proc.txt

《嵌入式系统设计与实例开发实验教材二源码》Linux内核移植与编译实验

 dummy      Unused                                                             
 dummy      Unused                                                             
..............................................................................

nfract
------

This parameter  governs  the  maximum  number  of  dirty buffers in the buffer
cache. Dirty means that the contents of the buffer still have to be written to
disk (as  opposed  to  a  clean  buffer,  which  can just be forgotten about).
Setting this  to  a  higher value means that Linux can delay disk writes for a
long time, but it also means that it will have to do a lot of I/O at once when
memory becomes short. A lower value will spread out disk I/O more evenly.

ndirty
------

Ndirty gives the maximum number of dirty buffers that bdflush can write to the
disk at  one  time.  A high value will mean delayed, bursty I/O, while a small
value can lead to memory shortage when bdflush isn't woken up often enough.

nrefill
-------

This is  the  number  of  buffers  that  bdflush  will add to the list of free
buffers when  refill_freelist()  is  called.  It is necessary to allocate free
buffers beforehand,  since  the  buffers  are  often  different sizes than the
memory pages  and some bookkeeping needs to be done beforehand. The higher the
number, the  more  memory  will be wasted and the less often refill_freelist()
will need to run.

nref_dirt
---------

When refill_freelist() comes across more than nref_dirt dirty buffers, it will
wake up bdflush.

age_buffer and age_super
------------------------

Finally, the age_buffer and age_super parameters govern the maximum time Linux
waits before  writing  out  a  dirty buffer to disk. The value is expressed in
jiffies (clockticks),  the  number of jiffies per second is 100. Age_buffer is
the maximum age for data blocks, while age_super is for filesystems meta data.

buffermem
---------

The three  values  in  this  file  control  how much memory should be used for
buffer memory.  The  percentage  is calculated as a percentage of total system
memory.

The values are:

min_percent
-----------

This is  the  minimum  percentage  of  memory  that  should be spent on buffer
memory.

borrow_percent
--------------

When Linux is short on memory, and the buffer cache uses more than it has been
allotted, the  memory  management  (MM)  subsystem will prune the buffer cache
more heavily than other memory to compensate.

max_percent
-----------

This is the maximum amount of memory that can be used for buffer memory.

freepages
---------

This file contains three values: min, low and high:

min
---
When the  number  of  free  pages  in the system reaches this number, only the
kernel can allocate more memory.

low
---
If the number of free pages falls below this point, the kernel starts swapping
aggressively.

high
----
The kernel  tries  to  keep  up to this amount of memory free; if memory falls
below this point, the kernel starts gently swapping in the hopes that it never
has to do really aggressive swapping.

kswapd
------

Kswapd is  the  kernel  swap  out daemon. That is, kswapd is that piece of the
kernel that  frees  memory when it gets fragmented or full. Since every system
is different, you'll probably want some control over this piece of the system.

The file contains three numbers:

tries_base
----------

The maximum  number  of  pages kswapd tries to free in one round is calculated
from this  number.  Usually  this  number  will  be  divided  by  4  or 8 (see
mm/vmscan.c), so it isn't as big as it looks.

When you  need to increase the bandwidth to/from swap, you'll want to increase
this number.

tries_min
---------

This is  the  minimum number of times kswapd tries to free a page each time it
is called. Basically it's just there to make sure that kswapd frees some pages
even when it's being called with minimum priority.

swap_cluster
------------

This is probably the greatest influence on system performance.

swap_cluster is  the  number  of  pages kswapd writes in one turn. You'll want
this value  to  be  large  so that kswapd does its I/O in large chunks and the
disk doesn't  have  to  seek  as  often, but you don't want it to be too large
since that would flood the request queue.

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.

pagecache
---------

This file  does exactly the same job as buffermem, only this file controls the
amount of memory allowed for memory mapping and generic caching of files.

You don't  want  the  minimum level to be too low, otherwise your system might
thrash when memory is tight or fragmentation is high.

pagetable_cache
---------------

The kernel  keeps a number of page tables in a per-processor cache (this helps
a lot  on  SMP systems). The cache size for each processor will be between the
low and the high value.

On a  low-memory,  single  CPU system, you can safely set these values to 0 so
you don't  waste  memory.  It  is  used  on SMP systems so that the system can
perform fast  pagetable allocations without having to acquire the kernel memory
lock.

For large  systems,  the  settings  are probably fine. For normal systems they
won't hurt  a  bit.  For  small  systems  (  less  than  16MB ram) it might be
advantageous to set both values to 0.

swapctl
-------

This file  contains  no less than 8 variables. All of these values are used by
kswapd.

The first four variables
* sc_max_page_age,
* sc_page_advance,
* sc_page_decline and
* sc_page_initial_age
are used  to  keep  track  of  Linux's page aging. Page aging is a bookkeeping
method to  track  which pages of memory are often used, and which pages can be
swapped out without consequences.

When a  page  is  swapped in, it starts at sc_page_initial_age (default 3) and
when the  page  is  scanned  by  kswapd,  its age is adjusted according to the
following scheme:

* If  the  page  was used since the last time we scanned, its age is increased
  by sc_page_advance  (default  3).  Where  the  maximum  value  is  given  by
  sc_max_page_age (default 20).
* Otherwise  (meaning  it wasn't used) its age is decreased by sc_page_decline
  (default 1).

When a page reaches age 0, it's ready to be swapped out.

The variables  sc_age_cluster_fract, sc_age_cluster_min, sc_pageout_weight and
sc_bufferout_weight, can  be  used  to  control  kswapd's  aggressiveness  in
swapping out pages.

Sc_age_cluster_fract is used to calculate how many pages from a process are to
be scanned by kswapd. The formula used is

(sc_age_cluster_fract divided by 1024) times resident set size

So if you want kswapd to scan the whole process, sc_age_cluster_fract needs to
have a  value  of  1024.  The  minimum  number  of  pages  kswapd will scan is
represented by sc_age_cluster_min, which is done so that kswapd will also scan
small processes.

The values  of  sc_pageout_weight  and sc_bufferout_weight are used to control
how many  tries  kswapd  will make in order to swap out one page/buffer. These
values can  be used to fine-tune the ratio between user pages and buffer/cache
memory. When  you find that your Linux system is swapping out too many process
pages in  order  to  satisfy  buffer  memory  demands,  you may want to either
increase sc_bufferout_weight, or decrease the value of sc_pageout_weight.

2.5 /proc/sys/dev - Device specific parameters
----------------------------------------------

Currently there is only support for CDROM drives, and for those, there is only
one read-only  file containing information about the CD-ROM drives attached to
the system:

  >cat /proc/sys/dev/cdrom/info 
  CD-ROM information, Id: cdrom.c 2.55 1999/04/25 
   
  drive name:             sr0     hdb 
  drive speed:            32      40 
  drive # of slots:       1       0 
  Can close tray:         1       1 
  Can open tray:          1       1 
  Can lock tray:          1       1 
  Can change speed:       1       1 
  Can select disk:        0       1 
  Can read multisession:  1       1 
  Can read MCN:           1       1 
  Reports media changed:  1       1 
  Can play audio:         1       1 


You see two drives, sr0 and hdb, along with a list of their features.

2.6 /proc/sys/sunrpc - Remote procedure calls
---------------------------------------------

This directory  contains four files, which enable or disable debugging for the
RPC functions NFS, NFS-daemon, RPC and NLM. The default values are 0. They can
be set to one to turn debugging on. (The default value is 0 for each)

2.7 /proc/sys/net - Networking stuff
------------------------------------

The interface  to  the  networking  parts  of  the  kernel  is  located  in
/proc/sys/net. Table  2-3  shows all possible subdirectories. You may see only
some of them, depending on your kernel's configuration.


Table 2-3: Subdirectories in /proc/sys/net 
..............................................................................
 Directory Content             Directory  Content            
 core      General parameter   appletalk  Appletalk protocol 
 unix      Unix domain sockets netrom     NET/ROM            
 802       E802 protocol       ax25       AX25               
 ethernet  Ethernet protocol   rose       X.25 PLP layer     
 ipv4      IP version 4        x25        X.25 protocol      
 ipx       IPX                 token-ring IBM token ring     
 bridge    Bridging            decnet     DEC net            
 ipv6      IP version 6                   
..............................................................................

We will  concentrate  on IP networking here. Since AX15, X.25, and DEC Net are
only minor players in the Linux world, we'll skip them in this chapter. You'll
find some  short  info on Appletalk and IPX further on in this chapter. Review
the online  documentation  and the kernel source to get a detailed view of the
parameters for  those  protocols.  In  this  section  we'll  discuss  the
subdirectories printed  in  bold letters in the table above. As default values
are suitable for most needs, there is no need to change these values.

/proc/sys/net/core - Network core options
-----------------------------------------

rmem_default
------------

The default setting of the socket receive buffer in bytes.

rmem_max
--------

The maximum receive socket buffer size in bytes.

wmem_default
------------

The default setting (in bytes) of the socket send buffer.

wmem_max
--------

The maximum send socket buffer size in bytes.

message_burst and message_cost
------------------------------

These parameters  are used to limit the warning messages written to the kernel
log from  the  networking  code.  They  enforce  a  rate  limit  to  make  a
denial-of-service attack  impossible. A higher message_cost factor, results in
fewer messages that will be written. Message_burst controls when messages will
be dropped.  The  default  settings  limit  warning messages to one every five
seconds.

netdev_max_backlog
------------------

Maximum number  of  packets,  queued  on  the  INPUT  side, when the interface
receives packets faster than kernel can process them.

optmem_max
----------

Maximum ancillary buffer size allowed per socket. Ancillary data is a sequence
of struct cmsghdr structures with appended data.

/proc/sys/net/unix - Parameters for Unix domain sockets