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documentation.suse.com / SUSE Linux Enterprise Server Documentation / System Analysis and Tuning Guide / Resource Management / Kernel Control Groups
Applies to SUSE Linux Enterprise Server 15 SP1

9 Kernel Control Groups

  

Kernel Control Groups (“cgroups”) are a kernel feature that allows assigning and limiting hardware and system resources for processes. Processes can also be organized in a hierarchical tree structure.

9.1 Overview

  

Every process is assigned exactly one administrative cgroup. cgroups are ordered in a hierarchical tree structure. You can set resource limitations, such as CPU, memory, disk I/O, or network bandwidth usage,for single processes or for whole branches of the hierarchy tree.

On SUSE Linux Enterprise Server, systemd uses cgroups to organize all processes in groups, which systemd calls slices. systemd also provides an interface for setting cgroup properties.

The command systemd-cgls displays the hierarchy tree.

This chapter is an overview. For more details, refer to the listed references.

9.2 Setting Resource Limits

  
Note
Note: Implicit Resource Consumption

Be aware that resource consumption implicitly depends on the environment where your workload executes (e.g. size of data structures in libraries/kernel, forking behavior of utilities, computational efficiency), hence it is recommended to (re)calibrate your limits should the environment change.

Limitations to cgroups can be set with the systemctl set-property command. The syntax is: 

# systemctl set-property [--runtime] NAME PROPERTY1=VALUE [PROPERTY2=VALUE]

Optionally, use the --runtime option. With this option, set limits do not persist after the next reboot.

Replace NAME with a systemd service slice, scope, socket, mount, or swap name. Replace properties with one or more of the following:

CPUAccounting=[yes|no]

Turns on CPU usage accounting. This property takes yes and no as arguments.

Example: 

# systemctl set-property user.slice CPUAccounting=yes
CPUQuota=PERCENTAGE

Assigns a CPU time to processes. The value is a percentage followed by a % as suffix. This implies CPUAccounting=yes.

Example: 

# systemctl set-property user.slice CPUQuota=50%
MemoryAccounting=[yes|no]

Turns on memory usage accounting. This property takes yes and no as arguments.

Example: 

# systemctl set-property user.slice MemoryAccounting=yes
MemoryLow=BYTES

Unused memory from processes below this limit will not be reclaimed for other use. Use suffixes K, M, G or T for BYTES. This implies MemoryAccounting=yes.

Example: 

# systemctl set-property nginx.service MemoryLow=512M
Note
Note: Unified Control Group Hierarchy

This setting is available only if the unified control group hierarchy is used, and disables MemoryLimit=. To enable the unified control group hierarchy, append systemd.unified_cgroup_hierarchy=1 as a kernel command line parameter to the GRUB 2 boot loader. Refer to Chapter 14, The Boot Loader GRUB 2 for more details about configuring GRUB 2.

MemoryHigh=BYTES

If more memory above this limit is used, memory is aggressively taken away from the processes. Use suffixes K, M, G or T for BYTES. This implies MemoryAccounting=yes. For example: 

# systemctl set-property nginx.service MemoryHigh=2G
Note
Note: Unified Control Group Hierarchy

This setting is available only if the unified control group hierarchy is used, and disables MemoryLimit=. To enable the unified control group hierarchy, append systemd.unified_cgroup_hierarchy=1 as a kernel command line parameter to the GRUB 2 boot loader. For more details about configuring GRUB 2, see Chapter 14, The Boot Loader GRUB 2.

MemoryMax=BYTES

Sets a maximum limit for used memory. Processes will be killed if they use more memory than allowed. Use suffixes K, M, G or T for BYTES. This implies MemoryAccounting=yes.

Example: 

# systemctl set-property nginx.service MemoryMax=4G
DeviceAllow=

Allows read (r), write (w) and mknod (m) access. The command takes a device node specifier and a list of r, w or m, separated by a white space.

Example: 

# systemctl set-property system.slice DeviceAllow="/dev/sdb1 r"
DevicePolicy=[auto|closed|strict]

When set to strict, only access to devices that are listed in DeviceAllow is allowed. closed additionally allows access to standard pseudo devices including /dev/null, /dev/zero, /dev/full, /dev/random, and /dev/urandom. auto allows access to all devices if no specific rule is defined in DeviceAllow. auto is the default setting.

For more details and a complete list of properties, see man systemd.resource-control.

9.3 Preventing Fork Bombs with TasksMax

  

systemd 228 shipped with a DefaultTasksMax limit of 512. This limited the number of processes any system unit can create at one time to 512. Previous versions had no default limit. The goal was to improve security by preventing runaway processes from creating excessive forks, or spawning enough threads to exhaust system resources.

However, it soon became apparent that there is not a single default that applies to all use cases. 512 is not low enough to prevent a runaway process from crashing a system, especially when other resources such as CPU and RAM are not restricted, and not high enough for processes that create a lot of threads, such as databases. In systemd 234 the default was changed to 15%, which is 4915 tasks (15% of the kernel limit of 32768; see cat /proc/sys/kernel/pid_max). This default is compiled, and can be changed in configuration files. The compiled defaults are documented in /etc/systemd/system.conf. You can edit this file to override the defaults, though there are other methods we will show in the following sections.

9.3.1 Finding the Current Default TasksMax Values

  

SUSE Linux Enterprise Server ships with two custom configurations that override the upstream defaults for system units and for user slices, and sets them both to infinity. /usr/lib/systemd/system.conf.d/20-suse-defaults.conf contains these lines: 

[Manager]
DefaultTasksMax=infinity

/usr/lib/systemd/system/user-.slice.d/20-suse-defaults.conf contains these lines: 

[Slice]
TasksMax=infinity

infinity means having no limit. It is not a requirement to change the default, but setting some limits may help to prevent system crashes from runaway processes.

9.3.2 Overriding the DefaultTasksMax Value

  

Change the global DefaultTasksMax value by creating a new override file, /etc/systemd/system.conf.d/10-system-tasksmax.conf, and write the following lines to set new default limit of 256 tasks per system unit: 

[Manager]
DefaultTasksMax=256

Load the new setting, then verify that it changed: 

> sudo systemctl daemon-reload
> systemctl show --property DefaultTasksMax
DefaultTasksMax=256

Adjust this default value to suit your needs. You can set higher limits on individual services as needed. This example is for MariaDB. First check the current active value: 

> systemctl status mariadb.service
  ● mariadb.service - MariaDB database server
   Loaded: loaded (/usr/lib/systemd/system/mariadb.service; disabled; vendor preset>
   Active: active (running) since Tue 2020-05-26 14:15:03 PDT; 27min ago
     Docs: man:mysqld(8)
           https://mariadb.com/kb/en/library/systemd/
 Main PID: 11845 (mysqld)
   Status: "Taking your SQL requests now..."
    Tasks: 30 (limit: 256)
   CGroup: /system.slice/mariadb.service
           └─11845 /usr/sbin/mysqld --defaults-file=/etc/my.cnf --user=mysql

The Tasks line shows that MariaDB currently has 30 tasks running, and has an upper limit of the default 256, which is inadequate for a database. The following example demonstrates how to raise MariaDB's limit to 8192. Create a new override file with systemctl edit, and enter the new value: 

> sudo systemctl edit mariadb.service

[Service]
TasksMax=8192

> systemctl status mariadb.service 
● mariadb.service - MariaDB database server
   Loaded: loaded (/usr/lib/systemd/system/mariadb.service; disabled; vendor preset: disab>
  Drop-In: /etc/systemd/system/mariadb.service.d
           └─override.conf
   Active: active (running) since Tue 2020-06-02 17:57:48 PDT; 7min ago
     Docs: man:mysqld(8)
           https://mariadb.com/kb/en/library/systemd/
  Process: 3446 ExecStartPre=/usr/lib/mysql/mysql-systemd-helper upgrade (code=exited, sta>
  Process: 3440 ExecStartPre=/usr/lib/mysql/mysql-systemd-helper install (code=exited, sta>
 Main PID: 3452 (mysqld)
   Status: "Taking your SQL requests now..."
    Tasks: 30 (limit: 8192)
   CGroup: /system.slice/mariadb.service
           └─3452 /usr/sbin/mysqld --defaults-file=/etc/my.cnf --user=mysql

systemctl edit creates an override file, /etc/systemd/system/mariadb.service.d/override.conf, that contains only the changes you want to apply to the existing unit file. The value does not have to be 8192, but should be whatever limit is appropriate for your workloads.

9.3.3 Default TasksMax Limit on Users

  

The default limit on users should be fairly high, because user sessions need more resources. Set your own default for users by creating a new file, for example /etc/systemd/system/user-.slice.d/user-taskmask.conf. The following example sets a default of 16284: 

[Slice]
TasksMax=16284

Then reload systemd to load the new value, and verify the change by querying the root slice and a user slice: 

> sudo systemctl daemon-reload

> systemctl show --property TasksMax user-.slice
TasksMax=16284

> systemctl show --property TasksMax user-1000.slice
TasksMax=16284

How do you know what values to use? This varies according to your workloads, system resources, and other resource configurations. When your TasksMax value is too low, you will see error messages such as Failed to fork (Resources temporarily unavailable), Can't create thread to handle new connection, and Error: Function call 'fork' failed with error code 11, 'Resource temporarily unavailable'.

For more information on configuring system resources in systemd, see systemd.resource-control (5).

9.4 For More Information