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documentation.suse.com / SUSE Linux Enterprise Desktop Documentation / System Analysis and Tuning Guide / Resource management / Kernel control groups
Applies to SUSE Linux Enterprise Desktop 15 SP4

9 Kernel control groups

Kernel Control Groups (cgroups) are a kernel feature for 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 Desktop, 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.

The kernel cgroup API comes in two variants, v1 and v2. Additionally, there can be multiple cgroup hierarchies exposing different APIs. From the numerous possible combinations, there are two practical choices:

  • hybrid: v2 hierarchy without controllers, and the controllers are on v1 hierarchies

  • unified: v2 hierarchy with controllers

The current default mode is hybrid (as of SLE SP3). This provides backwards compatibility for applications that need it. The following features are available only with the unified v2 hierarchy:

  • memory controller: reclaim protection (aka memory.low), memory.high, PSI (pressure stall information)

  • io controller: writeback control, new control policies

  • controller delegation to non-privileged users (rootless containers)

  • freezer support in systemd

  • simpler handling of the single hierararchy

You may set only one mode.

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 18, The boot loader GRUB 2 for more details about configuring GRUB 2.)

9.2 Resource accounting

Organizing processes into different cgroups can be used to obtain per-cgroup resource consumption data.

The accounting has relatively small but non-zero overhead, whose impact depends on the workload. Activating accounting for one unit will also implicitly activate it for all units in the same slice, and for all its parent slices, and the units contained in them.

The accounting can be set on a per-unit basis with directives such as MemoryAccounting= or globally for all units in /etc/systemd/system.conf with the directive DefaultMemoryAccounting=. Refer to man systemd.resource-control for the exhaustive list of possible directives.

9.3 Setting resource limits

Note
Note: Implicit resource consumption

Be aware that resource consumption implicitly depends on the environment where your workload executes (for example, 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]

The configured value is applied immediately. Optionally, use the --runtime option, so that the new values do not persist after reboot.

Replace NAME with a systemd service, scope, or slice name.

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

9.4 Preventing fork bombs with TasksMax

systemd supports configuring task count limits both for each individual leaf unit, or aggregated on slices. Upstream systemd ships with defaults that limit the number of tasks in each unit (15% of the kernel global limit, run /usr/sbin/sysctl kernel.pid_max to see the total limit). Each user's slice is limited to 33% of the kernel limit. However, this is different for SLE.

9.4.1 Finding the current default TasksMax values

It became apparent, in practice, that there is not a single default that applies to all use cases. SUSE Linux Enterprise Desktop 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/__25-defaults-SLE.conf contains these lines:

[Manager]
DefaultTasksMax=infinity

/usr/lib/systemd/system/user-.slice.d/25-defaults-SLE.conf contains these lines:

[Slice]
TasksMax=infinity

Use systemctl to verify the DefaultTasksMax value:

> systemctl show --property DefaultTasksMax
DefaultTasksMax=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.

9.4.2 Overriding the DefaultTasksMax value

Change the global DefaultTasksMax value by creating a new override file, /etc/systemd/system.conf.d/90-system-tasksmax.conf, and write the following lines to set a 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 different 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.

> sudo systemctl set-property mariadb.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
           └─50-TasksMax.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 set-property applies the new limit and creates a drop-in file for persistence, /etc/systemd/system/mariadb.service.d/50-TasksMax.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.

9.4.3 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 any user by creating a new file, for example /etc/systemd/system/user-.slice.d/40-user-taskmask.conf. The following example sets a default of 16284:

[Slice]
TasksMax=16284
Note
Note: Numeric prefixes reference

See https://documentation.suse.com/sles/15-SP3/html/SLES-all/cha-systemd.html#sec-boot-systemd-custom-drop-in to learn what numeric prefixes are expected for drop-in files.

Then reload systemd to load the new value, and verify the change:

> sudo systemctl daemon-reload
> 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).

9.5 I/O control with cgroups

This section introduces using the Linux kernel's block I/O controller to prioritize or throttle I/O operations. This leverages the means provided by systemd to configure cgroups, and discusses probable pitfalls when dealing with proportional I/O control.

9.5.1 Prerequisites

The following subsections describe steps that you must take in advance when you design and configure your system, since those aspects cannot be changed during runtime.

9.5.1.1 File system

You should use a cgroup-writeback-aware file system (otherwise writeback charging is not possible). The recommended SLES file systems added support in the following upstream releases:

  • Btrfs (v4.3)

  • Ext4 (v4.3)

  • XFS (v5.3)

As of SUSE Linux Enterprise Desktop 15 SP3, any of the named file systems can be used.

9.5.1.2 Unified cgroup hierarchy

To properly account writeback I/O, it is necessary to have equal I/O and memory controller cgroup hierarchies, and to use the cgroup v2 I/O controller. Together, this means that one has to use the unified cgroup hierarchy. This has to be requested explicitly in SLES by passing a kernel command line option, systemd.unified_cgroup_hierarchy=1.

9.5.1.3 Block I/O scheduler

The throttling policy is implemented higher in the stack, therefore it does not require any additional adjustments. The proportional I/O control policies have two different implementations: the BFQ controller, and the cost-based model. We describe the BFQ controller here. In order to exert its proportional implementation for a particular device, we must make sure that BFQ is the chosen scheduler. Check the current scheduler:

> cat /sys/class/block/sda/queue/scheduler
mq-deadline kyber bfq [none]

Switch the scheduler to BFQ:

 # echo bfq > /sys/class/block/sda/queue/scheduler

You must specify the disk device (not a partition). The optimal way to set this attribute is a udev rule specific to the device (note that SLES ships udev rules that already enable BFQ for rotational disk drives).

9.5.1.4 Cgroup hierarchy layout

Normally, all tasks reside in the root cgroup and they compete against each other. When the tasks are distributed into the cgroup tree the competition occurs between sibling cgroups only. This applies to the proportional I/O control; the throttling hierarchically aggregates throughput of all descendants (see the following diagram).

r
`-  a      IOWeight=100
    `- [c] IOWeight=300
    `-  d  IOWeight=100
`- [b]     IOWeight=200

I/O is originating only from cgroups c and b. Even though c has a higher weight, it will be treated with lower priority because it is level-competing with b.

9.5.2 Configuring control quantities

You can apply the values to (long running) services permanently.

> sudo systemctl set-property fast.service IOWeight=400
> sudo systemctl set-property slow.service IOWeight=50
> sudo systemctl set-property throttled.service IOReadBandwidthMax="/dev/sda 1M"

Alternatively, you can apply I/O control to individual commands, for example:

> sudo systemd-run --scope -p IOWeight=400 high_prioritized_command
> sudo systemd-run --scope -p IOWeight=50 low_prioritized_command
> sudo systemd-run --scope -p IOReadBandwidthMax="/dev/sda 1M" dd if=/dev/sda of=/dev/null bs=1M count=10

9.5.3 I/O control behavior and setting expectations

The following list items describe I/O control behavior, and what you should expect under various conditions.

  • I/O control works best for direct I/O operations (bypassing page cache), the situations where the actual I/O is decoupled from the caller (typically writeback via page cache) may manifest variously. For example, delayed I/O control or even no observed I/O control (consider little bursts or competing workloads that happen to never "meet", submitting I/O at the same time, and saturating the bandwidth). For these reasons, the resulting ratio of I/O throughputs does not strictly follow the ratio of configured weights.

  • systemd performs scaling of configured weights (to adjust for narrower BFQ weight range), hence the resulting throughput ratios also differ.

  • The writeback activity depends on the amount of dirty pages, besides the global sysctl knobs (vm.dirty_background_ratio and vm.dirty_ratio)). Memory limits of individual cgroups come into play when the dirty limits are distributed among cgroups, and this in turn may affect I/O intensity of affected cgroups.

  • Not all storages are equal. The I/O control happens at the I/O scheduler layer, which has ramifications for setups with devices stacked on these that do no actual scheduling. Consider device mapper logical volumes spanning multiple physical devices, MD RAID, or even Btrfs RAID. I/O control over such setups may be challenging.

  • There is no separate setting for proportional I/O control of reads and writes.

  • Proportional I/O control is only one of the policies that can interact with each other (but responsible resource design perhaps avoids that).

  • The I/O device bandwidth is not the only shared resource on the I/O path. Global file system structures are involved, which is relevant when I/O control is meant to guarantee certain bandwidth; it will not, and it may even lead to priority inversion (prioritized cgroup waiting for a transaction of slower cgroup).

  • So far, we have been discussing only explicit I/O of file system data, but swap-in and swap-out can also be controlled. Although if such a need arises, it usually points out to improperly provisioned memory (or memory limits).

9.6 More information