The value in file-max denotes the maximum number of file-handles that the Linux kernel will allocate. When you get lots of error messages about running out of file handles, you might want to increase this limit.
Historically,the kernel was able to allocate file handles dynamically, but not to free them again. The three values in file-nr denote the number of allocated file handles, the number of allocated but unused file handles, and the maximum number of file handles. Linux 2.6 always reports 0 as the number of free file handles -- this is not an error, it just means that the number of allocated file handles exactly matches the number of used file handles.
Attempts to allocate more file descriptors than file-max are reported with printk, look for "VFS: file-max limit / reached".
nr_open (ulimit -n)
This denotes the maximum number of file-handles a process can allocate. Default value is 1024*1024 (1048576) which should be enough for most machines. Actual limit depends on RLIMIT_NOFILE resource limit.
inode-max, inode-nr & inode-state
As with file handles, the kernel allocates the inode structures dynamically, but can't free them yet.
The value in inode-max denotes the maximum number of inode handlers. This value should be 3-4 times larger than the value in file-max, since stdin, stdout and network sockets also need an inode struct to handle them. When you regularly run out of inodes, you need to increase this value.
The file inode-nr contains the first two items from inode-state, so we'll skip to that file...
Inode-state contains three actual numbers and four dummies. The actual numbers are, in order of appearance, nr_inodes, nr_free_inodes and preshrink.
Nr_inodes stands for the number of inodes the system has allocated, this can be slightly more than inode-max because Linux allocates them one pageful at a time.
Nr_free_inodes represents the number of free inodes (?) and preshrink is nonzero when the nr_inodes > inode-max and the system needs to prune the inode list instead of allocating more.
overflowgid & overflowuid:
Some filesystems only support 16-bit UIDs and GIDs, although in Linux UIDs and GIDs are 32 bits. When one of these filesystems is mounted with writes enabled, any UID or GID that would exceed 65535 is translated
to a fixed value before being written to disk.
These sysctls allow you to change the value of the fixed UID and GID. The default is 65534.
suid_dumpable
This value can be used to query and set the core dump mode for setuid or otherwise protected/tainted binaries. The modes are
0 - (default) - traditional behaviour. Any process which has changed privilege levels or is execute only will not be dumped
1 - (debug) - all processes dump core when possible. The core dump is owned by the current user and no security is applied. This is intended for system debugging situations only. Ptrace is unchecked.
2 - (suidsafe) - any binary which normally would not be dumped is dumped readable by root only. This allows the end user to remove such a dump but not access it directly. For security reasons core dumps in this mode will not overwrite one another or other files. This mode is appropriate when administrators are attempting to debug problems in a normal environment.
super-max & super-nr
These numbers control the maximum number of superblocks, and thus the maximum number of mounted filesystems the kernel can have. You only need to increase super-max if you need to mount more filesystems than the current value in super-max allows you to.
aio-nr & aio-max-nr
aio-nr shows the current system-wide number of asynchronous io requests. aio-max-nr allows you to change the maximum value aio-nr can grow to.
/proc/sys/fs/mqueue - POSIX message queues filesystem
The "mqueue" filesystem provides the necessary kernel features to enable the creation of a user space library that implements the POSIX message queues API (as noted by the MSG tag in the POSIX 1003.1-2001 version of the System Interfaces specification.)
The "mqueue" filesystem contains values for determining/setting the amount of resources used by the file system.
/proc/sys/fs/mqueue/queues_max is a read/write file for setting/getting the maximum number of message queues allowed on the system.
/proc/sys/fs/mqueue/msg_max is a read/write file for setting/getting the maximum number of messages in a queue value. In fact it is the limiting value for another (user) limit which is set in mq_open invocation. This attribute of a queue must be less or equal then msg_max.
/proc/sys/fs/mqueue/msgsize_max is a read/write file for setting/getting the maximum message size value (it is every message queue's attribute set during its creation).
/proc/sys/fs/epoll/max_user_watches - Configuration options for the epoll interface
This directory contains configuration options for the epoll(7) interface.
Every epoll file descriptor can store a number of files to be monitored for event readiness. Each one of these monitored files constitutes a "watch".
This configuration option sets the maximum number of "watches" that are allowed for each user.
Each "watch" costs roughly 90 bytes on a 32bit kernel, and roughly 160 bytes on a 64bit one. The current default value for max_user_watches is the 1/32 of the available low memory, divided for the "watch" cost in bytes.
