1 Documentation for /proc/sys/kernel/* kernel version 2.2.10
2 (c) 1998, 1999, Rik van Riel <riel@nl.linux.org>
3 (c) 2009, Shen Feng<shen@cn.fujitsu.com>
5 For general info and legal blurb, please look in README.
7 ==============================================================
9 This file contains documentation for the sysctl files in
10 /proc/sys/kernel/ and is valid for Linux kernel version 2.2.
12 The files in this directory can be used to tune and monitor
13 miscellaneous and general things in the operation of the Linux
14 kernel. Since some of the files _can_ be used to screw up your
15 system, it is advisable to read both documentation and source
16 before actually making adjustments.
18 Currently, these files might (depending on your configuration)
19 show up in /proc/sys/kernel:
24 - bootloader_type [ X86 only ]
25 - bootloader_version [ X86 only ]
26 - callhome [ S390 only ]
37 - kstack_depth_to_print [ X86 only ]
39 - modprobe ==> Documentation/debugging-modules.txt
41 - msg_next_id [ sysv ipc ]
52 - panic_on_unrecovered_nmi
53 - panic_on_stackoverflow
55 - powersave-nap [ PPC only ]
59 - printk_ratelimit_burst
61 - real-root-dev ==> Documentation/initrd.txt
62 - reboot-cmd [ SPARC only ]
66 - sem_next_id [ sysv ipc ]
67 - sg-big-buff [ generic SCSI device (sg) ]
68 - shm_next_id [ sysv ipc ]
73 - stop-a [ SPARC only ]
74 - sysrq ==> Documentation/sysrq.txt
81 ==============================================================
85 highwater lowwater frequency
87 If BSD-style process accounting is enabled these values control
88 its behaviour. If free space on filesystem where the log lives
89 goes below <lowwater>% accounting suspends. If free space gets
90 above <highwater>% accounting resumes. <Frequency> determines
91 how often do we check the amount of free space (value is in
94 That is, suspend accounting if there left <= 2% free; resume it
95 if we got >=4%; consider information about amount of free space
98 ==============================================================
104 See Doc*/kernel/power/video.txt, it allows mode of video boot to be
107 ==============================================================
111 Enables/Disables automatic recomputing of msgmni upon memory add/remove
112 or upon ipc namespace creation/removal (see the msgmni description
113 above). Echoing "1" into this file enables msgmni automatic recomputing.
114 Echoing "0" turns it off. auto_msgmni default value is 1.
117 ==============================================================
121 x86 bootloader identification
123 This gives the bootloader type number as indicated by the bootloader,
124 shifted left by 4, and OR'd with the low four bits of the bootloader
125 version. The reason for this encoding is that this used to match the
126 type_of_loader field in the kernel header; the encoding is kept for
127 backwards compatibility. That is, if the full bootloader type number
128 is 0x15 and the full version number is 0x234, this file will contain
129 the value 340 = 0x154.
131 See the type_of_loader and ext_loader_type fields in
132 Documentation/x86/boot.txt for additional information.
134 ==============================================================
138 x86 bootloader version
140 The complete bootloader version number. In the example above, this
141 file will contain the value 564 = 0x234.
143 See the type_of_loader and ext_loader_ver fields in
144 Documentation/x86/boot.txt for additional information.
146 ==============================================================
150 Controls the kernel's callhome behavior in case of a kernel panic.
152 The s390 hardware allows an operating system to send a notification
153 to a service organization (callhome) in case of an operating system panic.
155 When the value in this file is 0 (which is the default behavior)
156 nothing happens in case of a kernel panic. If this value is set to "1"
157 the complete kernel oops message is send to the IBM customer service
158 organization in case the mainframe the Linux operating system is running
159 on has a service contract with IBM.
161 ==============================================================
165 Highest valid capability of the running kernel. Exports
166 CAP_LAST_CAP from the kernel.
168 ==============================================================
172 core_pattern is used to specify a core dumpfile pattern name.
173 . max length 128 characters; default value is "core"
174 . core_pattern is used as a pattern template for the output filename;
175 certain string patterns (beginning with '%') are substituted with
177 . backward compatibility with core_uses_pid:
178 If core_pattern does not include "%p" (default does not)
179 and core_uses_pid is set, then .PID will be appended to
181 . corename format specifiers:
182 %<NUL> '%' is dropped
187 %d dump mode, matches PR_SET_DUMPABLE and
188 /proc/sys/fs/suid_dumpable
192 %e executable filename (may be shortened)
194 %<OTHER> both are dropped
195 . If the first character of the pattern is a '|', the kernel will treat
196 the rest of the pattern as a command to run. The core dump will be
197 written to the standard input of that program instead of to a file.
199 ==============================================================
203 This sysctl is only applicable when core_pattern is configured to pipe
204 core files to a user space helper (when the first character of
205 core_pattern is a '|', see above). When collecting cores via a pipe
206 to an application, it is occasionally useful for the collecting
207 application to gather data about the crashing process from its
208 /proc/pid directory. In order to do this safely, the kernel must wait
209 for the collecting process to exit, so as not to remove the crashing
210 processes proc files prematurely. This in turn creates the
211 possibility that a misbehaving userspace collecting process can block
212 the reaping of a crashed process simply by never exiting. This sysctl
213 defends against that. It defines how many concurrent crashing
214 processes may be piped to user space applications in parallel. If
215 this value is exceeded, then those crashing processes above that value
216 are noted via the kernel log and their cores are skipped. 0 is a
217 special value, indicating that unlimited processes may be captured in
218 parallel, but that no waiting will take place (i.e. the collecting
219 process is not guaranteed access to /proc/<crashing pid>/). This
222 ==============================================================
226 The default coredump filename is "core". By setting
227 core_uses_pid to 1, the coredump filename becomes core.PID.
228 If core_pattern does not include "%p" (default does not)
229 and core_uses_pid is set, then .PID will be appended to
232 ==============================================================
236 When the value in this file is 0, ctrl-alt-del is trapped and
237 sent to the init(1) program to handle a graceful restart.
238 When, however, the value is > 0, Linux's reaction to a Vulcan
239 Nerve Pinch (tm) will be an immediate reboot, without even
240 syncing its dirty buffers.
242 Note: when a program (like dosemu) has the keyboard in 'raw'
243 mode, the ctrl-alt-del is intercepted by the program before it
244 ever reaches the kernel tty layer, and it's up to the program
245 to decide what to do with it.
247 ==============================================================
251 This toggle indicates whether unprivileged users are prevented
252 from using dmesg(8) to view messages from the kernel's log buffer.
253 When dmesg_restrict is set to (0) there are no restrictions. When
254 dmesg_restrict is set set to (1), users must have CAP_SYSLOG to use
257 The kernel config option CONFIG_SECURITY_DMESG_RESTRICT sets the
258 default value of dmesg_restrict.
260 ==============================================================
262 domainname & hostname:
264 These files can be used to set the NIS/YP domainname and the
265 hostname of your box in exactly the same way as the commands
266 domainname and hostname, i.e.:
267 # echo "darkstar" > /proc/sys/kernel/hostname
268 # echo "mydomain" > /proc/sys/kernel/domainname
269 has the same effect as
270 # hostname "darkstar"
271 # domainname "mydomain"
273 Note, however, that the classic darkstar.frop.org has the
274 hostname "darkstar" and DNS (Internet Domain Name Server)
275 domainname "frop.org", not to be confused with the NIS (Network
276 Information Service) or YP (Yellow Pages) domainname. These two
277 domain names are in general different. For a detailed discussion
278 see the hostname(1) man page.
280 ==============================================================
284 Path for the hotplug policy agent.
285 Default value is "/sbin/hotplug".
287 ==============================================================
291 This toggle indicates whether restrictions are placed on
292 exposing kernel addresses via /proc and other interfaces. When
293 kptr_restrict is set to (0), there are no restrictions. When
294 kptr_restrict is set to (1), the default, kernel pointers
295 printed using the %pK format specifier will be replaced with 0's
296 unless the user has CAP_SYSLOG. When kptr_restrict is set to
297 (2), kernel pointers printed using %pK will be replaced with 0's
298 regardless of privileges.
300 ==============================================================
302 kstack_depth_to_print: (X86 only)
304 Controls the number of words to print when dumping the raw
307 ==============================================================
311 This flag controls the L2 cache of G3 processor boards. If
312 0, the cache is disabled. Enabled if nonzero.
314 ==============================================================
318 A toggle value indicating if modules are allowed to be loaded
319 in an otherwise modular kernel. This toggle defaults to off
320 (0), but can be set true (1). Once true, modules can be
321 neither loaded nor unloaded, and the toggle cannot be set back
324 ==============================================================
326 msg_next_id, sem_next_id, and shm_next_id:
328 These three toggles allows to specify desired id for next allocated IPC
329 object: message, semaphore or shared memory respectively.
331 By default they are equal to -1, which means generic allocation logic.
332 Possible values to set are in range {0..INT_MAX}.
335 1) kernel doesn't guarantee, that new object will have desired id. So,
336 it's up to userspace, how to handle an object with "wrong" id.
337 2) Toggle with non-default value will be set back to -1 by kernel after
338 successful IPC object allocation.
340 ==============================================================
344 Enables/Disables the NMI watchdog on x86 systems. When the value is
345 non-zero the NMI watchdog is enabled and will continuously test all
346 online cpus to determine whether or not they are still functioning
347 properly. Currently, passing "nmi_watchdog=" parameter at boot time is
348 required for this function to work.
350 If LAPIC NMI watchdog method is in use (nmi_watchdog=2 kernel
351 parameter), the NMI watchdog shares registers with oprofile. By
352 disabling the NMI watchdog, oprofile may have more registers to
355 ==============================================================
357 osrelease, ostype & version:
364 #5 Wed Feb 25 21:49:24 MET 1998
366 The files osrelease and ostype should be clear enough. Version
367 needs a little more clarification however. The '#5' means that
368 this is the fifth kernel built from this source base and the
369 date behind it indicates the time the kernel was built.
370 The only way to tune these values is to rebuild the kernel :-)
372 ==============================================================
374 overflowgid & overflowuid:
376 if your architecture did not always support 32-bit UIDs (i.e. arm,
377 i386, m68k, sh, and sparc32), a fixed UID and GID will be returned to
378 applications that use the old 16-bit UID/GID system calls, if the
379 actual UID or GID would exceed 65535.
381 These sysctls allow you to change the value of the fixed UID and GID.
382 The default is 65534.
384 ==============================================================
388 The value in this file represents the number of seconds the kernel
389 waits before rebooting on a panic. When you use the software watchdog,
390 the recommended setting is 60.
392 ==============================================================
394 panic_on_unrecovered_nmi:
396 The default Linux behaviour on an NMI of either memory or unknown is
397 to continue operation. For many environments such as scientific
398 computing it is preferable that the box is taken out and the error
399 dealt with than an uncorrected parity/ECC error get propagated.
401 A small number of systems do generate NMI's for bizarre random reasons
402 such as power management so the default is off. That sysctl works like
403 the existing panic controls already in that directory.
405 ==============================================================
409 Controls the kernel's behaviour when an oops or BUG is encountered.
411 0: try to continue operation
413 1: panic immediately. If the `panic' sysctl is also non-zero then the
414 machine will be rebooted.
416 ==============================================================
418 panic_on_stackoverflow:
420 Controls the kernel's behavior when detecting the overflows of
421 kernel, IRQ and exception stacks except a user stack.
422 This file shows up if CONFIG_DEBUG_STACKOVERFLOW is enabled.
424 0: try to continue operation.
426 1: panic immediately.
428 ==============================================================
430 perf_cpu_time_max_percent:
432 Hints to the kernel how much CPU time it should be allowed to
433 use to handle perf sampling events. If the perf subsystem
434 is informed that its samples are exceeding this limit, it
435 will drop its sampling frequency to attempt to reduce its CPU
438 Some perf sampling happens in NMIs. If these samples
439 unexpectedly take too long to execute, the NMIs can become
440 stacked up next to each other so much that nothing else is
443 0: disable the mechanism. Do not monitor or correct perf's
444 sampling rate no matter how CPU time it takes.
446 1-100: attempt to throttle perf's sample rate to this
447 percentage of CPU. Note: the kernel calculates an
448 "expected" length of each sample event. 100 here means
449 100% of that expected length. Even if this is set to
450 100, you may still see sample throttling if this
451 length is exceeded. Set to 0 if you truly do not care
452 how much CPU is consumed.
454 ==============================================================
459 PID allocation wrap value. When the kernel's next PID value
460 reaches this value, it wraps back to a minimum PID value.
461 PIDs of value pid_max or larger are not allocated.
463 ==============================================================
467 The last pid allocated in the current (the one task using this sysctl
468 lives in) pid namespace. When selecting a pid for a next task on fork
469 kernel tries to allocate a number starting from this one.
471 ==============================================================
473 powersave-nap: (PPC only)
475 If set, Linux-PPC will use the 'nap' mode of powersaving,
476 otherwise the 'doze' mode will be used.
478 ==============================================================
482 The four values in printk denote: console_loglevel,
483 default_message_loglevel, minimum_console_loglevel and
484 default_console_loglevel respectively.
486 These values influence printk() behavior when printing or
487 logging error messages. See 'man 2 syslog' for more info on
488 the different loglevels.
490 - console_loglevel: messages with a higher priority than
491 this will be printed to the console
492 - default_message_loglevel: messages without an explicit priority
493 will be printed with this priority
494 - minimum_console_loglevel: minimum (highest) value to which
495 console_loglevel can be set
496 - default_console_loglevel: default value for console_loglevel
498 ==============================================================
502 Delay each printk message in printk_delay milliseconds
504 Value from 0 - 10000 is allowed.
506 ==============================================================
510 Some warning messages are rate limited. printk_ratelimit specifies
511 the minimum length of time between these messages (in jiffies), by
512 default we allow one every 5 seconds.
514 A value of 0 will disable rate limiting.
516 ==============================================================
518 printk_ratelimit_burst:
520 While long term we enforce one message per printk_ratelimit
521 seconds, we do allow a burst of messages to pass through.
522 printk_ratelimit_burst specifies the number of messages we can
523 send before ratelimiting kicks in.
525 ==============================================================
529 This option can be used to select the type of process address
530 space randomization that is used in the system, for architectures
531 that support this feature.
533 0 - Turn the process address space randomization off. This is the
534 default for architectures that do not support this feature anyways,
535 and kernels that are booted with the "norandmaps" parameter.
537 1 - Make the addresses of mmap base, stack and VDSO page randomized.
538 This, among other things, implies that shared libraries will be
539 loaded to random addresses. Also for PIE-linked binaries, the
540 location of code start is randomized. This is the default if the
541 CONFIG_COMPAT_BRK option is enabled.
543 2 - Additionally enable heap randomization. This is the default if
544 CONFIG_COMPAT_BRK is disabled.
546 There are a few legacy applications out there (such as some ancient
547 versions of libc.so.5 from 1996) that assume that brk area starts
548 just after the end of the code+bss. These applications break when
549 start of the brk area is randomized. There are however no known
550 non-legacy applications that would be broken this way, so for most
551 systems it is safe to choose full randomization.
553 Systems with ancient and/or broken binaries should be configured
554 with CONFIG_COMPAT_BRK enabled, which excludes the heap from process
555 address space randomization.
557 ==============================================================
559 reboot-cmd: (Sparc only)
561 ??? This seems to be a way to give an argument to the Sparc
562 ROM/Flash boot loader. Maybe to tell it what to do after
565 ==============================================================
567 rtsig-max & rtsig-nr:
569 The file rtsig-max can be used to tune the maximum number
570 of POSIX realtime (queued) signals that can be outstanding
573 rtsig-nr shows the number of RT signals currently queued.
575 ==============================================================
579 This file shows the size of the generic SCSI (sg) buffer.
580 You can't tune it just yet, but you could change it on
581 compile time by editing include/scsi/sg.h and changing
582 the value of SG_BIG_BUFF.
584 There shouldn't be any reason to change this value. If
585 you can come up with one, you probably know what you
588 ==============================================================
592 This parameter sets the total amount of shared memory pages that
593 can be used system wide. Hence, SHMALL should always be at least
594 ceil(shmmax/PAGE_SIZE).
596 If you are not sure what the default PAGE_SIZE is on your Linux
597 system, you can run the following command:
601 ==============================================================
605 This value can be used to query and set the run time limit
606 on the maximum shared memory segment size that can be created.
607 Shared memory segments up to 1Gb are now supported in the
608 kernel. This value defaults to SHMMAX.
610 ==============================================================
614 Linux lets you set resource limits, including how much memory one
615 process can consume, via setrlimit(2). Unfortunately, shared memory
616 segments are allowed to exist without association with any process, and
617 thus might not be counted against any resource limits. If enabled,
618 shared memory segments are automatically destroyed when their attach
619 count becomes zero after a detach or a process termination. It will
620 also destroy segments that were created, but never attached to, on exit
621 from the process. The only use left for IPC_RMID is to immediately
622 destroy an unattached segment. Of course, this breaks the way things are
623 defined, so some applications might stop working. Note that this
624 feature will do you no good unless you also configure your resource
625 limits (in particular, RLIMIT_AS and RLIMIT_NPROC). Most systems don't
628 Note that if you change this from 0 to 1, already created segments
629 without users and with a dead originative process will be destroyed.
631 ==============================================================
635 Non-zero if the kernel has been tainted. Numeric values, which
636 can be ORed together:
638 1 - A module with a non-GPL license has been loaded, this
639 includes modules with no license.
640 Set by modutils >= 2.4.9 and module-init-tools.
641 2 - A module was force loaded by insmod -f.
642 Set by modutils >= 2.4.9 and module-init-tools.
643 4 - Unsafe SMP processors: SMP with CPUs not designed for SMP.
644 8 - A module was forcibly unloaded from the system by rmmod -f.
645 16 - A hardware machine check error occurred on the system.
646 32 - A bad page was discovered on the system.
647 64 - The user has asked that the system be marked "tainted". This
648 could be because they are running software that directly modifies
649 the hardware, or for other reasons.
650 128 - The system has died.
651 256 - The ACPI DSDT has been overridden with one supplied by the user
652 instead of using the one provided by the hardware.
653 512 - A kernel warning has occurred.
654 1024 - A module from drivers/staging was loaded.
655 2048 - The system is working around a severe firmware bug.
656 4096 - An out-of-tree module has been loaded.
658 ==============================================================
662 The value in this file affects behavior of handling NMI. When the
663 value is non-zero, unknown NMI is trapped and then panic occurs. At
664 that time, kernel debugging information is displayed on console.
666 NMI switch that most IA32 servers have fires unknown NMI up, for
667 example. If a system hangs up, try pressing the NMI switch.
669 ==============================================================
673 This value can be used to control the frequency of hrtimer and NMI
674 events and the soft and hard lockup thresholds. The default threshold
677 The softlockup threshold is (2 * watchdog_thresh). Setting this
678 tunable to zero will disable lockup detection altogether.
680 ==============================================================