3.5. Schedule Hosts and Cells
The Compute scheduling service determines on which cell or host (or host aggregate), an instance will be placed. As an administrator, you can influence where the scheduler will place an instance. For example, you might want to limit scheduling to hosts in a certain group or with the right RAM.
You can configure the following components:
- Filters - Determine the initial set of hosts on which an instance might be placed (see Section 3.5.1, “Configure Scheduling Filters”).
- Weights - When filtering is complete, the resulting set of hosts are prioritized using the weighting system. The highest weight has the highest priority (see Section 3.5.2, “Configure Scheduling Weights”).
- Scheduler service - There are a number of configuration options in the
/etc/nova/nova.conffile (on the scheduler host), which determine how the scheduler executes its tasks, and handles weights and filters. There is both a host and a cell scheduler. For a list of these options, refer to the "Configuration Reference" (RHEL OpenStack Platform Documentation).
In the following diagram, both host 1 and 3 are eligible after filtering. Host 1 has the highest weight and therefore has the highest priority for scheduling.
Figure 3.3. Scheduling Hosts
3.5.1. Configure Scheduling Filters
You define which filters you would like the scheduler to use in the
scheduler_default_filters option (/etc/nova/nova.conf file; you must have either root or nova user permissions). Filters can be added or removed.
By default, the following filters are configured to run in the scheduler:
scheduler_default_filters=RetryFilter,AvailabilityZoneFilter,RamFilter,ComputeFilter,ComputeCapabilitiesFilter,ImagePropertiesFilter,ServerGroupAntiAffinityFilter,ServerGroupAffinityFilter
Some filters use information in parameters passed to the instance in:
- The nova boot command, see the "Command-Line Interface Reference" in https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux_OpenStack_Platform/.
- The instance's flavor (see Section 3.3.4, “Update Flavor Metadata”)
- The instance's image (see Appendix A, Image Configuration Parameters).
All available filters are listed in the following table.
Table 3.4. Scheduling Filters
| Filter | Description |
|---|---|
| AggregateCoreFilter |
Uses the host-aggregate metadata key
cpu_allocation_ratio to filter out hosts exceeding the over-commit ratio (virtual CPU to physical CPU allocation ratio); only valid if a host aggregate is specified for the instance.
If this ratio is not set, the filter uses the
cpu_allocation_ratio value in the /etc/nova/nova.conf file. The default value is '16.0' (16 virtual CPU can be allocated per physical CPU).
|
| AggregateDiskFilter |
Uses the host-aggregate metadata key
disk_allocation_ratio to filter out hosts exceeding the over-commit ratio (virtual disk to physical disk allocation ratio); only valid if a host aggregate is specified for the instance.
If this ratio is not set, the filter uses the
disk_allocation_ratio value in the /etc/nova/nova.conf file. The default value is '1.0' (one virtual disk can be allocated for each physical disk).
|
| AggregateImagePropertiesIsolation | Only passes hosts in host aggregates whose metadata matches the instance's image metadata; only valid if a host aggregate is specified for the instance. For more information, see Section 4.1.1, “Create an Image”. |
| AggregateInstanceExtraSpecsFilter | Metadata in the host aggregate must match the host's flavor metadata. For more information, see Section 3.3.4, “Update Flavor Metadata”. |
| AggregateMultiTenancyIsolation |
A host with the specified filter_tenant_id can only contain instances from that tenant (project). Note: The tenant can still place instances on other hosts.
|
| AggregateRamFilter |
Uses the host-aggregate metadata key
ram_allocation_ratio to filter out hosts exceeding the over commit ratio (virtual RAM to physical RAM allocation ratio); only valid if a host aggregate is specified for the instance.
If this ratio is not set, the filter uses the
ram_allocation_ratio value in the /etc/nova/nova.conf file. The default value is '1.5' (1.5 RAM can be allocated for each physical RAM).
|
| AllHostsFilter | Passes all available hosts (however, does not disable other filters). |
| AvailabilityZoneFilter | Filters using the instance's specified availability zone. |
| ComputeCapabilitiesFilter | Ensures Compute metadata is read correctly. Anything before the ':' is read as a namespace. For example, 'quota:cpu_period' uses 'quota' as the namespace and 'cpu_period' as the key. |
| ComputeFilter | Passes only hosts that are operational and enabled. |
| CoreFilter |
Uses the cpu_allocation_ratio in the /etc/nova/nova.conf file to filter out hosts exceeding the over commit ratio(virtual CPU to physical CPU allocation ratio). The default value is '16.0' (16 virtual CPU can be allocated per physical CPU).
|
| DifferentHostFilter | Enables an instance to build on a host that is different from one or more specified hosts. Specify 'different' hosts using the nova boot option --different_host option. |
| DiskFilter |
Uses disk_allocation_ratio in the /etc/nova/nova.conf file to filter out hosts exceeding the over commit ratio(virtual disk to physical disk allocation ratio). The default value is '1.0' (one virtual disk can be allocated for each physical disk).
|
| ImagePropertiesFilter | Only passes hosts that match the instance's image properties. For more information, see Section 4.1.1, “Create an Image”. |
| IsolatedHostsFilter |
Passes only isolated hosts running isolated images that are specified in the /etc/nova/nova.conf file using isolated_hosts and isolated_images (comma-separated values).
|
| JsonFilter |
Recognises and uses an instance's custom JSON filters:
The filter is specfied as a query hint in the nova boot command. For example:
--hint query='['>=', '$free_disk_mb', 200 * 1024]'
|
| MetricFilter | Filters out hosts with unavailable metrics. |
| NUMATopologyFilter | Filters out hosts based on its NUMA topology; if the instance has no topology defined, any host can be used. The filter tries to match the exact NUMA topology of the instance to those of the host (it does not attempt to pack the instance onto the host). The filter also looks at the standard over-subscription limits for each NUMA node, and provides limits to the compute host accordingly. |
| RamFilter |
Uses ram_allocation_ratio in the /etc/nova/nova.conf file to filter out hosts exceeding the over commit ratio (virtual RAM to physical RAM allocation ratio). The default value is '1.5' (1.5 RAM can be allocated for each physical RAM).
|
| RetryFilter |
Filters out hosts that have failed a scheduling attempt; valid if scheduler_max_attempts is greater than zero (by default,scheduler_max_attempts=3).
|
| SameHostFilter |
Passes one or more specified hosts; specify hosts for the instance using the
--hint same_host option for nova boot.
|
| ServerGroupAffinityFilter |
Only passes hosts for a specific server group:
|
| ServerGroupAntiAffinityFilter |
Only passes hosts in a server group that do not already host an instance:
|
| SimpleCIDRAffinityFilter |
Only passes hosts on the specified IP subnet range specified by the instance's cidr and build_new_host_ip hints. Example:
--hint build_near_host_ip=192.0.2.0 --hint cidr=/24
|
3.5.2. Configure Scheduling Weights
Both cells and hosts can be weighted for scheduling; the host or cell with the largest weight (after filtering) is selected. All weighers are given a multiplier that is applied after normalising the node's weight. A node's weight is calculated as:
w1_multiplier * norm(w1) + w2_multiplier * norm(w2) + ...
You can configure weight options in the scheduler host's
/etc/nova/nova.conf file (must have either root or nova user permissions).
3.5.2.1. Configure Weight Options for Hosts
You can define the host weighers you would like the scheduler to use in the
[DEFAULT] scheduler_weight_classes option. Valid weighers are:
nova.scheduler.weights.ram- Weighs the host's available RAM.nova.scheduler.weights.metrics- Weighs the host's metrics.nova.scheduler.weights.all_weighers- Uses all host weighers (default).
Table 3.5. Host Weight Options
| Weigher | Option | Description |
|---|---|---|
| All | [DEFAULT] scheduler_host_subset_size | Defines the subset size from which a host is selected (integer); must be at least 1. A value of 1 selects the first host returned by the weighing functions. Any value less than 1 is ignored and 1 is used instead (integer value). |
| metrics | [metrics] required |
Specifies how to handle metrics in [metrics] weight_setting that are unavailable:
|
| metrics | [metrics] weight_of_unavailable |
Used as the weight if any metric in [metrics] weight_setting is unavailable; valid if [metrics]required=False.
|
| metrics | [metrics] weight_multiplier |
Mulitplier used for weighing metrics. By default, weight_multiplier=1.0 and spreads instances across possible hosts. If this value is negative, the host with lower metrics is prioritized, and instances are stacked in hosts.
|
| metrics | [metrics] weight_setting |
Specifies metrics and the ratio with which they are weighed; use a comma-separated list of 'metric=ratio' pairs. Valid metric names are:
Example:
weight_setting=cpu.user.time=1.0
|
| ram | [DEFAULT] ram_weight_multiplier |
Multiplier for RAM (floating point). By default, ram_weight_multiplier=1.0 and spreads instances across possible hosts. If this value is negative, the host with less RAM is prioritized, and instances are stacked in hosts.
|
3.5.2.2. Configure Weight Options for Cells
You define which cell weighers you would like the scheduler to use in the
[cells] scheduler_weight_classes option (/etc/nova/nova.conf file; you must have either root or nova user permissions)
Valid weighers are:
- nova.cells.weights.all_weighers - Uses all cell weighers(default).
- nova.cells.weights.mute_child - Weighs whether a child cell has not sent capacity or capability updates for some time.
- nova.cells.weights.ram_by_instance_type - Weighs the cell's available RAM.
- nova.cells.weights.weight_offset - Evaluates a cell's weight offset. Note: A cell's weight offset is specified using --woffset in the nova-manage cell create command.
Table 3.6. Cell Weight Options
| Weighers | Option | Description |
|---|---|---|
| mute_child | [cells] mute_weight_multiplier | Multiplier for hosts which have been silent for some time (negative floating point). By default, this value is '-10.0'. |
| mute_child | [cells] mute_weight_value | Weight value given to silent hosts (positive floating point). By default, this value is '1000.0'. |
| ram_by_instance_type | [cells] ram_weight_multiplier | Multiplier for weighing RAM (floating point). By default, this value is '1.0', and spreads instances across possible cells. If this value is negative, the cell with fewer RAM is prioritized, and instances are stacked in cells. |
| weight_offset | [cells] offset_weight_multiplier |
Multiplier for weighing cells (floating point). Enables the instance to specify a preferred cell (floating point) by setting its weight offset to 999999999999999 (highest weight is prioritized). By default, this value is '1.0'.
|