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
185 %P global pid (init PID namespace)
188 %d dump mode, matches PR_SET_DUMPABLE and
189 /proc/sys/fs/suid_dumpable
193 %e executable filename (may be shortened)
195 %<OTHER> both are dropped
196 . If the first character of the pattern is a '|', the kernel will treat
197 the rest of the pattern as a command to run. The core dump will be
198 written to the standard input of that program instead of to a file.
200 ==============================================================
204 This sysctl is only applicable when core_pattern is configured to pipe
205 core files to a user space helper (when the first character of
206 core_pattern is a '|', see above). When collecting cores via a pipe
207 to an application, it is occasionally useful for the collecting
208 application to gather data about the crashing process from its
209 /proc/pid directory. In order to do this safely, the kernel must wait
210 for the collecting process to exit, so as not to remove the crashing
211 processes proc files prematurely. This in turn creates the
212 possibility that a misbehaving userspace collecting process can block
213 the reaping of a crashed process simply by never exiting. This sysctl
214 defends against that. It defines how many concurrent crashing
215 processes may be piped to user space applications in parallel. If
216 this value is exceeded, then those crashing processes above that value
217 are noted via the kernel log and their cores are skipped. 0 is a
218 special value, indicating that unlimited processes may be captured in
219 parallel, but that no waiting will take place (i.e. the collecting
220 process is not guaranteed access to /proc/<crashing pid>/). This
223 ==============================================================
227 The default coredump filename is "core". By setting
228 core_uses_pid to 1, the coredump filename becomes core.PID.
229 If core_pattern does not include "%p" (default does not)
230 and core_uses_pid is set, then .PID will be appended to
233 ==============================================================
237 When the value in this file is 0, ctrl-alt-del is trapped and
238 sent to the init(1) program to handle a graceful restart.
239 When, however, the value is > 0, Linux's reaction to a Vulcan
240 Nerve Pinch (tm) will be an immediate reboot, without even
241 syncing its dirty buffers.
243 Note: when a program (like dosemu) has the keyboard in 'raw'
244 mode, the ctrl-alt-del is intercepted by the program before it
245 ever reaches the kernel tty layer, and it's up to the program
246 to decide what to do with it.
248 ==============================================================
252 This toggle indicates whether unprivileged users are prevented
253 from using dmesg(8) to view messages from the kernel's log buffer.
254 When dmesg_restrict is set to (0) there are no restrictions. When
255 dmesg_restrict is set set to (1), users must have CAP_SYSLOG to use
258 The kernel config option CONFIG_SECURITY_DMESG_RESTRICT sets the
259 default value of dmesg_restrict.
261 ==============================================================
263 domainname & hostname:
265 These files can be used to set the NIS/YP domainname and the
266 hostname of your box in exactly the same way as the commands
267 domainname and hostname, i.e.:
268 # echo "darkstar" > /proc/sys/kernel/hostname
269 # echo "mydomain" > /proc/sys/kernel/domainname
270 has the same effect as
271 # hostname "darkstar"
272 # domainname "mydomain"
274 Note, however, that the classic darkstar.frop.org has the
275 hostname "darkstar" and DNS (Internet Domain Name Server)
276 domainname "frop.org", not to be confused with the NIS (Network
277 Information Service) or YP (Yellow Pages) domainname. These two
278 domain names are in general different. For a detailed discussion
279 see the hostname(1) man page.
281 ==============================================================
285 Path for the hotplug policy agent.
286 Default value is "/sbin/hotplug".
288 ==============================================================
292 This toggle indicates whether restrictions are placed on
293 exposing kernel addresses via /proc and other interfaces. When
294 kptr_restrict is set to (0), there are no restrictions. When
295 kptr_restrict is set to (1), the default, kernel pointers
296 printed using the %pK format specifier will be replaced with 0's
297 unless the user has CAP_SYSLOG. When kptr_restrict is set to
298 (2), kernel pointers printed using %pK will be replaced with 0's
299 regardless of privileges.
301 ==============================================================
303 kstack_depth_to_print: (X86 only)
305 Controls the number of words to print when dumping the raw
308 ==============================================================
312 This flag controls the L2 cache of G3 processor boards. If
313 0, the cache is disabled. Enabled if nonzero.
315 ==============================================================
319 A toggle value indicating if modules are allowed to be loaded
320 in an otherwise modular kernel. This toggle defaults to off
321 (0), but can be set true (1). Once true, modules can be
322 neither loaded nor unloaded, and the toggle cannot be set back
325 ==============================================================
327 msg_next_id, sem_next_id, and shm_next_id:
329 These three toggles allows to specify desired id for next allocated IPC
330 object: message, semaphore or shared memory respectively.
332 By default they are equal to -1, which means generic allocation logic.
333 Possible values to set are in range {0..INT_MAX}.
336 1) kernel doesn't guarantee, that new object will have desired id. So,
337 it's up to userspace, how to handle an object with "wrong" id.
338 2) Toggle with non-default value will be set back to -1 by kernel after
339 successful IPC object allocation.
341 ==============================================================
345 Enables/Disables the NMI watchdog on x86 systems. When the value is
346 non-zero the NMI watchdog is enabled and will continuously test all
347 online cpus to determine whether or not they are still functioning
348 properly. Currently, passing "nmi_watchdog=" parameter at boot time is
349 required for this function to work.
351 If LAPIC NMI watchdog method is in use (nmi_watchdog=2 kernel
352 parameter), the NMI watchdog shares registers with oprofile. By
353 disabling the NMI watchdog, oprofile may have more registers to
356 ==============================================================
360 Enables/disables automatic page fault based NUMA memory
361 balancing. Memory is moved automatically to nodes
362 that access it often.
364 Enables/disables automatic NUMA memory balancing. On NUMA machines, there
365 is a performance penalty if remote memory is accessed by a CPU. When this
366 feature is enabled the kernel samples what task thread is accessing memory
367 by periodically unmapping pages and later trapping a page fault. At the
368 time of the page fault, it is determined if the data being accessed should
369 be migrated to a local memory node.
371 The unmapping of pages and trapping faults incur additional overhead that
372 ideally is offset by improved memory locality but there is no universal
373 guarantee. If the target workload is already bound to NUMA nodes then this
374 feature should be disabled. Otherwise, if the system overhead from the
375 feature is too high then the rate the kernel samples for NUMA hinting
376 faults may be controlled by the numa_balancing_scan_period_min_ms,
377 numa_balancing_scan_delay_ms, numa_balancing_scan_period_max_ms,
378 numa_balancing_scan_size_mb, numa_balancing_settle_count sysctls and
379 numa_balancing_migrate_deferred.
381 ==============================================================
383 numa_balancing_scan_period_min_ms, numa_balancing_scan_delay_ms,
384 numa_balancing_scan_period_max_ms, numa_balancing_scan_size_mb
386 Automatic NUMA balancing scans tasks address space and unmaps pages to
387 detect if pages are properly placed or if the data should be migrated to a
388 memory node local to where the task is running. Every "scan delay" the task
389 scans the next "scan size" number of pages in its address space. When the
390 end of the address space is reached the scanner restarts from the beginning.
392 In combination, the "scan delay" and "scan size" determine the scan rate.
393 When "scan delay" decreases, the scan rate increases. The scan delay and
394 hence the scan rate of every task is adaptive and depends on historical
395 behaviour. If pages are properly placed then the scan delay increases,
396 otherwise the scan delay decreases. The "scan size" is not adaptive but
397 the higher the "scan size", the higher the scan rate.
399 Higher scan rates incur higher system overhead as page faults must be
400 trapped and potentially data must be migrated. However, the higher the scan
401 rate, the more quickly a tasks memory is migrated to a local node if the
402 workload pattern changes and minimises performance impact due to remote
403 memory accesses. These sysctls control the thresholds for scan delays and
404 the number of pages scanned.
406 numa_balancing_scan_period_min_ms is the minimum time in milliseconds to
407 scan a tasks virtual memory. It effectively controls the maximum scanning
410 numa_balancing_scan_delay_ms is the starting "scan delay" used for a task
411 when it initially forks.
413 numa_balancing_scan_period_max_ms is the maximum time in milliseconds to
414 scan a tasks virtual memory. It effectively controls the minimum scanning
417 numa_balancing_scan_size_mb is how many megabytes worth of pages are
418 scanned for a given scan.
420 numa_balancing_settle_count is how many scan periods must complete before
421 the schedule balancer stops pushing the task towards a preferred node. This
422 gives the scheduler a chance to place the task on an alternative node if the
423 preferred node is overloaded.
425 numa_balancing_migrate_deferred is how many page migrations get skipped
426 unconditionally, after a page migration is skipped because a page is shared
427 with other tasks. This reduces page migration overhead, and determines
428 how much stronger the "move task near its memory" policy scheduler becomes,
429 versus the "move memory near its task" memory management policy, for workloads
432 ==============================================================
434 osrelease, ostype & version:
441 #5 Wed Feb 25 21:49:24 MET 1998
443 The files osrelease and ostype should be clear enough. Version
444 needs a little more clarification however. The '#5' means that
445 this is the fifth kernel built from this source base and the
446 date behind it indicates the time the kernel was built.
447 The only way to tune these values is to rebuild the kernel :-)
449 ==============================================================
451 overflowgid & overflowuid:
453 if your architecture did not always support 32-bit UIDs (i.e. arm,
454 i386, m68k, sh, and sparc32), a fixed UID and GID will be returned to
455 applications that use the old 16-bit UID/GID system calls, if the
456 actual UID or GID would exceed 65535.
458 These sysctls allow you to change the value of the fixed UID and GID.
459 The default is 65534.
461 ==============================================================
465 The value in this file represents the number of seconds the kernel
466 waits before rebooting on a panic. When you use the software watchdog,
467 the recommended setting is 60.
469 ==============================================================
471 panic_on_unrecovered_nmi:
473 The default Linux behaviour on an NMI of either memory or unknown is
474 to continue operation. For many environments such as scientific
475 computing it is preferable that the box is taken out and the error
476 dealt with than an uncorrected parity/ECC error get propagated.
478 A small number of systems do generate NMI's for bizarre random reasons
479 such as power management so the default is off. That sysctl works like
480 the existing panic controls already in that directory.
482 ==============================================================
486 Controls the kernel's behaviour when an oops or BUG is encountered.
488 0: try to continue operation
490 1: panic immediately. If the `panic' sysctl is also non-zero then the
491 machine will be rebooted.
493 ==============================================================
495 panic_on_stackoverflow:
497 Controls the kernel's behavior when detecting the overflows of
498 kernel, IRQ and exception stacks except a user stack.
499 This file shows up if CONFIG_DEBUG_STACKOVERFLOW is enabled.
501 0: try to continue operation.
503 1: panic immediately.
505 ==============================================================
507 perf_cpu_time_max_percent:
509 Hints to the kernel how much CPU time it should be allowed to
510 use to handle perf sampling events. If the perf subsystem
511 is informed that its samples are exceeding this limit, it
512 will drop its sampling frequency to attempt to reduce its CPU
515 Some perf sampling happens in NMIs. If these samples
516 unexpectedly take too long to execute, the NMIs can become
517 stacked up next to each other so much that nothing else is
520 0: disable the mechanism. Do not monitor or correct perf's
521 sampling rate no matter how CPU time it takes.
523 1-100: attempt to throttle perf's sample rate to this
524 percentage of CPU. Note: the kernel calculates an
525 "expected" length of each sample event. 100 here means
526 100% of that expected length. Even if this is set to
527 100, you may still see sample throttling if this
528 length is exceeded. Set to 0 if you truly do not care
529 how much CPU is consumed.
531 ==============================================================
536 PID allocation wrap value. When the kernel's next PID value
537 reaches this value, it wraps back to a minimum PID value.
538 PIDs of value pid_max or larger are not allocated.
540 ==============================================================
544 The last pid allocated in the current (the one task using this sysctl
545 lives in) pid namespace. When selecting a pid for a next task on fork
546 kernel tries to allocate a number starting from this one.
548 ==============================================================
550 powersave-nap: (PPC only)
552 If set, Linux-PPC will use the 'nap' mode of powersaving,
553 otherwise the 'doze' mode will be used.
555 ==============================================================
559 The four values in printk denote: console_loglevel,
560 default_message_loglevel, minimum_console_loglevel and
561 default_console_loglevel respectively.
563 These values influence printk() behavior when printing or
564 logging error messages. See 'man 2 syslog' for more info on
565 the different loglevels.
567 - console_loglevel: messages with a higher priority than
568 this will be printed to the console
569 - default_message_loglevel: messages without an explicit priority
570 will be printed with this priority
571 - minimum_console_loglevel: minimum (highest) value to which
572 console_loglevel can be set
573 - default_console_loglevel: default value for console_loglevel
575 ==============================================================
579 Delay each printk message in printk_delay milliseconds
581 Value from 0 - 10000 is allowed.
583 ==============================================================
587 Some warning messages are rate limited. printk_ratelimit specifies
588 the minimum length of time between these messages (in jiffies), by
589 default we allow one every 5 seconds.
591 A value of 0 will disable rate limiting.
593 ==============================================================
595 printk_ratelimit_burst:
597 While long term we enforce one message per printk_ratelimit
598 seconds, we do allow a burst of messages to pass through.
599 printk_ratelimit_burst specifies the number of messages we can
600 send before ratelimiting kicks in.
602 ==============================================================
606 This option can be used to select the type of process address
607 space randomization that is used in the system, for architectures
608 that support this feature.
610 0 - Turn the process address space randomization off. This is the
611 default for architectures that do not support this feature anyways,
612 and kernels that are booted with the "norandmaps" parameter.
614 1 - Make the addresses of mmap base, stack and VDSO page randomized.
615 This, among other things, implies that shared libraries will be
616 loaded to random addresses. Also for PIE-linked binaries, the
617 location of code start is randomized. This is the default if the
618 CONFIG_COMPAT_BRK option is enabled.
620 2 - Additionally enable heap randomization. This is the default if
621 CONFIG_COMPAT_BRK is disabled.
623 There are a few legacy applications out there (such as some ancient
624 versions of libc.so.5 from 1996) that assume that brk area starts
625 just after the end of the code+bss. These applications break when
626 start of the brk area is randomized. There are however no known
627 non-legacy applications that would be broken this way, so for most
628 systems it is safe to choose full randomization.
630 Systems with ancient and/or broken binaries should be configured
631 with CONFIG_COMPAT_BRK enabled, which excludes the heap from process
632 address space randomization.
634 ==============================================================
636 reboot-cmd: (Sparc only)
638 ??? This seems to be a way to give an argument to the Sparc
639 ROM/Flash boot loader. Maybe to tell it what to do after
642 ==============================================================
644 rtsig-max & rtsig-nr:
646 The file rtsig-max can be used to tune the maximum number
647 of POSIX realtime (queued) signals that can be outstanding
650 rtsig-nr shows the number of RT signals currently queued.
652 ==============================================================
656 This file shows the size of the generic SCSI (sg) buffer.
657 You can't tune it just yet, but you could change it on
658 compile time by editing include/scsi/sg.h and changing
659 the value of SG_BIG_BUFF.
661 There shouldn't be any reason to change this value. If
662 you can come up with one, you probably know what you
665 ==============================================================
669 This parameter sets the total amount of shared memory pages that
670 can be used system wide. Hence, SHMALL should always be at least
671 ceil(shmmax/PAGE_SIZE).
673 If you are not sure what the default PAGE_SIZE is on your Linux
674 system, you can run the following command:
678 ==============================================================
682 This value can be used to query and set the run time limit
683 on the maximum shared memory segment size that can be created.
684 Shared memory segments up to 1Gb are now supported in the
685 kernel. This value defaults to SHMMAX.
687 ==============================================================
691 Linux lets you set resource limits, including how much memory one
692 process can consume, via setrlimit(2). Unfortunately, shared memory
693 segments are allowed to exist without association with any process, and
694 thus might not be counted against any resource limits. If enabled,
695 shared memory segments are automatically destroyed when their attach
696 count becomes zero after a detach or a process termination. It will
697 also destroy segments that were created, but never attached to, on exit
698 from the process. The only use left for IPC_RMID is to immediately
699 destroy an unattached segment. Of course, this breaks the way things are
700 defined, so some applications might stop working. Note that this
701 feature will do you no good unless you also configure your resource
702 limits (in particular, RLIMIT_AS and RLIMIT_NPROC). Most systems don't
705 Note that if you change this from 0 to 1, already created segments
706 without users and with a dead originative process will be destroyed.
708 ==============================================================
712 Non-zero if the kernel has been tainted. Numeric values, which
713 can be ORed together:
715 1 - A module with a non-GPL license has been loaded, this
716 includes modules with no license.
717 Set by modutils >= 2.4.9 and module-init-tools.
718 2 - A module was force loaded by insmod -f.
719 Set by modutils >= 2.4.9 and module-init-tools.
720 4 - Unsafe SMP processors: SMP with CPUs not designed for SMP.
721 8 - A module was forcibly unloaded from the system by rmmod -f.
722 16 - A hardware machine check error occurred on the system.
723 32 - A bad page was discovered on the system.
724 64 - The user has asked that the system be marked "tainted". This
725 could be because they are running software that directly modifies
726 the hardware, or for other reasons.
727 128 - The system has died.
728 256 - The ACPI DSDT has been overridden with one supplied by the user
729 instead of using the one provided by the hardware.
730 512 - A kernel warning has occurred.
731 1024 - A module from drivers/staging was loaded.
732 2048 - The system is working around a severe firmware bug.
733 4096 - An out-of-tree module has been loaded.
735 ==============================================================
739 The value in this file affects behavior of handling NMI. When the
740 value is non-zero, unknown NMI is trapped and then panic occurs. At
741 that time, kernel debugging information is displayed on console.
743 NMI switch that most IA32 servers have fires unknown NMI up, for
744 example. If a system hangs up, try pressing the NMI switch.
746 ==============================================================
750 This value can be used to control the frequency of hrtimer and NMI
751 events and the soft and hard lockup thresholds. The default threshold
754 The softlockup threshold is (2 * watchdog_thresh). Setting this
755 tunable to zero will disable lockup detection altogether.
757 ==============================================================