Documentation for /proc/sys/kernel/* kernel version 2.2.10 (c) 1998, 1999, Rik van Riel (c) 2009, Shen Feng For general info and legal blurb, please look in README. ============================================================== This file contains documentation for the sysctl files in /proc/sys/kernel/ and is valid for Linux kernel version 2.2. The files in this directory can be used to tune and monitor miscellaneous and general things in the operation of the Linux kernel. Since some of the files _can_ be used to screw up your system, it is advisable to read both documentation and source before actually making adjustments. Currently, these files might (depending on your configuration) show up in /proc/sys/kernel: - acct - acpi_video_flags - auto_msgmni - bootloader_type [ X86 only ] - bootloader_version [ X86 only ] - callhome [ S390 only ] - cap_last_cap - core_pattern - core_pipe_limit - core_uses_pid - ctrl-alt-del - dmesg_restrict - domainname - hostname - hotplug - hung_task_panic - hung_task_check_count - hung_task_timeout_secs - hung_task_warnings - kexec_load_disabled - kptr_restrict - kstack_depth_to_print [ X86 only ] - l2cr [ PPC only ] - modprobe ==> Documentation/debugging-modules.txt - modules_disabled - msg_next_id [ sysv ipc ] - msgmax - msgmnb - msgmni - nmi_watchdog - osrelease - ostype - overflowgid - overflowuid - panic - panic_on_oops - panic_on_stackoverflow - panic_on_unrecovered_nmi - panic_on_warn - pid_max - powersave-nap [ PPC only ] - printk - printk_delay - printk_ratelimit - printk_ratelimit_burst - randomize_va_space - real-root-dev ==> Documentation/initrd.txt - reboot-cmd [ SPARC only ] - rtsig-max - rtsig-nr - sem - sem_next_id [ sysv ipc ] - sg-big-buff [ generic SCSI device (sg) ] - shm_next_id [ sysv ipc ] - shm_rmid_forced - shmall - shmmax [ sysv ipc ] - shmmni - softlockup_all_cpu_backtrace - stop-a [ SPARC only ] - sysrq ==> Documentation/sysrq.txt - sysctl_writes_strict - tainted - threads-max - unknown_nmi_panic - watchdog_thresh - version ============================================================== acct: highwater lowwater frequency If BSD-style process accounting is enabled these values control its behaviour. If free space on filesystem where the log lives goes below % accounting suspends. If free space gets above % accounting resumes. determines how often do we check the amount of free space (value is in seconds). Default: 4 2 30 That is, suspend accounting if there left <= 2% free; resume it if we got >=4%; consider information about amount of free space valid for 30 seconds. ============================================================== acpi_video_flags: flags See Doc*/kernel/power/video.txt, it allows mode of video boot to be set during run time. ============================================================== auto_msgmni: This variable has no effect and may be removed in future kernel releases. Reading it always returns 0. Up to Linux 3.17, it enabled/disabled automatic recomputing of msgmni upon memory add/remove or upon ipc namespace creation/removal. Echoing "1" into this file enabled msgmni automatic recomputing. Echoing "0" turned it off. auto_msgmni default value was 1. ============================================================== bootloader_type: x86 bootloader identification This gives the bootloader type number as indicated by the bootloader, shifted left by 4, and OR'd with the low four bits of the bootloader version. The reason for this encoding is that this used to match the type_of_loader field in the kernel header; the encoding is kept for backwards compatibility. That is, if the full bootloader type number is 0x15 and the full version number is 0x234, this file will contain the value 340 = 0x154. See the type_of_loader and ext_loader_type fields in Documentation/x86/boot.txt for additional information. ============================================================== bootloader_version: x86 bootloader version The complete bootloader version number. In the example above, this file will contain the value 564 = 0x234. See the type_of_loader and ext_loader_ver fields in Documentation/x86/boot.txt for additional information. ============================================================== callhome: Controls the kernel's callhome behavior in case of a kernel panic. The s390 hardware allows an operating system to send a notification to a service organization (callhome) in case of an operating system panic. When the value in this file is 0 (which is the default behavior) nothing happens in case of a kernel panic. If this value is set to "1" the complete kernel oops message is send to the IBM customer service organization in case the mainframe the Linux operating system is running on has a service contract with IBM. ============================================================== cap_last_cap Highest valid capability of the running kernel. Exports CAP_LAST_CAP from the kernel. ============================================================== core_pattern: core_pattern is used to specify a core dumpfile pattern name. . max length 128 characters; default value is "core" . core_pattern is used as a pattern template for the output filename; certain string patterns (beginning with '%') are substituted with their actual values. . backward compatibility with core_uses_pid: If core_pattern does not include "%p" (default does not) and core_uses_pid is set, then .PID will be appended to the filename. . corename format specifiers: % '%' is dropped %% output one '%' %p pid %P global pid (init PID namespace) %i tid %I global tid (init PID namespace) %u uid %g gid %d dump mode, matches PR_SET_DUMPABLE and /proc/sys/fs/suid_dumpable %s signal number %t UNIX time of dump %h hostname %e executable filename (may be shortened) %E executable path % both are dropped . If the first character of the pattern is a '|', the kernel will treat the rest of the pattern as a command to run. The core dump will be written to the standard input of that program instead of to a file. ============================================================== core_pipe_limit: This sysctl is only applicable when core_pattern is configured to pipe core files to a user space helper (when the first character of core_pattern is a '|', see above). When collecting cores via a pipe to an application, it is occasionally useful for the collecting application to gather data about the crashing process from its /proc/pid directory. In order to do this safely, the kernel must wait for the collecting process to exit, so as not to remove the crashing processes proc files prematurely. This in turn creates the possibility that a misbehaving userspace collecting process can block the reaping of a crashed process simply by never exiting. This sysctl defends against that. It defines how many concurrent crashing processes may be piped to user space applications in parallel. If this value is exceeded, then those crashing processes above that value are noted via the kernel log and their cores are skipped. 0 is a special value, indicating that unlimited processes may be captured in parallel, but that no waiting will take place (i.e. the collecting process is not guaranteed access to /proc//). This value defaults to 0. ============================================================== core_uses_pid: The default coredump filename is "core". By setting core_uses_pid to 1, the coredump filename becomes core.PID. If core_pattern does not include "%p" (default does not) and core_uses_pid is set, then .PID will be appended to the filename. ============================================================== ctrl-alt-del: When the value in this file is 0, ctrl-alt-del is trapped and sent to the init(1) program to handle a graceful restart. When, however, the value is > 0, Linux's reaction to a Vulcan Nerve Pinch (tm) will be an immediate reboot, without even syncing its dirty buffers. Note: when a program (like dosemu) has the keyboard in 'raw' mode, the ctrl-alt-del is intercepted by the program before it ever reaches the kernel tty layer, and it's up to the program to decide what to do with it. ============================================================== dmesg_restrict: This toggle indicates whether unprivileged users are prevented from using dmesg(8) to view messages from the kernel's log buffer. When dmesg_restrict is set to (0) there are no restrictions. When dmesg_restrict is set set to (1), users must have CAP_SYSLOG to use dmesg(8). The kernel config option CONFIG_SECURITY_DMESG_RESTRICT sets the default value of dmesg_restrict. ============================================================== domainname & hostname: These files can be used to set the NIS/YP domainname and the hostname of your box in exactly the same way as the commands domainname and hostname, i.e.: # echo "darkstar" > /proc/sys/kernel/hostname # echo "mydomain" > /proc/sys/kernel/domainname has the same effect as # hostname "darkstar" # domainname "mydomain" Note, however, that the classic darkstar.frop.org has the hostname "darkstar" and DNS (Internet Domain Name Server) domainname "frop.org", not to be confused with the NIS (Network Information Service) or YP (Yellow Pages) domainname. These two domain names are in general different. For a detailed discussion see the hostname(1) man page. ============================================================== hotplug: Path for the hotplug policy agent. Default value is "/sbin/hotplug". ============================================================== hung_task_panic: Controls the kernel's behavior when a hung task is detected. This file shows up if CONFIG_DETECT_HUNG_TASK is enabled. 0: continue operation. This is the default behavior. 1: panic immediately. ============================================================== hung_task_check_count: The upper bound on the number of tasks that are checked. This file shows up if CONFIG_DETECT_HUNG_TASK is enabled. ============================================================== hung_task_timeout_secs: Check interval. When a task in D state did not get scheduled for more than this value report a warning. This file shows up if CONFIG_DETECT_HUNG_TASK is enabled. 0: means infinite timeout - no checking done. Possible values to set are in range {0..LONG_MAX/HZ}. ============================================================== hung_task_warnings: The maximum number of warnings to report. During a check interval if a hung task is detected, this value is decreased by 1. When this value reaches 0, no more warnings will be reported. This file shows up if CONFIG_DETECT_HUNG_TASK is enabled. -1: report an infinite number of warnings. ============================================================== kexec_load_disabled: A toggle indicating if the kexec_load syscall has been disabled. This value defaults to 0 (false: kexec_load enabled), but can be set to 1 (true: kexec_load disabled). Once true, kexec can no longer be used, and the toggle cannot be set back to false. This allows a kexec image to be loaded before disabling the syscall, allowing a system to set up (and later use) an image without it being altered. Generally used together with the "modules_disabled" sysctl. ============================================================== kptr_restrict: This toggle indicates whether restrictions are placed on exposing kernel addresses via /proc and other interfaces. When kptr_restrict is set to (0), the default, there are no restrictions. When kptr_restrict is set to (1), kernel pointers printed using the %pK format specifier will be replaced with 0's unless the user has CAP_SYSLOG and effective user and group ids are equal to the real ids. This is because %pK checks are done at read() time rather than open() time, so if permissions are elevated between the open() and the read() (e.g via a setuid binary) then %pK will not leak kernel pointers to unprivileged users. Note, this is a temporary solution only. The correct long-term solution is to do the permission checks at open() time. Consider removing world read permissions from files that use %pK, and using dmesg_restrict to protect against uses of %pK in dmesg(8) if leaking kernel pointer values to unprivileged users is a concern. When kptr_restrict is set to (2), kernel pointers printed using %pK will be replaced with 0's regardless of privileges. ============================================================== kstack_depth_to_print: (X86 only) Controls the number of words to print when dumping the raw kernel stack. ============================================================== l2cr: (PPC only) This flag controls the L2 cache of G3 processor boards. If 0, the cache is disabled. Enabled if nonzero. ============================================================== modules_disabled: A toggle value indicating if modules are allowed to be loaded in an otherwise modular kernel. This toggle defaults to off (0), but can be set true (1). Once true, modules can be neither loaded nor unloaded, and the toggle cannot be set back to false. Generally used with the "kexec_load_disabled" toggle. ============================================================== msg_next_id, sem_next_id, and shm_next_id: These three toggles allows to specify desired id for next allocated IPC object: message, semaphore or shared memory respectively. By default they are equal to -1, which means generic allocation logic. Possible values to set are in range {0..INT_MAX}. Notes: 1) kernel doesn't guarantee, that new object will have desired id. So, it's up to userspace, how to handle an object with "wrong" id. 2) Toggle with non-default value will be set back to -1 by kernel after successful IPC object allocation. ============================================================== nmi_watchdog: Enables/Disables the NMI watchdog on x86 systems. When the value is non-zero the NMI watchdog is enabled and will continuously test all online cpus to determine whether or not they are still functioning properly. Currently, passing "nmi_watchdog=" parameter at boot time is required for this function to work. If LAPIC NMI watchdog method is in use (nmi_watchdog=2 kernel parameter), the NMI watchdog shares registers with oprofile. By disabling the NMI watchdog, oprofile may have more registers to utilize. ============================================================== numa_balancing Enables/disables automatic page fault based NUMA memory balancing. Memory is moved automatically to nodes that access it often. Enables/disables automatic NUMA memory balancing. On NUMA machines, there is a performance penalty if remote memory is accessed by a CPU. When this feature is enabled the kernel samples what task thread is accessing memory by periodically unmapping pages and later trapping a page fault. At the time of the page fault, it is determined if the data being accessed should be migrated to a local memory node. The unmapping of pages and trapping faults incur additional overhead that ideally is offset by improved memory locality but there is no universal guarantee. If the target workload is already bound to NUMA nodes then this feature should be disabled. Otherwise, if the system overhead from the feature is too high then the rate the kernel samples for NUMA hinting faults may be controlled by the numa_balancing_scan_period_min_ms, numa_balancing_scan_delay_ms, numa_balancing_scan_period_max_ms, numa_balancing_scan_size_mb, and numa_balancing_settle_count sysctls. ============================================================== numa_balancing_scan_period_min_ms, numa_balancing_scan_delay_ms, numa_balancing_scan_period_max_ms, numa_balancing_scan_size_mb Automatic NUMA balancing scans tasks address space and unmaps pages to detect if pages are properly placed or if the data should be migrated to a memory node local to where the task is running. Every "scan delay" the task scans the next "scan size" number of pages in its address space. When the end of the address space is reached the scanner restarts from the beginning. In combination, the "scan delay" and "scan size" determine the scan rate. When "scan delay" decreases, the scan rate increases. The scan delay and hence the scan rate of every task is adaptive and depends on historical behaviour. If pages are properly placed then the scan delay increases, otherwise the scan delay decreases. The "scan size" is not adaptive but the higher the "scan size", the higher the scan rate. Higher scan rates incur higher system overhead as page faults must be trapped and potentially data must be migrated. However, the higher the scan rate, the more quickly a tasks memory is migrated to a local node if the workload pattern changes and minimises performance impact due to remote memory accesses. These sysctls control the thresholds for scan delays and the number of pages scanned. numa_balancing_scan_period_min_ms is the minimum time in milliseconds to scan a tasks virtual memory. It effectively controls the maximum scanning rate for each task. numa_balancing_scan_delay_ms is the starting "scan delay" used for a task when it initially forks. numa_balancing_scan_period_max_ms is the maximum time in milliseconds to scan a tasks virtual memory. It effectively controls the minimum scanning rate for each task. numa_balancing_scan_size_mb is how many megabytes worth of pages are scanned for a given scan. ============================================================== osrelease, ostype & version: # cat osrelease 2.1.88 # cat ostype Linux # cat version #5 Wed Feb 25 21:49:24 MET 1998 The files osrelease and ostype should be clear enough. Version needs a little more clarification however. The '#5' means that this is the fifth kernel built from this source base and the date behind it indicates the time the kernel was built. The only way to tune these values is to rebuild the kernel :-) ============================================================== overflowgid & overflowuid: if your architecture did not always support 32-bit UIDs (i.e. arm, i386, m68k, sh, and sparc32), a fixed UID and GID will be returned to applications that use the old 16-bit UID/GID system calls, if the actual UID or GID would exceed 65535. These sysctls allow you to change the value of the fixed UID and GID. The default is 65534. ============================================================== panic: The value in this file represents the number of seconds the kernel waits before rebooting on a panic. When you use the software watchdog, the recommended setting is 60. ============================================================== panic_on_oops: Controls the kernel's behaviour when an oops or BUG is encountered. 0: try to continue operation 1: panic immediately. If the `panic' sysctl is also non-zero then the machine will be rebooted. ============================================================== panic_on_stackoverflow: Controls the kernel's behavior when detecting the overflows of kernel, IRQ and exception stacks except a user stack. This file shows up if CONFIG_DEBUG_STACKOVERFLOW is enabled. 0: try to continue operation. 1: panic immediately. ============================================================== panic_on_unrecovered_nmi: The default Linux behaviour on an NMI of either memory or unknown is to continue operation. For many environments such as scientific computing it is preferable that the box is taken out and the error dealt with than an uncorrected parity/ECC error get propagated. A small number of systems do generate NMI's for bizarre random reasons such as power management so the default is off. That sysctl works like the existing panic controls already in that directory. ============================================================== panic_on_warn: Calls panic() in the WARN() path when set to 1. This is useful to avoid a kernel rebuild when attempting to kdump at the location of a WARN(). 0: only WARN(), default behaviour. 1: call panic() after printing out WARN() location. ============================================================== perf_cpu_time_max_percent: Hints to the kernel how much CPU time it should be allowed to use to handle perf sampling events. If the perf subsystem is informed that its samples are exceeding this limit, it will drop its sampling frequency to attempt to reduce its CPU usage. Some perf sampling happens in NMIs. If these samples unexpectedly take too long to execute, the NMIs can become stacked up next to each other so much that nothing else is allowed to execute. 0: disable the mechanism. Do not monitor or correct perf's sampling rate no matter how CPU time it takes. 1-100: attempt to throttle perf's sample rate to this percentage of CPU. Note: the kernel calculates an "expected" length of each sample event. 100 here means 100% of that expected length. Even if this is set to 100, you may still see sample throttling if this length is exceeded. Set to 0 if you truly do not care how much CPU is consumed. ============================================================== pid_max: PID allocation wrap value. When the kernel's next PID value reaches this value, it wraps back to a minimum PID value. PIDs of value pid_max or larger are not allocated. ============================================================== ns_last_pid: The last pid allocated in the current (the one task using this sysctl lives in) pid namespace. When selecting a pid for a next task on fork kernel tries to allocate a number starting from this one. ============================================================== powersave-nap: (PPC only) If set, Linux-PPC will use the 'nap' mode of powersaving, otherwise the 'doze' mode will be used. ============================================================== printk: The four values in printk denote: console_loglevel, default_message_loglevel, minimum_console_loglevel and default_console_loglevel respectively. These values influence printk() behavior when printing or logging error messages. See 'man 2 syslog' for more info on the different loglevels. - console_loglevel: messages with a higher priority than this will be printed to the console - default_message_loglevel: messages without an explicit priority will be printed with this priority - minimum_console_loglevel: minimum (highest) value to which console_loglevel can be set - default_console_loglevel: default value for console_loglevel ============================================================== printk_delay: Delay each printk message in printk_delay milliseconds Value from 0 - 10000 is allowed. ============================================================== printk_ratelimit: Some warning messages are rate limited. printk_ratelimit specifies the minimum length of time between these messages (in jiffies), by default we allow one every 5 seconds. A value of 0 will disable rate limiting. ============================================================== printk_ratelimit_burst: While long term we enforce one message per printk_ratelimit seconds, we do allow a burst of messages to pass through. printk_ratelimit_burst specifies the number of messages we can send before ratelimiting kicks in. ============================================================== randomize_va_space: This option can be used to select the type of process address space randomization that is used in the system, for architectures that support this feature. 0 - Turn the process address space randomization off. This is the default for architectures that do not support this feature anyways, and kernels that are booted with the "norandmaps" parameter. 1 - Make the addresses of mmap base, stack and VDSO page randomized. This, among other things, implies that shared libraries will be loaded to random addresses. Also for PIE-linked binaries, the location of code start is randomized. This is the default if the CONFIG_COMPAT_BRK option is enabled. 2 - Additionally enable heap randomization. This is the default if CONFIG_COMPAT_BRK is disabled. There are a few legacy applications out there (such as some ancient versions of libc.so.5 from 1996) that assume that brk area starts just after the end of the code+bss. These applications break when start of the brk area is randomized. There are however no known non-legacy applications that would be broken this way, so for most systems it is safe to choose full randomization. Systems with ancient and/or broken binaries should be configured with CONFIG_COMPAT_BRK enabled, which excludes the heap from process address space randomization. ============================================================== reboot-cmd: (Sparc only) ??? This seems to be a way to give an argument to the Sparc ROM/Flash boot loader. Maybe to tell it what to do after rebooting. ??? ============================================================== rtsig-max & rtsig-nr: The file rtsig-max can be used to tune the maximum number of POSIX realtime (queued) signals that can be outstanding in the system. rtsig-nr shows the number of RT signals currently queued. ============================================================== sg-big-buff: This file shows the size of the generic SCSI (sg) buffer. You can't tune it just yet, but you could change it on compile time by editing include/scsi/sg.h and changing the value of SG_BIG_BUFF. There shouldn't be any reason to change this value. If you can come up with one, you probably know what you are doing anyway :) ============================================================== shmall: This parameter sets the total amount of shared memory pages that can be used system wide. Hence, SHMALL should always be at least ceil(shmmax/PAGE_SIZE). If you are not sure what the default PAGE_SIZE is on your Linux system, you can run the following command: # getconf PAGE_SIZE ============================================================== shmmax: This value can be used to query and set the run time limit on the maximum shared memory segment size that can be created. Shared memory segments up to 1Gb are now supported in the kernel. This value defaults to SHMMAX. ============================================================== shm_rmid_forced: Linux lets you set resource limits, including how much memory one process can consume, via setrlimit(2). Unfortunately, shared memory segments are allowed to exist without association with any process, and thus might not be counted against any resource limits. If enabled, shared memory segments are automatically destroyed when their attach count becomes zero after a detach or a process termination. It will also destroy segments that were created, but never attached to, on exit from the process. The only use left for IPC_RMID is to immediately destroy an unattached segment. Of course, this breaks the way things are defined, so some applications might stop working. Note that this feature will do you no good unless you also configure your resource limits (in particular, RLIMIT_AS and RLIMIT_NPROC). Most systems don't need this. Note that if you change this from 0 to 1, already created segments without users and with a dead originative process will be destroyed. ============================================================== sysctl_writes_strict: Control how file position affects the behavior of updating sysctl values via the /proc/sys interface: -1 - Legacy per-write sysctl value handling, with no printk warnings. Each write syscall must fully contain the sysctl value to be written, and multiple writes on the same sysctl file descriptor will rewrite the sysctl value, regardless of file position. 0 - (default) Same behavior as above, but warn about processes that perform writes to a sysctl file descriptor when the file position is not 0. 1 - Respect file position when writing sysctl strings. Multiple writes will append to the sysctl value buffer. Anything past the max length of the sysctl value buffer will be ignored. Writes to numeric sysctl entries must always be at file position 0 and the value must be fully contained in the buffer sent in the write syscall. ============================================================== softlockup_all_cpu_backtrace: This value controls the soft lockup detector thread's behavior when a soft lockup condition is detected as to whether or not to gather further debug information. If enabled, each cpu will be issued an NMI and instructed to capture stack trace. This feature is only applicable for architectures which support NMI. 0: do nothing. This is the default behavior. 1: on detection capture more debug information. ============================================================== tainted: Non-zero if the kernel has been tainted. Numeric values, which can be ORed together: 1 - A module with a non-GPL license has been loaded, this includes modules with no license. Set by modutils >= 2.4.9 and module-init-tools. 2 - A module was force loaded by insmod -f. Set by modutils >= 2.4.9 and module-init-tools. 4 - Unsafe SMP processors: SMP with CPUs not designed for SMP. 8 - A module was forcibly unloaded from the system by rmmod -f. 16 - A hardware machine check error occurred on the system. 32 - A bad page was discovered on the system. 64 - The user has asked that the system be marked "tainted". This could be because they are running software that directly modifies the hardware, or for other reasons. 128 - The system has died. 256 - The ACPI DSDT has been overridden with one supplied by the user instead of using the one provided by the hardware. 512 - A kernel warning has occurred. 1024 - A module from drivers/staging was loaded. 2048 - The system is working around a severe firmware bug. 4096 - An out-of-tree module has been loaded. 8192 - An unsigned module has been loaded in a kernel supporting module signature. 16384 - A soft lockup has previously occurred on the system. 32768 - The kernel has been live patched. ============================================================== unknown_nmi_panic: The value in this file affects behavior of handling NMI. When the value is non-zero, unknown NMI is trapped and then panic occurs. At that time, kernel debugging information is displayed on console. NMI switch that most IA32 servers have fires unknown NMI up, for example. If a system hangs up, try pressing the NMI switch. ============================================================== watchdog_thresh: This value can be used to control the frequency of hrtimer and NMI events and the soft and hard lockup thresholds. The default threshold is 10 seconds. The softlockup threshold is (2 * watchdog_thresh). Setting this tunable to zero will disable lockup detection altogether. ==============================================================