I think it's worth mentioning (I may have said this before xD) that when we talk about Open Files in Linux systems we are basically talking about File descriptors (aka file handles). A file descriptor is a data structure used by processes to access: files, Unix sockets, Networking sockets, pipes, etc. Every new process comes by default with 3 file descriptors:
- FD 0: standard input
- FD 1: standard output
- FD 2: standard error
juan@test:~$ ps aux|wc -l 142 juan@test:~$about 142 processes running on the system, we should expect at least around 426 file descriptors to be in use (142x3=426). What if there was a way to know how many file descriptors a particular process is using?
File descriptors a process is using:
Yeahp, of course that's possible! And as always in Linux, there are at least two different ways. The first approach will be of course the easiest one, each process has a folder under /proc that will provide loads of information, in this case will focus on subfolder fd, where guess what's going to show up? indeed, the file descriptors for the process:
root@test:/home/juan# ls /proc/1/fd | wc -l 24 root@test:/home/juan#init process (PID=1) has 24 file descriptors in use! We can see more details about them in the next output:
root@test:/home/juan# ls -la /proc/1/fd total 0 dr-x------ 2 root root 0 Oct 15 10:39 . dr-xr-xr-x 9 root root 0 Oct 15 10:38 .. lrwx------ 1 root root 64 Oct 15 10:39 0 -> /dev/null lrwx------ 1 root root 64 Oct 15 10:39 1 -> /dev/null lrwx------ 1 root root 64 Oct 15 10:39 10 -> socket:[8662] lrwx------ 1 root root 64 Oct 15 10:39 11 -> socket:[9485] l-wx------ 1 root root 64 Oct 15 10:39 12 -> /var/log/upstart/network-manager.log.1 (deleted) lrwx------ 1 root root 64 Oct 15 10:39 14 -> socket:[10329] l-wx------ 1 root root 64 Oct 15 10:39 16 -> /var/log/upstart/systemd-logind.log.1 (deleted) lrwx------ 1 root root 64 Oct 15 10:39 17 -> socket:[8637] lrwx------ 1 root root 64 Oct 15 10:39 18 -> /dev/ptmx lrwx------ 1 root root 64 Oct 15 10:39 2 -> /dev/null lrwx------ 1 root root 64 Oct 15 10:39 20 -> /dev/ptmx lrwx------ 1 root root 64 Oct 15 10:39 22 -> /dev/ptmx l-wx------ 1 root root 64 Oct 15 10:39 24 -> /var/log/upstart/modemmanager.log.1 (deleted) lrwx------ 1 root root 64 Oct 15 10:39 29 -> /dev/ptmx lr-x------ 1 root root 64 Oct 15 10:39 3 -> pipe:[8403] lrwx------ 1 root root 64 Oct 15 10:39 30 -> /dev/ptmx l-wx------ 1 root root 64 Oct 15 10:39 31 -> /var/log/upstart/mysql.log.1 (deleted) lrwx------ 1 root root 64 Oct 15 10:39 34 -> /dev/ptmx lrwx------ 1 root root 64 Oct 15 10:39 36 -> /dev/ptmx l-wx------ 1 root root 64 Oct 15 10:39 4 -> pipe:[8403] lr-x------ 1 root root 64 Oct 15 10:39 5 -> anon_inode:inotify lr-x------ 1 root root 64 Oct 15 10:39 6 -> anon_inode:inotify lrwx------ 1 root root 64 Oct 15 10:39 7 -> socket:[8404] lrwx------ 1 root root 64 Oct 15 10:39 9 -> socket:[12675] root@test:/home/juan#we can see how the file descriptors are represented as links, a brief of the output would be:
- Default file descriptors (0,1 and 2) have been pointed to /dev/null, which is ok for a process like init that isn't an interactive process.
- There are a couple of UNIX sockets "socket:[XXXX]" opened (7,9,10, 11, etc), probably to connect to other processes.
- There's a pipe "pipe:[8403]" as well using two fd (3 and 4) that's normal, pipes provide a fd to write and one to read while data is buffered on the kernel.
- The rest of the fs point to:
- /dev/ptmx pseudo terminal device.
- inotify a way to monitor changes on files, this means init is interested in the events on two particular fd.
- some deleted log files like /var/log/upstart/mysql.log.1 this is odd. Probably files were rotated or something like that.
Lets list all the open files for a particular process using lsof, init in this case:
root@test:/home/juan# lsof -p 1
lsof: WARNING: can't stat() fuse.gvfsd-fuse file system /run/user/112/gvfs
Output information may be incomplete.
COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME
init 1 root cwd DIR 8,1 4096 2 /
init 1 root rtd DIR 8,1 4096 2 /
init 1 root txt REG 8,1 265848 261189 /sbin/init
init 1 root mem REG 8,1 43616 581960 /lib/x86_64-linux-gnu/libnss_files-2.19.so
init 1 root mem REG 8,1 47760 555508 /lib/x86_64-linux-gnu/libnss_nis-2.19.so
init 1 root mem REG 8,1 97296 555504 /lib/x86_64-linux-gnu/libnsl-2.19.so
init 1 root mem REG 8,1 39824 555503 /lib/x86_64-linux-gnu/libnss_compat-2.19.so
init 1 root mem REG 8,1 14664 555500 /lib/x86_64-linux-gnu/libdl-2.19.so
init 1 root mem REG 8,1 252032 540246 /lib/x86_64-linux-gnu/libpcre.so.3.13.1
init 1 root mem REG 8,1 141574 555505 /lib/x86_64-linux-gnu/libpthread-2.19.so
init 1 root mem REG 8,1 1840928 581957 /lib/x86_64-linux-gnu/libc-2.19.so
init 1 root mem REG 8,1 31792 581956 /lib/x86_64-linux-gnu/librt-2.19.so
init 1 root mem REG 8,1 43464 527349 /lib/x86_64-linux-gnu/libjson-c.so.2.0.0
init 1 root mem REG 8,1 134296 527439 /lib/x86_64-linux-gnu/libselinux.so.1
init 1 root mem REG 8,1 281552 527323 /lib/x86_64-linux-gnu/libdbus-1.so.3.7.6
init 1 root mem REG 8,1 38920 527371 /lib/x86_64-linux-gnu/libnih-dbus.so.1.0.0
init 1 root mem REG 8,1 96280 527373 /lib/x86_64-linux-gnu/libnih.so.1.0.0
init 1 root mem REG 8,1 149120 555506 /lib/x86_64-linux-gnu/ld-2.19.so
init 1 root 0u CHR 1,3 0t0 1029 /dev/null
init 1 root 1u CHR 1,3 0t0 1029 /dev/null
init 1 root 2u CHR 1,3 0t0 1029 /dev/null
init 1 root 3r FIFO 0,9 0t0 8403 pipe
init 1 root 4w FIFO 0,9 0t0 8403 pipe
init 1 root 5r 0000 0,10 0 7661 anon_inode
init 1 root 6r 0000 0,10 0 7661 anon_inode
init 1 root 7u unix 0xffff8800b37c8780 0t0 8404 @/com/ubuntu/upstart
init 1 root 9u unix 0xffff8800a14e7c00 0t0 12675 @/com/ubuntu/upstart
init 1 root 10u unix 0xffff8800b37c9a40 0t0 8662 @/com/ubuntu/upstart
init 1 root 11u unix 0xffff8800b37a1e00 0t0 9485 @/com/ubuntu/upstart
init 1 root 12w REG 8,1 283 551619 /var/log/upstart/network-manager.log.1 (deleted)
init 1 root 14u unix 0xffff8800b37a3c00 0t0 10329 @/com/ubuntu/upstart
init 1 root 16w REG 8,1 451 522345 /var/log/upstart/systemd-logind.log.1 (deleted)
init 1 root 17u unix 0xffff8800b37cb0c0 0t0 8637 socket
init 1 root 18u CHR 5,2 0t0 1932 /dev/ptmx
init 1 root 20u CHR 5,2 0t0 1932 /dev/ptmx
init 1 root 22u CHR 5,2 0t0 1932 /dev/ptmx
init 1 root 24w REG 8,1 502 527289 /var/log/upstart/modemmanager.log.1 (deleted)
init 1 root 29u CHR 5,2 0t0 1932 /dev/ptmx
init 1 root 30u CHR 5,2 0t0 1932 /dev/ptmx
init 1 root 31w REG 8,1 881 552236 /var/log/upstart/mysql.log.1 (deleted)
init 1 root 34u CHR 5,2 0t0 1932 /dev/ptmx
init 1 root 36u CHR 5,2 0t0 1932 /dev/ptmx
root@test:/home/juan#
now we can see a few more things, like:- Details of the FD, like its Type, Device it belongs to, etc.
- We can see also some things that aren't really opened fd but some extra process information:
- cwd current working directory
- rtd root directory
- txt init's binary code file
- memory mapped files in this case bunch of system libraries. YES, these had a fd when they were mapped, but the fd was closed right after the mmap call was successful (you can see that checking this entry about strace).
Max open files, system wide:
If the answer for the previous question was now, I wouldn't have a reason to write this article in the first place I guess xD, therefore the answer is YES :P. The kernel is cool, and in order to play safe it has to set limits (I sound like a father now...) to avoid bigger problems.
The maximum number of open files the kernel can handle can be obtained from our beloved /proc, particularly in file file-nr under sys/fs directory. Here we can see the numbers for my current system:
root@test:/home/juan# cat /proc/sys/fs/file-nr 2944 0 298505 root@test:/home/juan#These values mean the following:
- First value (2944) indicates the number of allocated file descriptors, these are allocated dynamically by the kernel.
- Second value (0) is the number of allocated but unused file descriptor. Kernels from 2.6.something free any unused fd, so this value should always be 0.
- Third value (298505) indicates the maximum number of file descriptors that the kernel can allocate (also visible on file-max file).
Just for the sake of it, lets track down the process using the most number of file descriptors:
root@test:/home/juan# for i in `ps -Ao pid|grep -v PID`;do count=`ls /proc/$i/fd/ 2> /dev/null|wc -l`; echo "$count $i";done | sort -nr | head 61 1075 48 393 32 1264 31 1365 27 1132 24 1 21 440 20 1265 19 1325 19 1311 root@test:/home/juan#there we see the top ten (first column is the number of FD and the second is the PID). Interestingly enough there's a process using 61 file descriptors, turns out I had mysqld installed on this VM (had no idea...):
root@test:/home/juan# ps aux|grep 1075 mysql 1075 0.1 1.9 624040 57792 ? Ssl 10:38 0:07 /usr/sbin/mysqld root 9664 0.0 0.0 15948 2232 pts/1 S+ 12:35 0:00 grep --color=auto 1075 root@test:/home/juan#
Increasing the limit
If by any chance the almost 300k file descriptors the kernel allows to open is not enough (some busy systems may reach that limit) you will notice logs like
- iminproductionpainchangeitrightnowgoddamnit way, by just updating /proc with the new value, like
root@test:/home/juan# echo 400000 > /proc/sys/fs/file-max root@test:/home/juan# cat /proc/sys/fs/file-nr 3072 0 400000 root@test:/home/juan#
- Or you can be more elegant and use sysctl command:
root@test:/home/juan# sysctl -w fs.file-max=500000 fs.file-max = 500000 root@test:/home/juan# cat /proc/sys/fs/file-nr 3072 0 500000 root@test:/home/juan#
In any case, don't forget to make the change persistent by doing:
root@test:/home/juan# echo "fs.file-max=500000" >> /etc/sysctl.conf root@test:/home/juan#
If the error showing on your logs is instead
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