A quota group sets resource limits on services running inside snaps.
Resource limits can be set for the following attributes:
- Journal log messages: as a maximum size for the log and as a rate limit
- Memory: as a maximum amount or RAM
- CPU usage: as a percentage across a number - or specific - CPU cores, and as a given number of threads
Quota groups are created, removed and queried with the following snap commands:
- set-quota: create or update a quota group
- quota: show quota group for a set of snaps
- quotas: show all quota groups
- remove-quota: remove a quota group
Quota groups require snap version 2.52+ and they’re currently considered experimental. To experiment with quota groups, an experimental feature-flag must first be enabled:
$ sudo snap set system experimental.quota-groups=true
Creating a quota group
- Viewing quota groups
- Nested quota groups
- Updating quota groups
- Group members and resource usage
- Removing snaps and quota groups
Creating a quota group
To create a quota group, use the
snap set-quota command with a group name and a limiting attribute, such as
snap set-quota mygroup --memory=64M
Snaps that contain services can optionally be listed to be included within the quota group.
snap set-quota mygroup --memory=64M snap1 snap2
See below for details on which limiting attributes are supported by quota groups.
Journal log limits
snap logs command is used to retrieve the systemd journal logs for a specific service, or for all services within a snap (see Inspecting logs for further details), a quota can be used to limit the output.
Log output can be limited by both size and rate within a quota group:
1. Journal size
Limit the size of the aggregated journal log for all the snaps in the quota group. This is useful when service log output is verbose on devices with limited storage capacity.
Accepts a value in either kilobytes(KB), megabytes(MB) or gigabytes(GB), including aggregated values:
$ sudo snap set-quota loggroup --journal-size=64MB
2. Journal rate limit
Limit the number of messages logged to a maximum per time period. This is a useful way of limiting the number of messages for snaps.
The time period is can be nanoseconds(us), milliseconds (ms), seconds(s), minutes (m), and hours(h), including aggregated periods:
$ sudo snap set-quota loggroup --journal-rate-limit=10/1h2m3s4ms5us
Memory units can be ‘B’, ‘KB’, 'MB or ‘GB’ with a size greater than 4KB:
$ sudo snap set-quota highmem --memory=2GB
The memory limit for a quota group can only be increased. Increasing the memory limit for a quota group does not restart any services associated with snaps in the quota group.
To decrease the memory limit for a quota group, the entire group must be removed with the remove-quota command and recreated with a lower limit.
CPU limits can be set in 4 different ways, some of which can be combined depending on requirements:
- Percentage of total CPU resources
- Percentage of a given number of CPU cores
- Percentage for specific cores
- Limit the number of threads
Without a specified CPU limit, all quota groups can use the full system capacity.
1. Percentage of total CPU resources
Sets a relative percentage limit on how much execution time a process receives, regardless of the number of cores available, up to 100%.
2. Percentage of a given number of CPU cores
--cpu=<number of cores>x<percentage>%
One or more cores can be specified alongside a CPU percentage to allocate relative CPU resources across only that number of cores.
--cpu=2x100% on a 4 core system would permit a process to run at full capacity on only two cores, which is equivalent to 50% of full system capacity.
3. Percentage for specific cores
cpu-set=<core number 1>,<core number 2> --cpu=<percentage>%
Using an additional
--cpu-set argument with
--cpu , it’s possible to specify exactly which cores to be used as the CPU resources for that quote group.
--cpu-set=0,1 --cpu=100% will permit up to 100% CPU resource usage on 2 specific cores (0 and 1).
The following example will allow up to a maximum of half usage combined on four specific cores which means the sum of usage across all 4 specific cores cannot exceed 200%:
$ snap set-quota max-two-specific-cores --cpu-set=0,1,2,3 --cpu=50%
In the above example, services in the max-two-specific-cores quota group could run using 100% of two cores and 0% of the other two cores, or 25% of each core, 33% of 3 of the cores and 0% of the last core, or any other combination that does not exceed 200%. The number 200% comes from the size of the
--cpu-set set multiplied by the
4. Limit the number of threads
--threads option is used to limit the number of threads or processes that can be created by the snaps within the quota group.
--threads=4096 will limit a quota group to no more than 4096 threads or processes.
The threads option can also be used with both
--cpu-set options to target specific requirements:
$ snap set-quota max-two-specific-cores --cpu-set=0,1,2,3 --cpu=50% --threads=8092
By default, quota groups are not limited in the number of threads they can create.
If the group already exists, set-quota will update its resource limits.
Use the quotas command to view created quota groups:
$ snap quotas Quota Parent Constraints Current highmem memory=2.00GB cpu=50%,cpu-set=0,1 cpuhalf cpu=50%,cpu-set=0,1 manycores cpu=2x,cpu=100%,threads=4096 maxcores cpu=100% memcpu memory=2.00GB,cpu=50% threads threads=4096 loggroup journal-size=64.0MB,journal-rate=10/1h2m3.004005s
Snaps can belong to only one quota group, but quota groups can be nested.
Nested quota groups
To create a nested quota group, add the
--parent=<parent group> argument to the set-quota command when creating a new quota group:
$ sudo snap set-quota lowmem --memory=1GB --parent=highmem
The quotas command will also show nested quota groups:
$ snap quotas Quota Parent Constraints Current highmem memory=2.00GB lowmem highmem memory=1.00GB
The total resource use of nested quota groups cannot exceed that of the parent group.
Updating a quota group
Add the names of one or more snaps to the set-quota command to include those snaps in the quota group when its created:
$ sudo snap set-quota lowmem go-example-webserver --parent=highmem
If the quota group already exists, snaps can be added by using set-quota without the resource limit:
$ sudo snap set-quota lowmem go-example-webserver
Adding new a snap to a quota group will result in all non-disabled services in that snap being restarted.
New quotas can be set on existing quota groups, but existing quotas cannot be removed from a quota group, without removing and recreating the entire group.
The CPU set limit for a quota group can be modified to include new cores, or to remove existing cores from the quota already set.
The threads limit for a quota group can be increased but not decreased. To decrease the threads limit for a quota group, the entire group must be removed with the remove-quota command and recreated with a lower limit.
Group members and resource usage
The quota command shows information about a quota group, including the set of snaps it includes and any subgroups it contains, as well as its resource constraints and the current usage of those constrained resources by snap services in those snaps:
$ snap quota lowmem name: lowmem parent: highmem constraints: memory: 1.00GB current: memory: 5.32MB snaps: - go-example-webserver
Use the quota command on the parent of any nested groups to view total resource usage and subgroups:
$ snap quota highmem name: highmem constraints: memory: 2.00GB current: memory: 5.35MB subgroups: - lowmem
Removing snaps and quota groups
To remove a snap from a quota group, the entire group must be removed with the remove-quota command and the quota group recreated without the snaps:
$ sudo snap remove-quota lowmem $ sudo snap set-quota lowmem --memory=1GB $ snap quota lowmem name: lowmem constraints: memory: 1.00GB current: memory: 0B
Currently, only quota groups with no subgroups can be removed. In order to remove a quota group with subgroups, the subgroups must be first removed until there are no further subgroups in the group, then the group itself can be removed.
An existing subgroup cannot be moved from one parent to another.
To remove a subgroup from a quota group, the subgroup must be first removed directly with the remove-quota command.