While administrating a box, you may wanted to find out what a processes is doing and find out how many file descriptors (fd) are being used. You will surprised to find out that process does open all sort of files:
=> Actual log file
=> /dev files
=> UNIX Sockets
=> Network sockets
=> Library files /lib /lib64
=> Executables and other programs etc
In this quick post, I will explain how to to count how many file descriptors are currently in use on your Linux server system.
Step # 1 Find Out PID
To find out PID for mysqld process, enter:
# ps aux | grep mysqld
OR
# pidof mysqld
Output:
28290
Step # 2 List File Opened By a PID # 28290
Use the lsof command or /proc/$PID/ file system to display open fds (file descriptors), run:
# lsof -p 28290
# lsof -a -p 28290
OR
# cd /proc/28290/fd
# ls -l | less
You can count open file, enter:
# ls -l | wc -l
Tip: Count All Open File Handles
To count the number of open file handles of any sort, type the following command:
# lsof | wc -l
Sample outputs:
5436
List File Descriptors in Kernel Memory
Type the following command:
# sysctl fs.file-nr
Sample outputs:
fs.file-nr = 1020 0 70000
Where,
- 1020 The number of allocated file handles.
- 0 The number of unused-but-allocated file handles.
- 70000 The system-wide maximum number of file handles.
You can use the following to find out or set the system-wide maximum number of file handles:
# sysctl fs.file-max
Sample outputs:
fs.file-max = 70000
See how to set the system-wide maximum number of file handles under Linux for more information.
More about /proc/PID/file & procfs File System
/proc (or procfs) is a pseudo-file system that it is dynamically generated after each reboot. It is used to access kernel information. procfs is also used by Solaris, BSD, AIX and other UNIX like operating systems. Now, you know how many file descriptors are being used by a process. You will find more interesting stuff in /proc/$PID/file directory:
- /proc/PID/cmdline : process arguments
- /proc/PID/cwd : process current working directory (symlink)
- /proc/PID/exe : path to actual process executable file (symlink)
- /proc/PID/environ : environment used by process
- /proc/PID/root : the root path as seen by the process. For most processes this will be a link to / unless the process is running in a chroot jail.
- /proc/PID/status : basic information about a process including its run state and memory usage.
- /proc/PID/task : hard links to any tasks that have been started by this (the parent) process.
See also: /proc related FAQ/Tips
/proc is an essentials file system for sys-admin work. Just browser through our previous article to get more information about /proc file system:
- /proc/filesystems: Find out what filesystems supported by kernel
- Howto: Linux detect or find out a dual-core cpu
- Linux display CPU information - number of CPUs and their speed
- How to Scan new LUNs on Linux with QLogic driver
- Linux command to gathers up information about a Linux system
- Linux increase the maximum number of open files or file descriptors
- How to display or show information about a Linux Kernel module or drivers
- Linux scan wireless card for information
- Linux disable or drop / block ping packets all together
- How do I find out if my server CPU can run a 64 bit kernel version (apps) or not?
- Display Linux kernel slab cache information in real time
- Making changes to /proc filesystem permanently
- Howto rebuilding a RAID array after a disk fails
- Find out if my server is capable of running para-virtualized guest ( PAE support )
- Linux configure Network Address Translation or NAT
- Howto: Linux see new fiber channel attached disk LUNs without rebooting
- I also recommend reading /proc file system related document, and lsof man page to get a better understanding about fd and files.
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{ 10 comments… read them below or add one }
hey thanks for quick n dirty procfs tutorial :)
thx, that was really useful!quick question: anybody has an idea why the value displayed by ‘lsof -p {procid} | wc -l’ is different to that of ‘ls -l /proc/{procid}/fd | wc -l’. lsof is usually higher…?
Because lsof counts files in /proc//fd and adds the opend shared libs from /proc//maps
Thanks man, this was really useful.
Thank you. I had a daemon that stopped functioning correctly six hours after I started it. Turns out, I had a file descriptor leak. It had never even crossed my mind until I put 2 and 2 together. This confirmed it. It’s all fixed now, thanks to you!
Can’t we use lsof | wc -l for the same
you can combine lsof with pidof:
lsof -p `pidof ruby`
I Have a files in /proc//fd which should be in a different pid fd folder! File Descriptor Leak! Closed files from another process are duplicating themselves (random number of duplicate content 5-75 times) into another process’s fd folder! Has anybody seen this before. Solaris 10.
Yeah you can combine them all!
below one gives all type of descriptor opened by “mysqld” process:
lsof -p `pidof mysqld` |wc -l
There is a way to set system level “max open file descriptors” and view current count of “open file descriptors” at system-level;
I’m unable to recollect :(
does anyone know it?
cat /proc/sys/fs/file-nr shows number of open files in the first column, max allowed open files in third column and 0 in the second column.
It doesn’t update the first column on a single file handle basis, but seems to increase and decrease in steps