[−][src]Module async_listen::errors

Documentation of the Error Hints

This page is the destination of the links shown by error_hint.

List of errors having a hint:

Too Many Open Files

Posix NameEMFILE
MessageToo many open files (os error 24)
HintIncrease per-process open file limit
Linkhttps://bit.ly/async-err#EMFILE

Common Causes

  1. File descriptor limit for the process is too low
  2. Limit of number of simultaneous connections is either too high or unimplemented

The (2) can be fixed by applying backpressure abstraction from this crate.

The rest of this section discusses how to change file decriptor limit.

Choosing a Limit

There is no one good strategy. But here are few hints:

  1. It must be lower than limit on simultaneous connections.
  2. Sometimes it's several times lower, like if you need to open a file or to open a backend connection for each client, it should be 2x lower plus some offset.
  3. Measure an average memory used by each client and divide memory available by that value.

We use 10000 as an example value later in the text.

Linux

Either use ulimit -n in the same shell:

$ ulimit -n 10000
$ ./your_app

If you get the error, you need superuser privileges:

$ ulimit -n 10000
ulimit: value exceeds hard limit
$ sudo -s
# ulimit -n 10000
# su your_user
$ ./your_app

On most systems there is /etc/security/limits.conf to make persistent changes:

your_user        hard nofile 10000
your_user        sort nofile 10000

Run su your_user to apply changes to your current shell without reboot.

More information

Docker

Docker uses limit of 65535 by default. To increase it add a parameter:

docker run --ulimit nofile=10000:10000 ...

MacOS

On MacOS raising ulimit doesn't require permissions:

$ ulimit -n 10000
$ ./your_app

More information

Too Many Open Files in System

Posix NameENFILE
MessageToo many open files in system (os error 23)
HintIncrease system open file limit
Linkhttps://bit.ly/async-err#ENFILE

Common Causes

  1. Per-process file descriptor limit is larger than system one
  2. File descriptor limit on the system is too small
  3. Limit of number of simultaneous connections is either too high or unimplemented

The (3) can be fixed by applying backpressure abstraction from this crate.

Changing (1) is described in the section above.

The rest of this section discusses how to change system file decriptor limit.

Choosing a Limit

Usually system limit depends on the memory and doesn't have to be increased. So be careful and consult your system docs for more info.

Linux

Checking the limit:

$ cat /proc/sys/fs/file-max
818354

Setting a limit:

$ sudo sysctl fs.file-max=1500000

This only works until reboot. On some systems, to preserve this setting after reboot you can run:

$ sudo sysctl -w fs.file-max=1500000

(Note -w)

More information

MacOS

Checking a limit:

launchctl limit maxfiles

Setting a limit, until reboot:

sudo sysctl -w kern.maxfiles=20480

To make the permanent change, add the following to /etc/sysctl.conf:

kern.maxfiles=65536
kern.maxfilesperproc=65536

More information