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A Mystery With open_files_limit

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A Mystery With open_files_limit

Every developer loves a good coding mystery, right? Well, sometimes... but don't worry — this mystery is sure to peak your interest!

· Performance Zone
Free Resource

In this blog, we'll look at a mystery around setting the open_file_limit variable in MySQL and Percona Server for MySQL.

MySQL Server needs file descriptors to run. It uses them to open new connections, store tables in the cache, create temporary tables to resolve complicated queries, and access persistent ones. If mysqld is not able to open new files when needed, it can stop functioning correctly. A common symptom of this issue is error 24: "Too many open files."

The number of file descriptors that can open simultaneously is defined by the configuration option. You would expect it to work like any other MySQL Server option — set in the configuration file, restart, and use more or fewer descriptors. All other configuration variables work this way. But it also depends on the operating system (OS) limits. This makes setting the variable more complicated.

mysqld

As a user, when you start an application, it cannot have limits set to be greater than the limits defined by the operating system for the user in question. Therefore, you would intuitively expect to set to any value that is less than the OS limit. This is not the case, however. No matter what value you set for the variable, the OS limit is used unless it is set to infinity.

sveta@Thinkie:~$ ulimit -n
32000
sveta@Thinkie:$ cat /etc/my.cnf
[mysqld]
open-files-limit=16000
...
sveta@Thinkie:$ ./bin/mysqld &
sveta@Thinkie:$ mysql -uroot
Welcome to the MySQL monitor. Commands end with ; or g.
Your MySQL connection id is 3
Server version: 5.7.19-17-debug-log Source distribution
Copyright (c) 2009-2017 Percona LLC and/or its affiliates
Copyright (c) 2000, 2017, Oracle and/or its affiliates. All rights reserved.
Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.
Type 'help;' or 'h' for help. Type 'c' to clear the current input statement.
mysql> select @@open_files_limit;
+--------------------+
| @@open_files_limit |
+--------------------+
|              32000 |
+--------------------+
1 row in set (0.00 sec)

The reason for this can be found in the code contained in the mysys/my_file.c file:

static uint set_max_open_files(uint max_file_limit)
{
  struct rlimit rlimit;
  uint old_cur;
  DBUG_ENTER("set_max_open_files");
  DBUG_PRINT("enter",("files: %u", max_file_limit));
  if (!getrlimit(RLIMIT_NOFILE,&rlimit))
  {
    old_cur= (uint) rlimit.rlim_cur;
    DBUG_PRINT("info", ("rlim_cur: %u  rlim_max: %u",
            (uint) rlimit.rlim_cur,
            (uint) rlimit.rlim_max));
    if (rlimit.rlim_cur == RLIM_INFINITY)
      rlimit.rlim_cur = max_file_limit;
    if (rlimit.rlim_cur >= max_file_limit)
      DBUG_RETURN(rlimit.rlim_cur);     /* purecov: inspected */
    rlimit.rlim_cur= rlimit.rlim_max= max_file_limit;
    if (setrlimit(RLIMIT_NOFILE, &rlimit))
      max_file_limit= old_cur;          /* Use original value */
    else
    { 
      rlimit.rlim_cur= 0;           /* Safety if next call fails */
      (void) getrlimit(RLIMIT_NOFILE,&rlimit);
      DBUG_PRINT("info", ("rlim_cur: %u", (uint) rlimit.rlim_cur));
      if (rlimit.rlim_cur)          /* If call didn't fail */
    max_file_limit= (uint) rlimit.rlim_cur;
    } 
  }
  DBUG_PRINT("exit",("max_file_limit: %u", max_file_limit));
  DBUG_RETURN(max_file_limit);
}

Particularly these lines:

if (rlimit.rlim_cur >= max_file_limit)
  DBUG_RETURN(rlimit.rlim_cur);    /* purecov: inspected */

This code tells to take the maximum value of what is specified in either the variable open_files_limit or the soft system user limit.

I reported this behavior as documentation bug #87681.

mysqld_safe

mysqld_safe has its own option. This option allows you to overwrite the system soft limit any way you want. However, on:

  • Red Hat Enterprise Linux 7
  • Oracle Linux 7
  • CentOS 7
  • SUSE Linux Enterprise Server 12
  • Fedora 25 and 26
  • Debian 8 or higher
  • Ubuntu 16.04 LTS or higher

This option as specified under the [mysqld_safe] header in the configuration file is not used when you start as a service. To explain the reason for this behavior, we need to step back into history.

init.d

For a long time, many Linux Operating Systems used init.d to start certain commands together with the OS. The Init daemon executes scripts (usually located in the directory /etc/init.d) at system startup, depending on the runlevel.

The different implementations of init.d vary, but they have known drawbacks. For example, init.d starts everything sequentially. This means a new process has to wait if another has already started. This makes the startup process on multi-core machine slow. Another drawback related to our topic is that daemons started by init.d inherit OS limits from the root user. If a program needs to be run by another user, the switch needs to happen in the startup script itself. But the order of option files that such users read can be different, depending if they are logged in via the sshsu, or sudo, or commands.

MySQL Server

MySQL Server's startup sequence for the service is as follow:

  1. <Perform another job>

  2. Start mysqld_safe as a mysql user:  su - mysql -s /bin/bash -c "mysqld_safe > /dev/null &" 

This behavior has existed at least since version 5.5.

Percona Server for MySQL

Before version 5.7, Percona Server for MySQL had a different startup sequence:

  1. <Perform another job>

  2. Start mysqld_safe as root and pass option --user=mysql to it: "${PERCONA_PREFIX}"/bin/mysqld_safe > /dev/null 2>&1 &

With this sequence, you only need to set a hard limit for a MySQL user in the file /etc/security/limits.conf, and mysqld_safe  will do the rest.

In version 5.7, Percona Server for MySQL backported the startup sequence from MySQL Server. Since then, setting a hard limit on the number of open files for MySQL users in /etc/security/limits.conf is not enough. You also need to have a row session required pam_limits.so  in the file /etc/pam/d/common-session. This is needed because the startup sequence for users changed due to the design of init.d.

SystemD

Linux developers performed several trials to find a better startup solution than init.d. Speaking for MySQL and Percona Server for MySQL startups, the most important innovation is SystemD. SystemD is becoming more and more popular. Therefore MySQL and Percona Server for MySQL do not use init.d on Red Hat Enterprise Linux 7, Oracle Linux 7, CentOS 7, SUSE Linux Enterprise Server 12, Fedora 25 and 26, Debian 8 or higher, and Ubuntu 16.04 LTS or higher. Instead, they use SystemD.

What does this mean for MySQL users?

Scripts started by SystemD start as required by the system user from the start. Therefore they do not inherit limits from the root user and use their own limits specified in /etc/security/limits.conf. If you need to have your process limits differ from the defaults for user mysql, you need to set the option limitNOFILE under the [Service] section in the service configuration file. Again, you cannot then lower this limit using open_files_limit, unless you set it to infinity.

Both Packages

To make things more complex, Percona Server for MySQL packages for Ubuntu contain both the mysql.server script (used by init.d) and the service description for SystemD. In fact, SystemD is used after install — but you might be confused when looking at only the package files.

Conclusion

You should set the open_files_limit variable together with the operating system limits. You should study how init.d or SystemD works if you see values that you don't expect.

How to change open_files_limit variable:

Operating System Startup daemon Where to put configuration
Red Hat Enterprise Linux 7, Oracle Linux 7, CentOS 7
SUSE Linux Enterprise Server 12
Fedora 25 and 26
Debian 8+
Ubuntu 16.04 LTS+
SystemD /etc/security/limits.conf and
/etc/pam.d/common-session
Service configuration: sudo systemctl edit mysql
[mysqld] section of the configuration file
Others init.d /etc/security/limits.conf and
/etc/pam.d/common-session
[mysqld_safe] section of the configuration file
[mysqld] section of the configuration file

Which values of the open_files_limit variable make sense?

Soft User Limit open_files_limit range
Infinity Any
Positive

Greater/equal than soft user limit and smaller than hard user limit

Topics:
open_files_limit ,mysqld ,mysql ,performance ,tutorial

Published at DZone with permission of Sveta Smirnova, DZone MVB. See the original article here.

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