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2022-09-24
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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