Documentation for /proc/sys/kernel/* version 0.1 (c) 1998, 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 2.1. 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 are in /proc/sys/kernel: - ctrl-alt-del - dentry-state - domainname - file-max - file-nr - hostname - inode-max - inode-nr - inode-state - osrelease - ostype - panic - printk - securelevel - version ============================================================== 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 it's 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. ============================================================== dentry-state: From linux/fs/dentry.c: -------------------------------------------------------------- struct { int nr_dentry; int nr_unused; int age_limit; /* age in seconds */ int want_pages; /* pages requested by system */ int dummy[2]; } dentry_stat = {0, 0, 45, 0,}; -------------------------------------------------------------- Dentries are dynamically allocated and deallocated, and nr_dentry seems to be 0 all the time. Hence it's safe to assume that only nr_unused, age_limit and want_pages are used. Nr_unused seems to be exactly what it's name says. Age_limit is the age in seconds after which dcache entries can be reclaimed when memory is short and want_pages is nonzero when shrink_dcache_pages() has been called and the dcache isn't pruned yet. ============================================================== domainname & hostname: These files can be controlled to set the domainname and hostname of your box. For the classic darkstar.frop.org a simple: # echo "darkstar" > /proc/sys/kernel/hostname # echo "frop.org" > /proc/sys/kernel/domainname would suffice to set your hostname and domainname. ============================================================== file-max & file-nr: The kernel allocates filehandles dynamically, but as yet it doesn't free them again... The value in file-max denotes the maximum number of file- handles that the Linux kernel will allocate. When you get lots of error messages about running out of file handles, you might want to increase this limit. The three values in file-nr denote the number of allocated file handles, the number of used file handles and the maximum number of file handles. When the allocated filehandles come close to the maximum, but the number of actually used ones is far behind, you've encountered a peek in your filehandle usage and you don't need to increase the maximum. ============================================================== inode-max, inode-nr & inode-state: As with filehandles, the kernel allocates the inode structures dynamically, but can't free them yet... The value in inode-max denotes the maximum number of inode handlers. This value should be 3-4 times larger as the value in file-max, since stdin, stdout and network sockets also need an inode struct to handle them. When you regularly run out of inodes, you need to increase this value. The file inode-nr contains the first two items from inode-state, so we'll skip to that file... Inode-state contains three actual numbers and four dummies. The actual numbers are, in order of appearance, nr_inodes, nr_free_inodes and preshrink. Nr_inodes stands for the number of inodes the system has allocated, this can be slightly more than inode-max because Linux allocates them one pagefull at a time. Nr_free_inodes represents the number of free inodes (?) and preshrink is nonzero when the nr_inodes > inode-max and the system needs to prune the inode list instead of allocating more. ============================================================== 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 :-) ============================================================== 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. ============================================================== 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. (???) ============================================================== securelevel: When the value in this file is nonzero, root is prohibited from: - changing the immutable and append-only flags on files - changing sysctl things (limited ???) ============================================================== real-root-dev: (CONFIG_INITRD only) This file is used to configure the real root device when using an initial ramdisk to configure the system before switching to the 'real' root device. See linux/Documentation/initrd.txt for more info. ============================================================== 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. ???