proc.txt

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Registering a new binary format
-------------------------------

To register a new binary format you have to issue the command

  echo :name:type:offset:magic:mask:interpreter: > /proc/sys/fs/binfmt_misc/register 



with appropriate  name (the name for the /proc-dir entry), offset (defaults to
0, if  omitted),  magic, mask (which can be omitted, defaults to all 0xff) and
last but  not  least,  the  interpreter that is to be invoked (for example and
testing /bin/echo).  Type  can be M for usual magic matching or E for filename
extension matching (give extension in place of magic).

Check or reset the status of the binary format handler
------------------------------------------------------

If you  do a cat on the file /proc/sys/fs/binfmt_misc/status, you will get the
current status (enabled/disabled) of binfmt_misc. Change the status by echoing
0 (disables)  or  1  (enables)  or  -1  (caution:  this  clears all previously
registered binary  formats)  to status. For example echo 0 > status to disable
binfmt_misc (temporarily).

Status of a single handler
--------------------------

Each registered  handler has an entry in /proc/sys/fs/binfmt_misc. These files
perform the  same function as status, but their scope is limited to the actual
binary format.  By  cating this file, you also receive all related information
about the interpreter/magic of the binfmt.

Example usage of binfmt_misc (emulate binfmt_java)
--------------------------------------------------

  cd /proc/sys/fs/binfmt_misc  
  echo ':Java:M::\xca\xfe\xba\xbe::/usr/local/java/bin/javawrapper:' > register  
  echo ':HTML:E::html::/usr/local/java/bin/appletviewer:' > register  
  echo ':Applet:M::<!--applet::/usr/local/java/bin/appletviewer:' > register 
  echo ':DEXE:M::\x0eDEX::/usr/bin/dosexec:' > register 


These four  lines  add  support  for  Java  executables and Java applets (like
binfmt_java, additionally  recognizing the .html extension with no need to put
<!--applet> to  every  applet  file).  You  have  to  install  the JDK and the
shell-script /usr/local/java/bin/javawrapper  too.  It  works  around  the
brokenness of  the Java filename handling. To add a Java binary, just create a
link to the class-file somewhere in the path.

2.3 /proc/sys/kernel - general kernel parameters
------------------------------------------------

This directory  reflects  general  kernel  behaviors. As I've said before, the
contents depend  on  your  configuration.  Here you'll find the most important
files, along with descriptions of what they mean and how to use them.

acct
----

The file contains three values; highwater, lowwater, and frequency.

It exists  only  when  BSD-style  process  accounting is enabled. These values
control its behavior. If the free space on the file system where the log lives
goes below  lowwater  percentage,  accounting  suspends.  If  it  goes  above
highwater percentage,  accounting  resumes. Frequency determines how often you
check the amount of free space (value is in seconds). Default settings are: 4,
2, and  30.  That is, suspend accounting if there is less than 2 percent free;
resume it  if we have a value of 3 or more percent; consider information about
the 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
program to  handle a graceful restart. However, when the value is greater that
zero, Linux's  reaction  to  this key combination will be an immediate reboot,
without 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 is up to the program to decide what to do with
    it.

domainname and hostname
-----------------------

These files  can  be controlled to set the NIS 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 NIS domainname.

osrelease, ostype and version
-----------------------------

The names make it pretty obvious what these fields contain:

  > cat /proc/sys/kernel/osrelease 
  2.2.12 
   
  > cat /proc/sys/kernel/ostype 
  Linux 
   
  > cat /proc/sys/kernel/version 
  #4 Fri Oct 1 12:41:14 PDT 1999 


The files  osrelease and ostype should be clear enough. Version needs a little
more clarification.  The  #4 means that this is the 4th kernel built from this
source base and the date after 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. If set to 0, the auto reboot after a kernel panic
is disabled, which is the default setting.

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, which  come  from  inside  the  kernel.  See  syslog(2)  for  more
information on the different log levels.

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 the console_loglevel can be set.

default_console_loglevel
------------------------

Default value for console_loglevel.

sg-big-buff
-----------

This file  shows  the size of the generic SCSI (sg) buffer. At this point, you
can't tune  it  yet,  but  you  can  change  it  at  compile  time  by editing
include/scsi/sg.h and changing the value of SG_BIG_BUFF.

If you use a scanner with SANE (Scanner Access Now Easy) you might want to set
this to a higher value. Refer to the SANE documentation on this issue.

modprobe
--------

The location  where  the  modprobe  binary  is  located.  The kernel uses this
program to load modules on demand.

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.

[NOTE]
   This function and oprofile share a NMI callback. Therefore this function
   cannot be enabled when oprofile is activated.
   And NMI watchdog will be disabled when the value in this file is set to
   non-zero.


2.4 /proc/sys/vm - The virtual memory subsystem
-----------------------------------------------

The files  in  this directory can be used to tune the operation of the virtual
memory (VM)  subsystem  of  the  Linux  kernel.

vfs_cache_pressure
------------------

Controls the tendency of the kernel to reclaim the memory which is used for
caching of directory and inode objects.

At the default value of vfs_cache_pressure=100 the kernel will attempt to
reclaim dentries and inodes at a "fair" rate with respect to pagecache and
swapcache reclaim.  Decreasing vfs_cache_pressure causes the kernel to prefer
to retain dentry and inode caches.  Increasing vfs_cache_pressure beyond 100
causes the kernel to prefer to reclaim dentries and inodes.

dirty_background_ratio
----------------------

Contains, as a percentage of total system memory, the number of pages at which
the pdflush background writeback daemon will start writing out dirty data.

dirty_ratio
-----------------

Contains, as a percentage of total system memory, the number of pages at which
a process which is generating disk writes will itself start writing out dirty
data.

dirty_writeback_centisecs
-------------------------

The pdflush writeback daemons will periodically wake up and write `old' data
out to disk.  This tunable expresses the interval between those wakeups, in
100'ths of a second.

Setting this to zero disables periodic writeback altogether.

dirty_expire_centisecs
----------------------

This tunable is used to define when dirty data is old enough to be eligible
for writeout by the pdflush daemons.  It is expressed in 100'ths of a second. 
Data which has been dirty in-memory for longer than this interval will be
written out next time a pdflush daemon wakes up.

legacy_va_layout
----------------

If non-zero, this sysctl disables the new 32-bit mmap mmap layout - the kernel
will use the legacy (2.4) layout for all processes.

lower_zone_protection
---------------------

For some specialised workloads on highmem machines it is dangerous for
the kernel to allow process memory to be allocated from the "lowmem"
zone.  This is because that memory could then be pinned via the mlock()
system call, or by unavailability of swapspace.

And on large highmem machines this lack of reclaimable lowmem memory
can be fatal.

So the Linux page allocator has a mechanism which prevents allocations
which _could_ use highmem from using too much lowmem.  This means that
a certain amount of lowmem is defended from the possibility of being
captured into pinned user memory.

(The same argument applies to the old 16 megabyte ISA DMA region.  This
mechanism will also defend that region from allocations which could use
highmem or lowmem).

The `lower_zone_protection' tunable determines how aggressive the kernel is
in defending these lower zones.  The default value is zero - no
protection at all.

If you have a machine which uses highmem or ISA DMA and your
applications are using mlock(), or if you are running with no swap then
you probably should increase the lower_zone_protection setting.

The units of this tunable are fairly vague.  It is approximately equal
to "megabytes".  So setting lower_zone_protection=100 will protect around 100
megabytes of the lowmem zone from user allocations.  It will also make
those 100 megabytes unavaliable for use by applications and by
pagecache, so there is a cost.

The effects of this tunable may be observed by monitoring
/proc/meminfo:LowFree.  Write a single huge file and observe the point
at which LowFree ceases to fall.

A reasonable value for lower_zone_protection is 100.

page-cluster
------------

page-cluster controls the number of pages which are written to swap in
a single attempt.  The swap I/O size.

It is a logarithmic value - setting it to zero means "1 page", setting
it to 1 means "2 pages", setting it to 2 means "4 pages", etc.

The default value is three (eight pages at a time).  There may be some
small benefits in tuning this to a different value if your workload is
swap-intensive.

overcommit_memory
-----------------

This file  contains  one  value.  The following algorithm is used to decide if
there's enough  memory:  if  the  value of overcommit_memory is positive, then
there's always  enough  memory. This is a useful feature, since programs often
malloc() huge  amounts  of  memory 'just in case', while they only use a small
part of  it.  Leaving  this value at 0 will lead to the failure of such a huge
malloc(), when in fact the system has enough memory for the program to run.

On the  other  hand,  enabling this feature can cause you to run out of memory
and thrash the system to death, so large and/or important servers will want to
set this value to 0.

nr_hugepages and hugetlb_shm_group