Does OpenBSD have a limit to the number of file descriptors

file-descriptorslimitopenbsd

What's the maximum number of file descriptors that a process can get on OpenBSD successfully?

For example, there is an openfiles limit in login.conf(5) in OpenBSD. If I want to have as many file descriptors as possible, but still don't let a runaway process to bog down the system, what would be a sensible value to set?

E.g. if I specify, say, 20000, will the kernel be capable of giving my application all of these 20000 FDs? What if I'm running multiple instances of a given application (e.g. multiple worker processes)?

Best Answer

Taking a look at the source code, to get the default value of max open files:

Well documented code

extern int maxfiles;                 /* kernel limit on number of open files */

maxfiles, on param.c defines the formula to maxfiles

int maxfiles = 5 * (NPROCESS + MAXUSERS) + 80;

OK, we found it.

NPROCESS =

#define NPROCESS (30 + 16 * MAXUSERS)

MAXUSERS = - Lets take amd64 architecture as an example:

machine         amd64
include         "../../../conf/GENERIC"
maxusers        80                      # estimated number of users

Lets sum all the stuff:

maxfiles = 5 * ((30 + 16 * 80) + 80) + 80
maxfiles = 5 * ((30 + 1280) + 80) + 80
maxfiles = 5 * (1390) + 80
maxfiles = 6950 + 80
maxfiles = 7030

To increase the total of max open files, you will need first of all to increase the max open files kernel limit with sysctl kern.maxfiles=20000 and increase the number of files a process/user can open, editing login.conf. This Tor daemon setup have both examples for you.

Related Solutions

How to set a limit on the number of files Linux can have open

Most systems use PAM, and have the pam_limits module set limits based on /etc/security/limits.conf. The per-user limit for open files is called nofile. You can set it for every user or for a particular user or group, and you can set a limit that the user can override (soft limit) and another that only root can override (hard limit). The documentation and the limits.conf man page have the details. For example, to raise the limit to 50000 for everyone, put this line in /etc/limits.conf (the setting takes effect when you log in):

* - nofile 50000

Linux – Limits on the number of file descriptors

According to the kernel documentation, /proc/sys/fs/file-max is the maximum, total, global number of file descriptors the kernel will allocate before choking. This is the kernel's limit, not your current user's. So you can open 590432, provided you're alone on an idle system (single-user mode, no daemons running).

Note that the documentation is out of date: the file has been /proc/sys/fs/file-max for a long time. Thanks to Martin Jambon for pointing this out.

The difference between soft and hard limits is answered here, on SE. You can raise or lower a soft limit as an ordinary user, provided you don't overstep the hard limit. You can also lower a hard limit (but you can't raise it again for that process). As the superuser, you can raise and lower both hard and soft limits. The dual limit scheme is used to enforce system policies, but also allow ordinary users to set temporary limits for themselves and later change them.

Note that if you try to lower a hard limit below the soft limit (and you're not the superuser), you'll get EINVAL back (Invalid Argument).

So, in your particular case, ulimit (which is the same as ulimit -Sf) says you don't have a soft limit on the size of files written by the shell and its subprocesses. (that's probably a good idea in most cases)

Your other invocation, ulimit -Hn reports on the -n limit (maximum number of open file descriptors), not the -f limit, which is why the soft limit seems higher than the hard limit. If you enter ulimit -Hf you'll also get unlimited.

Best Answer

Related Solutions

How to set a limit on the number of files Linux can have open

Linux – Limits on the number of file descriptors

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