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1Documentation for /proc/sys/kernel/* kernel version 2.2.10
2 (c) 1998, 1999, Rik van Riel <riel@nl.linux.org>
3
4For general info and legal blurb, please look in README.
5
6==============================================================
7
8This file contains documentation for the sysctl files in
9/proc/sys/kernel/ and is valid for Linux kernel version 2.2.
10
11The files in this directory can be used to tune and monitor
12miscellaneous and general things in the operation of the Linux
13kernel. Since some of the files _can_ be used to screw up your
14system, it is advisable to read both documentation and source
15before actually making adjustments.
16
17Currently, these files might (depending on your configuration)
18show up in /proc/sys/kernel:
19- acpi_video_flags
20- acct
21- core_pattern
22- core_uses_pid
23- ctrl-alt-del
24- dentry-state
25- domainname
26- hostname
27- hotplug
28- java-appletviewer [ binfmt_java, obsolete ]
29- java-interpreter [ binfmt_java, obsolete ]
30- l2cr [ PPC only ]
31- modprobe ==> Documentation/kmod.txt
32- msgmax
33- msgmnb
34- msgmni
35- osrelease
36- ostype
37- overflowgid
38- overflowuid
39- panic
40- pid_max
41- powersave-nap [ PPC only ]
42- printk
43- real-root-dev ==> Documentation/initrd.txt
44- reboot-cmd [ SPARC only ]
45- rtsig-max
46- rtsig-nr
47- sem
48- sg-big-buff [ generic SCSI device (sg) ]
49- shmall
50- shmmax [ sysv ipc ]
51- shmmni
52- stop-a [ SPARC only ]
53- suid_dumpable
54- sysrq ==> Documentation/sysrq.txt
55- tainted
56- threads-max
57- version
58
59==============================================================
60
61acpi_video_flags:
62
63flags
64
65See Doc*/kernel/power/video.txt, it allows mode of video boot to be
66set during run time.
67
68==============================================================
69
70acct:
71
72highwater lowwater frequency
73
74If BSD-style process accounting is enabled these values control
75its behaviour. If free space on filesystem where the log lives
76goes below <lowwater>% accounting suspends. If free space gets
77above <highwater>% accounting resumes. <Frequency> determines
78how often do we check the amount of free space (value is in
79seconds). Default:
804 2 30
81That is, suspend accounting if there left <= 2% free; resume it
82if we got >=4%; consider information about amount of free space
83valid for 30 seconds.
84
85==============================================================
86
87core_pattern:
88
89core_pattern is used to specify a core dumpfile pattern name.
90. max length 64 characters; default value is "core"
91. core_pattern is used as a pattern template for the output filename;
92 certain string patterns (beginning with '%') are substituted with
93 their actual values.
94. backward compatibility with core_uses_pid:
95 If core_pattern does not include "%p" (default does not)
96 and core_uses_pid is set, then .PID will be appended to
97 the filename.
98. corename format specifiers:
99 %<NUL> '%' is dropped
100 %% output one '%'
101 %p pid
102 %u uid
103 %g gid
104 %s signal number
105 %t UNIX time of dump
106 %h hostname
107 %e executable filename
108 %<OTHER> both are dropped
109
110==============================================================
111
112core_uses_pid:
113
114The default coredump filename is "core". By setting
115core_uses_pid to 1, the coredump filename becomes core.PID.
116If core_pattern does not include "%p" (default does not)
117and core_uses_pid is set, then .PID will be appended to
118the filename.
119
120==============================================================
121
122ctrl-alt-del:
123
124When the value in this file is 0, ctrl-alt-del is trapped and
125sent to the init(1) program to handle a graceful restart.
126When, however, the value is > 0, Linux's reaction to a Vulcan
127Nerve Pinch (tm) will be an immediate reboot, without even
128syncing its dirty buffers.
129
130Note: when a program (like dosemu) has the keyboard in 'raw'
131mode, the ctrl-alt-del is intercepted by the program before it
132ever reaches the kernel tty layer, and it's up to the program
133to decide what to do with it.
134
135==============================================================
136
137domainname & hostname:
138
139These files can be used to set the NIS/YP domainname and the
140hostname of your box in exactly the same way as the commands
141domainname and hostname, i.e.:
142# echo "darkstar" > /proc/sys/kernel/hostname
143# echo "mydomain" > /proc/sys/kernel/domainname
144has the same effect as
145# hostname "darkstar"
146# domainname "mydomain"
147
148Note, however, that the classic darkstar.frop.org has the
149hostname "darkstar" and DNS (Internet Domain Name Server)
150domainname "frop.org", not to be confused with the NIS (Network
151Information Service) or YP (Yellow Pages) domainname. These two
152domain names are in general different. For a detailed discussion
153see the hostname(1) man page.
154
155==============================================================
156
157hotplug:
158
159Path for the hotplug policy agent.
160Default value is "/sbin/hotplug".
161
162==============================================================
163
164l2cr: (PPC only)
165
166This flag controls the L2 cache of G3 processor boards. If
1670, the cache is disabled. Enabled if nonzero.
168
169==============================================================
170
171osrelease, ostype & version:
172
173# cat osrelease
1742.1.88
175# cat ostype
176Linux
177# cat version
178#5 Wed Feb 25 21:49:24 MET 1998
179
180The files osrelease and ostype should be clear enough. Version
181needs a little more clarification however. The '#5' means that
182this is the fifth kernel built from this source base and the
183date behind it indicates the time the kernel was built.
184The only way to tune these values is to rebuild the kernel :-)
185
186==============================================================
187
188overflowgid & overflowuid:
189
190if your architecture did not always support 32-bit UIDs (i.e. arm, i386,
191m68k, sh, and sparc32), a fixed UID and GID will be returned to
192applications that use the old 16-bit UID/GID system calls, if the actual
193UID or GID would exceed 65535.
194
195These sysctls allow you to change the value of the fixed UID and GID.
196The default is 65534.
197
198==============================================================
199
200panic:
201
202The value in this file represents the number of seconds the
203kernel waits before rebooting on a panic. When you use the
204software watchdog, the recommended setting is 60.
205
206==============================================================
207
208panic_on_oops:
209
210Controls the kernel's behaviour when an oops or BUG is encountered.
211
2120: try to continue operation
213
2141: delay a few seconds (to give klogd time to record the oops output) and
215 then panic. If the `panic' sysctl is also non-zero then the machine will
216 be rebooted.
217
218==============================================================
219
220pid_max:
221
222PID allocation wrap value. When the kenrel's next PID value
223reaches this value, it wraps back to a minimum PID value.
224PIDs of value pid_max or larger are not allocated.
225
226==============================================================
227
228powersave-nap: (PPC only)
229
230If set, Linux-PPC will use the 'nap' mode of powersaving,
231otherwise the 'doze' mode will be used.
232
233==============================================================
234
235printk:
236
237The four values in printk denote: console_loglevel,
238default_message_loglevel, minimum_console_loglevel and
239default_console_loglevel respectively.
240
241These values influence printk() behavior when printing or
242logging error messages. See 'man 2 syslog' for more info on
243the different loglevels.
244
245- console_loglevel: messages with a higher priority than
246 this will be printed to the console
247- default_message_level: messages without an explicit priority
248 will be printed with this priority
249- minimum_console_loglevel: minimum (highest) value to which
250 console_loglevel can be set
251- default_console_loglevel: default value for console_loglevel
252
253==============================================================
254
255printk_ratelimit:
256
257Some warning messages are rate limited. printk_ratelimit specifies
258the minimum length of time between these messages (in jiffies), by
259default we allow one every 5 seconds.
260
261A value of 0 will disable rate limiting.
262
263==============================================================
264
265printk_ratelimit_burst:
266
267While long term we enforce one message per printk_ratelimit
268seconds, we do allow a burst of messages to pass through.
269printk_ratelimit_burst specifies the number of messages we can
270send before ratelimiting kicks in.
271
272==============================================================
273
274reboot-cmd: (Sparc only)
275
276??? This seems to be a way to give an argument to the Sparc
277ROM/Flash boot loader. Maybe to tell it what to do after
278rebooting. ???
279
280==============================================================
281
282rtsig-max & rtsig-nr:
283
284The file rtsig-max can be used to tune the maximum number
285of POSIX realtime (queued) signals that can be outstanding
286in the system.
287
288rtsig-nr shows the number of RT signals currently queued.
289
290==============================================================
291
292sg-big-buff:
293
294This file shows the size of the generic SCSI (sg) buffer.
295You can't tune it just yet, but you could change it on
296compile time by editing include/scsi/sg.h and changing
297the value of SG_BIG_BUFF.
298
299There shouldn't be any reason to change this value. If
300you can come up with one, you probably know what you
301are doing anyway :)
302
303==============================================================
304
305shmmax:
306
307This value can be used to query and set the run time limit
308on the maximum shared memory segment size that can be created.
309Shared memory segments up to 1Gb are now supported in the
310kernel. This value defaults to SHMMAX.
311
312==============================================================
313
314suid_dumpable:
315
316This value can be used to query and set the core dump mode for setuid
317or otherwise protected/tainted binaries. The modes are
318
3190 - (default) - traditional behaviour. Any process which has changed
320 privilege levels or is execute only will not be dumped
3211 - (debug) - all processes dump core when possible. The core dump is
322 owned by the current user and no security is applied. This is
323 intended for system debugging situations only. Ptrace is unchecked.
3242 - (suidsafe) - any binary which normally would not be dumped is dumped
325 readable by root only. This allows the end user to remove
326 such a dump but not access it directly. For security reasons
327 core dumps in this mode will not overwrite one another or
328 other files. This mode is appropriate when adminstrators are
329 attempting to debug problems in a normal environment.
330
331==============================================================
332
333tainted:
334
335Non-zero if the kernel has been tainted. Numeric values, which
336can be ORed together:
337
338 1 - A module with a non-GPL license has been loaded, this
339 includes modules with no license.
340 Set by modutils >= 2.4.9 and module-init-tools.
341 2 - A module was force loaded by insmod -f.
342 Set by modutils >= 2.4.9 and module-init-tools.
343 4 - Unsafe SMP processors: SMP with CPUs not designed for SMP.
344