Documentation for /proc/sys/kernel/* kernel version 2.2.10 (c) 1998, 1999, Rik van Riel 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 - ctrl-alt-del - dentry-state - domainname - hostname - htab-reclaim [ PPC only ] - java-appletviewer [ binfmt_java, obsolete ] - java-interpreter [ binfmt_java, obsolete ] - l2cr [ PPC only ] - modprobe ==> Documentation/kmod.txt - osrelease - ostype - overflowgid - overflowuid - panic - powersave-nap [ PPC only ] - printk - real-root-dev ==> Documentation/initrd.txt - reboot-cmd [ SPARC only ] - rtsig-nr - rtsig-max - sg-big-buff [ generic SCSI device (sg) ] - shmmax [ sysv ipc ] - version - zero-paged [ PPC only ] ============================================================== 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. ============================================================== 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. ============================================================== 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" > /proc/sys/kernel/hostname # domainname "mydomain" > /proc/sys/kernel/domainname 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. ============================================================== htab-reclaim: (PPC only) Setting this to a non-zero value, the PowerPC htab (see Documentation/powerpc/ppc_htab.txt) is pruned each time the system hits the idle loop. ============================================================== l2cr: (PPC only) This flag controls the L2 cache of G3 processor boards. If 0, the cache is disabled. Enabled if nonzero. ============================================================== 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. ============================================================== 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_level 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_level: 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 Note: a quick look in linux/kernel/printk.c will reveal that these variables aren't put inside a structure, so their order in-core isn't formally guaranteed and garbage values _might_ occur when the compiler changes. (???) ============================================================== 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 :) ============================================================== 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. ============================================================== zero-paged: (PPC only) When enabled (non-zero), Linux-PPC will pre-zero pages in the idle loop, possibly speeding up get_free_pages. Since this only affects what the idle loop is doing, you should enable this and see if anything changes.