Nova service concurrency

For a long time nova services relied almost exclusively on the Eventlet library for processing multiple API requests, RPC requests and other tasks that needed concurrency. Since Eventlet is not expected to support the next major cPython version the OpenStack TC set a goal to replace Eventlet and therefore Nova has started transitioning its concurrency model to native threads. During this transition Nova maintains the Eventlet based concurrency mode while building up support for the native threading mode.

Note

The native threading mode is not ready yet. Do not use it in production.

Selecting concurrency mode for a service

Nova still uses Eventlet by default, but allows switching services to native threading mode at service startup via setting the environment variable OS_NOVA_DISABLE_EVENTLET_PATCHING=true.

Note

Since nova 32.0.0 (2025.2 Flamingo) the nova-scheduler can be switched to native threading mode.

Tunables for the native threading mode

As native threads are more expensive resources than greenthreads Nova provides a set of configuration options to allow fine tuning the deployment based on load and resource constraints. The default values are selected to support a basic, small deployment without consuming substantially more memory resources, than the legacy Eventlet mode. Increasing the size of the below thread pools means that the given service will consume more memory but will also allow more tasks to be executed concurrently.

  • cell_worker_thread_pool_size: Used to execute tasks across all the cells within the deployment.

    E.g. To generate the result of the openstack server list CLI command, the nova-api service will use one native thread for each cell to load the nova instances from the related cell database.

    So if the deployment has many cells then the size of this pool probably needs to be increased.

    This option is only relevant for nova-api, nova-metadata, nova-scheduler, and nova-conductor as these are the services doing cross cell operations.

  • executor_thread_pool_size: Used to handle incoming RPC requests. Services with many more inbound requests will need larger pools. For example, a single conductor serves requests from many computes as well as the scheduler. A compute node only serves requests from the API for lifecycle operations and other computes during migrations.

    This option is only relevant for nova-scheduler, nova-conductor, and nova-compute as these are the services acting as RPC servers.

  • default_thread_pool_size: Used by various concurrent tasks in the service that are not categorized into the above pools.

    This option is relevant to every nova service using nova.utils.spawn().

Seeing the usage of the pools

When new work is submitted to any of these pools in both concurrency modes Nova logs the statistics of the pool (work executed, threads available, work queued, etc). This can be useful when fine tuning of the pool size is needed. The parameter thread_pool_statistic_period defines how frequently such logging happens from a specific pool in seconds. A value of 60 seconds means that stats will be logged from a pool maximum once every 60 seconds. The value 0 means that logging happens every time work is submitted to the pool. The default value is -1 meaning that the stats logging is disabled.

Preventing hanging threads

Threads from a pool are not cancellable once they are executing a task, therefore it is important to ensure external dependencies cannot hold up a task execution indefinitely as that will lead to having fewer threads in the pool available for incoming work and therefore reduced overall capacity.

Nova’s RPC interface already uses proper timeout handling to avoid hanging threads. But adding timeout handling to the Nova’s database interface is database server and database client library dependent.

For mysql-server the max_execution_time configuration option can be used to limit the execution time of a database query on the server side. Similar options exist for other database servers.

For the pymysql database client a client side timeout can be implemented by adding the read_timeout connection parameter to the connection string.

We recommend using both in deployments where Nova services are running in native threading mode.