /proc/sys/. Procfs is required for sysctl support in Linux. [donotprint]
| Tutorial details | |
|---|---|
| Difficulty | Easy (rss) |
| Root privileges | Yes |
| Requirements | None |
| Time | 5m |
You must login as the root user to use any one of the following command on a Linux operating systems.
Display Linux kernel parameters
The syntax is:
# sysctl variable
####################################
### Display all sysctl variables ###
####################################
# sysctl -a
# sysctl -a | more
# sysctl -a | grep 'something'
# sysctl -a | grep memory
Sample outputs:
vm.memory_failure_early_kill = 0 vm.memory_failure_recovery = 1 vm.overcommit_memory = 0
To see value for kernel variable called kernel.hostname, enter:
# sysctl -n kernel.hostname
How do I set new values?
There are three methods to set new values for given kernel parameters as follows:
Method # 1: Setting value via procfs
You can use standard echo command to write data to variables (this temporary change):
# echo "value" > /proc/sys/location/variable
Method # 2: Temporary on the command line
Use sysctl command with -w option when you want to change a sysctl setting:
sysctl -w variable=value
Method # 3: Configuration file /etc/sysctl.conf
This is recommended way. First open /etc/sysctl.conf file, enter:
# vi /etc/sysctl.conf
Now add value:
variable = value
Close and save the changes. Type the following command to load sysctl settings from the file /etc/sysctl.conf file:
# sysctl -p
OR
# sysctl -p /etc/sysctl.conf
The last method will load settings permanently at boot time from /etc/sysctl.conf file. Read man page of sysctl for information:
$ man sysctl
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Can you make an example for “variable” and “value” ?
EXAMPLE….
[root@hostname1 /]# sysctl -a
kernel.sched_child_runs_first = 0
kernel.sched_min_granularity_ns = 4000000
kernel.sched_latency_ns = 20000000
kernel.sched_wakeup_granularity_ns = 4000000
kernel.sched_tunable_scaling = 1
kernel.sched_features = 3183
kernel.sched_migration_cost = 500000
kernel.sched_nr_migrate = 32
kernel.sched_time_avg = 1000
kernel.sched_shares_window = 10000000
kernel.timer_migration = 1
kernel.sched_rt_period_us = 1000000
kernel.sched_rt_runtime_us = 950000
kernel.sched_compat_yield = 0
kernel.sched_rr_timeslice_ms = 100
kernel.sched_autogroup_enabled = 0
kernel.sched_cfs_bandwidth_slice_us = 5000
kernel.panic = 0
kernel.exec-shield = 1
kernel.core_uses_pid = 1
kernel.core_pattern = |/usr/libexec/abrt-hook-ccpp %s %c %p %u %g %t e
kernel.core_pipe_limit = 4
kernel.tainted = 0
kernel.real-root-dev = 0
kernel.print-fatal-signals = 0
kernel.ctrl-alt-del = 0
kernel.ftrace_enabled = 1
kernel.stack_tracer_enabled = 0
kernel.ftrace_dump_on_oops = 0
kernel.modprobe = /sbin/modprobe
kernel.modules_disabled = 0
kernel.kexec_load_disabled = 0
kernel.hotplug =
kernel.acct = 4 2 30
kernel.sysrq = 0
kernel.cad_pid = 1
kernel.threads-max = 513406
kernel.random.poolsize = 4096
kernel.random.entropy_avail = 135
kernel.random.read_wakeup_threshold = 64
kernel.random.write_wakeup_threshold = 128
kernel.random.boot_id = 95a40261-6816-445f-b5ae-38660f15d7e3
kernel.random.uuid = 7a1d1848-0868-42f5-bb3b-21172b60996b
kernel.usermodehelper.bset = 4294967295 4294967295
kernel.usermodehelper.inheritable = 4294967295 4294967295
kernel.overflowuid = 65534
kernel.overflowgid = 65534
kernel.pid_max = 32768
kernel.panic_on_oops = 1
kernel.printk = 4 4 1 7
kernel.printk_ratelimit = 5
kernel.printk_ratelimit_burst = 10
kernel.printk_delay = 0
kernel.dmesg_restrict = 0
kernel.kptr_restrict = 1
kernel.ngroups_max = 65536
kernel.watchdog = 1
kernel.watchdog_thresh = 60
kernel.softlockup_panic = 0
kernel.nmi_watchdog = 1
kernel.unknown_nmi_panic = 0
kernel.panic_on_unrecovered_nmi = 0
kernel.panic_on_io_nmi = 0
kernel.bootloader_type = 113
kernel.bootloader_version = 1
kernel.kstack_depth_to_print = 12
kernel.io_delay_type = 0
kernel.randomize_va_space = 2
kernel.acpi_video_flags = 0
kernel.hung_task_panic = 0
kernel.hung_task_check_count = 4194304
kernel.hung_task_timeout_secs = 120
kernel.hung_task_warnings = 10
kernel.compat-log = 1
kernel.max_lock_depth = 1024
kernel.poweroff_cmd = /sbin/poweroff
kernel.keys.maxkeys = 200
kernel.keys.maxbytes = 20000
kernel.keys.root_maxkeys = 1000000
kernel.keys.root_maxbytes = 25000000
kernel.keys.gc_delay = 300
kernel.slow-work.min-threads = 2
kernel.slow-work.max-threads = 8
kernel.slow-work.vslow-percentage = 50
kernel.perf_event_paranoid = 1
kernel.perf_event_mlock_kb = 516
kernel.perf_event_max_sample_rate = 100000
(VARIABLE = VALUE)
sysctl -w variable=value
sysctl -w kernel.keys.maxkeys=400