The proc filesystem 4

网友投稿 823 2022-09-24

The proc filesystem 4

The proc filesystem 4

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 the dirtyable system memory (free pages + mapped pages + file cache, not including locked pages and HugePages), the number of pages at which the pdflush background writeback daemon will start writing out dirty data.

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

Contains, as a percentage of the dirtyable system memory (free pages + mapped pages + file cache, not including locked pages and HugePages), 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.

highmem_is_dirtyable --------------------

Only present if CONFIG_HIGHMEM is set.

This defaults to 0 (false), meaning that the ratios set above are calculated as a percentage of lowmem only.  This protects against excessive scanning in page reclaim, swapping and general VM distress.

Setting this to 1 can be useful on 32 bit machines where you want to make random changes within an MMAPed file that is larger than your available lowmem without causing large quantities of random IO.  Is is safe if the behavior of all programs running on the machine is known and memory will not be otherwise stressed.

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.

lowmem_reserve_ratio ---------------------

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 `lowmem_reserve_ratio' tunable determines how aggressive the kernel is in defending these lower zones.

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 change the lowmem_reserve_ratio setting.

The lowmem_reserve_ratio is an array. You can see them by reading this file. - % cat /proc/sys/vm/lowmem_reserve_ratio 256     256     32 - Note: # of this elements is one fewer than number of zones. Because the highest       zone's value is not necessary for following calculation.

But, these values are not used directly. The kernel calculates # of protection pages for each zones from them. These are shown as array of protection pages in /proc/zoneinfo like followings. (This is an example of x86-64 box). Each zone has an array of protection pages like this.

- Node 0, zone      DMA   pages free     1355         min      3         low      3         high     4         :         :     numa_other   0         protection: (0, 2004, 2004, 2004)         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^   pagesets     cpu: 0 pcp: 0         : - These protections are added to score to judge whether this zone should be used for page allocation or should be reclaimed.

In this example, if normal pages (index=2) are required to this DMA zone and pages_high is used for watermark, the kernel judges this zone should not be used because pages_free(1355) is smaller than watermark + protection[2] (4 + 2004 = 2008). If this protection value is 0, this zone would be used for normal page requirement. If requirement is DMA zone(index=0), protection[0] (=0) is used.

zone[i]'s protection[j] is calculated by following expression.

(i < j):   zone[i]->protection[j]   = (total sums of present_pages from zone[i+1] to zone[j] on the node)     / lowmem_reserve_ratio[i]; (i = j):    (should not be protected. = 0; (i > j):    (not necessary, but looks 0)

The default values of lowmem_reserve_ratio[i] are     256 (if zone[i] means DMA or DMA32 zone)     32  (others). As above expression, they are reciprocal number of ratio. 256 means 1/256. # of protection pages becomes about "0.39%" of total present pages of higher zones on the node.

If you would like to protect more pages, smaller values are effective. The minimum value is 1 (1/1 -> 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 -----------------

Controls overcommit of system memory, possibly allowing processes to allocate (but not use) more memory than is actually available.

0       -       Heuristic overcommit handling. Obvious overcommits of                 address space are refused. Used for a typical system. It                 ensures a seriously wild allocation fails while allowing                 overcommit to reduce swap usage.  root is allowed to                 allocate slightly more memory in this mode. This is the                 default.

1       -       Always overcommit. Appropriate for some scientific                 applications.

2       -       Don't overcommit. The total address space commit                 for the system is not permitted to exceed swap plus a                 configurable percentage (default is 50) of physical RAM.                 Depending on the percentage you use, in most situations                 this means a process will not be killed while attempting                 to use already-allocated memory but will receive errors                 on memory allocation as appropriate.

overcommit_ratio ----------------

Percentage of physical memory size to include in overcommit calculations (see above.)

Memory allocation limit = swapspace + physmem * (overcommit_ratio / 100)

swapspace = total size of all swap areas         physmem = size of physical memory in system

nr_hugepages and hugetlb_shm_group ----------------------------------

nr_hugepages configures number of hugetlb page reserved for the system.

hugetlb_shm_group contains group id that is allowed to create SysV shared memory segment using hugetlb page.

hugepages_treat_as_movable --------------------------

This parameter is only useful when kernelcore= is specified at boot time to create ZONE_MOVABLE for pages that may be reclaimed or migrated. Huge pages are not movable so are not normally allocated from ZONE_MOVABLE. A non-zero value written to hugepages_treat_as_movable allows huge pages to be allocated from ZONE_MOVABLE.

Once enabled, the ZONE_MOVABLE is treated as an area of memory the huge pages pool can easily grow or shrink within. Assuming that applications are not running that mlock() a lot of memory, it is likely the huge pages pool can grow to the size of ZONE_MOVABLE by repeatedly entering the desired value into nr_hugepages and triggering page reclaim.

laptop_mode -----------

laptop_mode is a knob that controls "laptop mode". All the things that are controlled by this knob are discussed in Documentation/laptops/laptop-mode.txt.

block_dump ----------

block_dump enables block I/O debugging when set to a nonzero value. More information on block I/O debugging is in Documentation/laptops/laptop-mode.txt.

swap_token_timeout ------------------

This file contains valid hold time of swap out protection token. The Linux VM has token based thrashing control mechanism and uses the token to prevent unnecessary page faults in thrashing situation. The unit of the value is second. The value would be useful to tune thrashing behavior.

drop_caches -----------

Writing to this will cause the kernel to drop clean caches, dentries and inodes from memory, causing that memory to become free.

To free pagecache:         echo 1 > /proc/sys/vm/drop_caches To free dentries and inodes:         echo 2 > /proc/sys/vm/drop_caches To free pagecache, dentries and inodes:         echo 3 > /proc/sys/vm/drop_caches

As this is a non-destructive operation and dirty objects are not freeable, the user should run `sync' first.

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.

warnings --------

This controls console messages from the networking stack that can occur because of problems on the network like duplicate address or bad checksums. Normally, this should be enabled, but if the problem persists the messages can be disabled.

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 -------------------------------------------------------

There are  only  two  files  in this subdirectory. They control the delays for deleting and destroying socket descriptors.

2.8 /proc/sys/net/ipv4 - IPV4 settings --------------------------------------

IP version  4  is  still the most used protocol in Unix networking. It will be replaced by  IP version 6 in the next couple of years, but for the moment it's the de  facto  standard  for  the  internet  and  is  used  in most networking environments around  the  world.  Because  of the importance of this protocol, we'll have a deeper look into the subtree controlling the behavior of the IPv4 subsystem of the Linux kernel.

Let's start with the entries in /proc/sys/net/ipv4.

ICMP settings -------------

icmp_echo_ignore_all and icmp_echo_ignore_broadcasts ----------------------------------------------------

Turn on (1) or off (0), if the kernel should ignore all ICMP ECHO requests, or just those to broadcast and multicast addresses.

Please note that if you accept ICMP echo requests with a broadcast/multi\-cast destination address  your  network  may  be  used as an exploder for denial of service packet flooding attacks to other hosts.

icmp_destunreach_rate, icmp_echoreply_rate, icmp_paramprob_rate and icmp_timeexeed_rate ---------------------------------------------------------------------------------------

Sets limits  for  sending  ICMP  packets  to specific targets. A value of zero disables all  limiting.  Any  positive  value sets the maximum package rate in hundredth of a second (on Intel systems).

IP settings -----------

ip_autoconfig -------------

This file contains the number one if the host received its IP configuration by RARP, BOOTP, DHCP or a similar mechanism. Otherwise it is zero.

ip_default_ttl --------------

TTL (Time  To  Live) for IPv4 interfaces. This is simply the maximum number of hops a packet may travel.

ip_dynaddr ----------

Enable dynamic  socket  address rewriting on interface address change. This is useful for dialup interface with changing IP addresses.

ip_forward ----------

Enable or  disable forwarding of IP packages between interfaces. Changing this value resets  all other parameters to their default values. They differ if the kernel is configured as host or router.

ip_local_port_range -------------------

Range of  ports  used  by  TCP  and UDP to choose the local port. Contains two numbers, the  first  number  is the lowest port, the second number the highest local port.  Default  is  1024-4999.  Should  be  changed  to  32768-61000 for high-usage systems.

ip_no_pmtu_disc ---------------

Global switch  to  turn  path  MTU  discovery off. It can also be set on a per socket basis by the applications or on a per route basis.

ip_masq_debug -------------

Enable/disable debugging of IP masquerading.

IP fragmentation settings -------------------------

ipfrag_high_trash and ipfrag_low_trash --------------------------------------

Maximum memory  used to reassemble IP fragments. When ipfrag_high_thresh bytes of memory  is  allocated  for  this  purpose,  the  fragment handler will toss packets until ipfrag_low_thresh is reached.

ipfrag_time -----------

Time in seconds to keep an IP fragment in memory.

TCP settings ------------

tcp_ecn -------

This file controls the use of the ECN bit in the IPv4 headers. This is a new feature about Explicit Congestion Notification, but some routers and firewalls block traffic that has this bit set, so it could be necessary to echo 0 to /proc/sys/net/ipv4/tcp_ecn if you want to talk to these sites. For more info you could read RFC2481.

tcp_retrans_collapse --------------------

Bug-to-bug compatibility with some broken printers. On retransmit, try to send larger packets to work around bugs in certain TCP stacks. Can be turned off by setting it to zero.

tcp_keepalive_probes --------------------

Number of  keep  alive  probes  TCP  sends  out,  until  it  decides  that the connection is broken.

tcp_keepalive_time ------------------

How often  TCP  sends out keep alive messages, when keep alive is enabled. The default is 2 hours.

tcp_syn_retries ---------------

Number of  times  initial  SYNs  for  a  TCP  connection  attempt  will  be retransmitted. Should  not  be  higher  than 255. This is only the timeout for outgoing connections,  for  incoming  connections the number of retransmits is defined by tcp_retries1.

tcp_sack --------

Enable select acknowledgments after RFC2018.

tcp_timestamps --------------

Enable timestamps as defined in RFC1323.

tcp_stdurg ----------

Enable the  strict  RFC793 interpretation of the TCP urgent pointer field. The default is  to  use  the  BSD  compatible interpretation of the urgent pointer pointing to the first byte after the urgent data. The RFC793 interpretation is to have  it  point  to  the last byte of urgent data. Enabling this option may lead to interoperability problems. Disabled by default.

tcp_syncookies --------------

Only valid  when  the  kernel  was  compiled  with CONFIG_SYNCOOKIES. Send out syncookies when  the  syn backlog queue of a socket overflows. This is to ward off the common 'syn flood attack'. Disabled by default.

Note that  the  concept  of a socket backlog is abandoned. This means the peer may not  receive  reliable  error  messages  from  an  over loaded server with syncookies enabled.

tcp_window_scaling ------------------

Enable window scaling as defined in RFC1323.

tcp_fin_timeout ---------------

The length  of  time  in  seconds  it  takes to receive a final FIN before the socket is  always  closed.  This  is  strictly  a  violation  of  the  TCP specification, but required to prevent denial-of-service attacks.

tcp_max_ka_probes -----------------

Indicates how  many  keep alive probes are sent per slow timer run. Should not be set too high to prevent bursts.

tcp_max_syn_backlog -------------------

Length of  the per socket backlog queue. Since Linux 2.2 the backlog specified in listen(2)  only  specifies  the  length  of  the  backlog  queue of already established sockets. When more connection requests arrive Linux starts to drop packets. When  syncookies  are  enabled the packets are still answered and the maximum queue is effectively ignored.

tcp_retries1 ------------

Defines how  often  an  answer  to  a  TCP connection request is retransmitted before giving up.

tcp_retries2 ------------

Defines how often a TCP packet is retransmitted before giving up.

Interface specific settings ---------------------------

In the directory /proc/sys/net/ipv4/conf you'll find one subdirectory for each interface the  system  knows about and one directory calls all. Changes in the all subdirectory  affect  all  interfaces,  whereas  changes  in  the  other subdirectories affect  only  one  interface.  All  directories  have  the same entries:

accept_redirects ----------------

This switch  decides  if the kernel accepts ICMP redirect messages or not. The default is 'yes' if the kernel is configured for a regular host and 'no' for a router configuration.

accept_source_route -------------------

Should source  routed  packages  be  accepted  or  declined.  The  default  is dependent on  the  kernel  configuration.  It's 'yes' for routers and 'no' for hosts.

bootp_relay ~~~~~~~~~~~

Accept packets  with source address 0.b.c.d with destinations not to this host as local ones. It is supposed that a BOOTP relay daemon will catch and forward such packets.

The default  is  0,  since this feature is not implemented yet (kernel version 2.2.12).

forwarding ----------

Enable or disable IP forwarding on this interface.

log_martians ------------

Log packets with source addresses with no known route to kernel log.

mc_forwarding -------------

Do multicast routing. The kernel needs to be compiled with CONFIG_MROUTE and a multicast routing daemon is required.

proxy_arp ---------

Does (1) or does not (0) perform proxy ARP.

rp_filter ---------

Integer value determines if a source validation should be made. 1 means yes, 0 means no.  Disabled by default, but local/broadcast address spoofing is always on.

If you  set this to 1 on a router that is the only connection for a network to the net,  it  will  prevent  spoofing  attacks  against your internal networks (external addresses  can  still  be  spoofed), without the need for additional firewall rules.

secure_redirects ----------------

Accept ICMP  redirect  messages  only  for gateways, listed in default gateway list. Enabled by default.

shared_media ------------

If it  is  not  set  the kernel does not assume that different subnets on this device can communicate directly. Default setting is 'yes'.

send_redirects --------------

Determines whether to send ICMP redirects to other hosts.

Routing settings ----------------

The directory  /proc/sys/net/ipv4/route  contains  several  file  to  control routing issues.

error_burst and error_cost --------------------------

These  parameters  are used to limit how many ICMP destination unreachable to send  from  the  host  in question. ICMP destination unreachable messages are sent  when  we  cannot reach  the next hop while trying to transmit a packet. It  will also print some error messages to kernel logs if someone is ignoring our   ICMP  redirects.  The  higher  the  error_cost  factor  is,  the  fewer destination  unreachable  and error messages will be let through. Error_burst controls  when  destination  unreachable  messages and error messages will be dropped. The default settings limit warning messages to five every second.

flush -----

Writing to this file results in a flush of the routing cache.

gc_elasticity, gc_interval, gc_min_interval_ms, gc_timeout, gc_thresh ---------------------------------------------------------------------

Values to  control  the  frequency  and  behavior  of  the  garbage collection algorithm for the routing cache. gc_min_interval is deprecated and replaced by gc_min_interval_ms.

max_size --------

Maximum size  of  the routing cache. Old entries will be purged once the cache reached has this size.

redirect_load, redirect_number ------------------------------

Factors which  determine  if  more ICPM redirects should be sent to a specific host. No  redirects  will be sent once the load limit or the maximum number of redirects has been reached.

redirect_silence ----------------

Timeout for redirects. After this period redirects will be sent again, even if this has been stopped, because the load or number limit has been reached.

Network Neighbor handling -------------------------

Settings about how to handle connections with direct neighbors (nodes attached to the same link) can be found in the directory /proc/sys/net/ipv4/neigh.

As we  saw  it  in  the  conf directory, there is a default subdirectory which holds the  default  values, and one directory for each interface. The contents of the  directories  are identical, with the single exception that the default settings contain additional options to set garbage collection parameters.

In the interface directories you'll find the following entries:

base_reachable_time, base_reachable_time_ms -------------------------------------------

A base  value  used for computing the random reachable time value as specified in RFC2461.

Expression of base_reachable_time, which is deprecated, is in seconds. Expression of base_reachable_time_ms is in milliseconds.

retrans_time, retrans_time_ms -----------------------------

The time between retransmitted Neighbor Solicitation messages. Used for address resolution and to determine if a neighbor is unreachable.

Expression of retrans_time, which is deprecated, is in 1/100 seconds (for IPv4) or in jiffies (for IPv6). Expression of retrans_time_ms is in milliseconds.

unres_qlen ----------

Maximum queue  length  for a pending arp request - the number of packets which are accepted from other layers while the ARP address is still resolved.

anycast_delay -------------

Maximum for  random  delay  of  answers  to  neighbor solicitation messages in jiffies (1/100  sec). Not yet implemented (Linux does not have anycast support yet).

ucast_solicit -------------

Maximum number of retries for unicast solicitation.

mcast_solicit -------------

Maximum number of retries for multicast solicitation.

delay_first_probe_time ----------------------

Delay for  the  first  time  probe  if  the  neighbor  is  reachable.  (see gc_stale_time)

locktime --------

An ARP/neighbor  entry  is only replaced with a new one if the old is at least locktime old. This prevents ARP cache thrashing.

proxy_delay -----------

Maximum time  (real  time is random [0..proxytime]) before answering to an ARP request for  which  we have an proxy ARP entry. In some cases, this is used to prevent network flooding.

proxy_qlen ----------

Maximum queue length of the delayed proxy arp timer. (see proxy_delay).

app_solicit ----------

Determines the  number of requests to send to the user level ARP daemon. Use 0 to turn off.

gc_stale_time -------------

Determines how  often  to  check  for stale ARP entries. After an ARP entry is stale it  will  be resolved again (which is useful when an IP address migrates to another  machine).  When  ucast_solicit is greater than 0 it first tries to send an  ARP  packet  directly  to  the  known  host  When  that  fails  and mcast_solicit is greater than 0, an ARP request is broadcasted.

2.9 Appletalk -------------

The /proc/sys/net/appletalk  directory  holds the Appletalk configuration data when Appletalk is loaded. The configurable parameters are:

aarp-expiry-time ----------------

The amount  of  time  we keep an ARP entry before expiring it. Used to age out old hosts.

aarp-resolve-time -----------------

The amount of time we will spend trying to resolve an Appletalk address.

aarp-retransmit-limit ---------------------

The number of times we will retransmit a query before giving up.

aarp-tick-time --------------

Controls the rate at which expires are checked.

The directory  /proc/net/appletalk  holds the list of active Appletalk sockets on a machine.

The fields  indicate  the DDP type, the local address (in network:node format) the remote  address,  the  size of the transmit pending queue, the size of the received queue  (bytes waiting for applications to read) the state and the uid owning the socket.

/proc/net/atalk_iface lists  all  the  interfaces  configured for appletalk.It shows the  name  of the interface, its Appletalk address, the network range on that address  (or  network number for phase 1 networks), and the status of the interface.

/proc/net/atalk_route lists  each  known  network  route.  It lists the target (network) that the route leads to, the router (may be directly connected), the route flags, and the device the route is using.

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