Tuning file descriptors limits on Linux
What is a file descriptor ?
Is a data structure what handle a real or pseudo file. Real files are filesystem files and pseudo files are the proc files, dev files etc. File descriptors are limited per process and a limit per entire system also exist. So your limits per process cannot surpass limit per entire system.
How many file descriptors can my system support ?
To see that just run the following command in bash:
cat /proc/sys/fs/file-max
202781
How can I increase the number of file descriptors on my system ?
Just run the following command:
echo 65535 > proc/sys/fs/file-max
Where 65535 is the new number of opened file descriptors. This value will persist until the next reboot. To save as persistent value in your system you need to add next value in /etc/sysctl.conf:
fs.file-max = 65535
After editing /etc/sysctl.conf just run:
systcl -p
to apply modification on your system.
How many file descriptors are being used ?
This information is stored in /proc/sys/fs/file-nr. Run following command to see the content of this file
cat /proc/sys/fs/file-nr
And you will get and output something like:
1500 210 65535
Where:
1500 is total alocated file descriptors
210 is total of free alocated file descriptors
65535 is the total limit of the system
Another method to see the number of file descriptors opened in your system is using lsof (list open files). Run the following command to get the number of file descriptors opened:
lsof | wc -l
Why we need to increase the number of file descriptors ?
Heavy loaded servers (but not only them) need that because they are opening a large number of files, sockets or other pseudo files. For each connection a server application will open a new socket and probably other files.
What is the limit of file descriptors per process ?
To see that just run the following command:
ulimit -n
and you will get the number of file descriptors per your session (process).
How cand I modify the limit of file descriptors per session ?
Edit /etc/security/limits.conf and add:
* soft nofile 65535 * hard nofile 65535
This options will increase the number of file descriptors for all users. If you want just for a specific user replace * with the username. For example if you want to increase the limit for the apache file server just replace * with www-data (Debian specific) and after that just restart the apache (on Debian run: /etc/init.d/apache2 restart)
How many file descriptors are opened on a specific process ?
For example if you want to see how many file descriptors are opened for apache process (www-data user) just run following command:
lsof | grep www-data | wc -l
and you will get the number of file descriptors for apache http server.
To see what files are opened and you only know the pid of the process just run:
lsof -p <pid>
and the result will be something like:
smbd 17533 root cwd DIR 8,2 608 29 /tmp smbd 17533 root rtd DIR 8,2 728 2 / smbd 17533 root txt REG 8,2 3936896 109985 /usr/sbin/smbd smbd 17533 root mem REG 8,2 43480 37497 /lib/libnss_nis-2.7.so smbd 17533 root mem REG 8,2 35632 37493 /lib/libnss_compat-2.7.so smbd 17533 root mem REG 8,2 47528 37495 /lib/libnss_files-2.7.so smbd 17533 root mem REG 8,2 10200 37728 /usr/lib/gconv/IBM850.so smbd 17533 root mem REG 8,2 14320 131965 /usr/lib/gconv/UTF-16.so smbd 17533 root mem REG 8,2 13144 15790 /lib/libgpg-error.so.0.3.0 smbd 17533 root mem REG 8,2 318608 30412 /lib/libgcrypt.so.11.2.3 smbd 17533 root mem REG 8,2 65800 48343 /usr/lib/libtasn1.so.3.0.12 smbd 17533 root mem REG 8,2 7344 77870 /lib/libkeyutils-1.2.so smbd 17533 root mem REG 8,2 30440 517492 /usr/lib/libkrb5support.so.0.1 smbd 17533 root mem REG 8,2 102784 28254 /usr/lib/libsasl2.so.2.0.22 smbd 17533 root mem REG 8,2 1436976 37486 /lib/libc-2.7.so smbd 17533 root mem REG 8,2 31992 48491 /lib/libpopt.so.0.0.0 smbd 17533 root mem REG 8,2 14624 37489 /lib/libdl-2.7.so smbd 17533 root mem REG 8,2 93080 37492 /lib/libnsl-2.7.so smbd 17533 root mem REG 8,2 80760 37501 /lib/libresolv-2.7.so smbd 17533 root mem REG 8,2 27600 65429 /lib/libacl.so.1.1.0 smbd 17533 root mem REG 8,2 16128 65415 /lib/libattr.so.1.1.0 smbd 17533 root mem REG 8,2 42928 65510 /lib/libpam.so.0.81.6 smbd 17533 root mem REG 8,2 39120 37488 /lib/libcrypt-2.7.so smbd 17533 root mem REG 8,2 526560 37490 /lib/libm-2.7.so smbd 17533 root mem REG 8,2 130224 37500 /lib/libpthread-2.7.so smbd 17533 root mem REG 8,2 93536 48299 /usr/lib/libz.so.1.2.3.3 smbd 17533 root mem REG 8,2 542040 48349 /usr/lib/libgnutls.so.13.9.1 smbd 17533 root mem REG 8,2 221960 7935 /usr/lib/libcups.so.2 smbd 17533 root mem REG 8,2 10104 106055 /lib/libcom_err.so.2.1 smbd 17533 root mem REG 8,2 147680 514871 /usr/lib/libk5crypto.so.3.1 smbd 17533 root mem REG 8,2 619336 517485 /usr/lib/libkrb5.so.3.3 smbd 17533 root mem REG 8,2 174984 514852 /usr/lib/libgssapi_krb5.so.2.2 smbd 17533 root mem REG 8,2 57064 1635793 /usr/lib/liblber-2.4.so.2.0.5 smbd 17533 root mem REG 8,2 274904 1635794 /usr/lib/libldap_r-2.4.so.2.0.5 smbd 17533 root mem REG 8,2 127480 37483 /lib/ld-2.7.so smbd 17533 root mem REG 8,2 4096 128425 /var/lib/samba/share_info.tdb
Missing trailing slash: echo 65535 > proc/sys/fs/file-max
Otherwise useful text. thx.
Thanks …
Short, concise and useful blog. Thanks.
tnkss very god….
Thanks for the post.
I’m using this command:
[root@testserver ~]#lsof | grep username | wc -l
4719
But I can’t understand it, because the limit is lower than result:
[root@testserver ~]# ulimit -a | grep open
open files (-n) 4096
What am I doing / reading wrong?
The fd number can differ depending on what type of login you made. If you do console, su or ssh login the number of FD can be different if you don’t have pam_limits enabled on /etc/pam.d/(ssh|su|login). Second, doing grep, probably you catch another processes which in a way or other is using your username. The correct way is to use lsof -u username and not grep.
Regards
[...] Take a look at file descriptors (Tuning file descriptors limits on Linux) under Linux if you deploy this configuration on a production [...]
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