How do ulimit settings impact Linux?
I have made my homework and (almost) found what each option does. Also, I've noted that there is more options in /etc/security/limits.conf than it appears with ulimit -a. Therefore, I've only documented the latter here. Of course, everyone is invited to enrich this answer!
core file size (blocks, -c)
The maximum size of core files created. Core dump is a system snapshot (RAM + context switch + processor registers).
https://en.wikipedia.org/wiki/Core_dump
data seg size (kbytes, -d)
The maximum size of a process's data segment. a data segment is a portion of an object file or the corresponding virtual address space of a program that contains initialized static variables.
https://en.wikipedia.org/wiki/Data_segment
scheduling priority (-e)
The maximum scheduling priority ("nice") a process can be given.
https://en.wikipedia.org/wiki/Scheduling_%28computing%29
file size (blocks, -f)
The maximum size of files written by the shell and its children.
pending signals (-i)
Set of signals that are pending for delivery to the calling thread.
https://unix.stackexchange.com/questions/197600/what-are-pending-signals
max locked memory (kbytes, -l)
The maximum size that may be locked into memory. Memory locking ensures the memory is always in RAM and never moved to the swap disk.
https://stackoverflow.com/questions/9818755/why-would-we-need-to-lock-a-processs-address-space-in-ram
max memory size (kbytes, -m)
How much memory a process currently has in main memory (RAM), opposed to how much virtual memory the process has in total.
https://en.wikipedia.org/wiki/Resident_set_size
open files (-n)
The maximum number of open file descriptors. A file descriptor is an abstract indicator used to access a file or other input/output resource, such as a pipe or network socket.
https://en.wikipedia.org/wiki/File_descriptor
List file descriptors: http://www.cyberciti.biz/tips/linux-procfs-file-descriptors.html
pipe size (512 bytes, -p)
Pipe's internal buffer size. See "pipe capacity" section in http://man7.org/linux/man-pages/man7/pipe.7.html
POSIX message queues (bytes, -q)
The maximum number of bytes in POSIX message queues. POSIX message queues allow processes to exchange data in the form of messages.
http://linux.die.net/man/7/mq_overview
Message queues in general https://en.wikipedia.org/wiki/Message_queue
real-time priority (-r)
The maximum real-time scheduling priority. A realtime priority thread can never be pre-empted by timer interrupts and runs at a higher priority than any other thread in the system.
https://stackoverflow.com/questions/1663993/what-is-the-realtime-setting-for-for-process-priority
stack size (kbytes, -s)
The maximum stack size. The stack size is a reserved region of memory that is used to store the location of function calls in order to allow return statements to return to the correct location.
https://en.wikipedia.org/wiki/Stack-based_memory_allocation
cpu time (seconds, -t)
The maximum amount of cpu time in seconds.
https://en.wikipedia.org/wiki/CPU_time
max user processes (-u)
The maximum number of processes a user can start or fork.
https://en.wikipedia.org/wiki/Process_%28computing%29
This command shows how much processes each user is currently using:
ps h -Led -o user | sort | uniq -c | sort -n
virtual memory (kbytes, -v)
The maximum amount of virtual memory available to the shell. Virtual memory maps memory addresses used by a program, called virtual addresses, into physical addresses in computer memory.
https://en.wikipedia.org/wiki/Virtual_memory
file locks (-x)
File locking is a mechanism that restricts access to a computer file by allowing only one user or process access at any specific time.
https://en.wikipedia.org/wiki/File_locking