qiskit-documentation/docs/guides/execution-modes.mdx

---
title: Introduction to execution modes
description: An overview of the available execution modes in Qiskit Runtime; sessions, batch, and single jobs.

---
# Introduction to Qiskit Runtime execution modes

There are several ways to run workloads, depending on your needs.

<Admonition type="note">
The `mode` option requires `qiskit-ibm-runtime` 0.24 or later.
</Admonition>

**Job mode**: A single primitive request of the estimator or the sampler made without a context manager. Circuits and inputs are packaged as primitive unified blocs (PUBs) and submitted as an execution task on the quantum computer. To run in job mode, specify `mode=backend` when instantiating a primitive. See [Primitives examples](primitives-examples) for examples.

[**Batch mode**](run-jobs-batch): A multi-job manager for efficiently running an experiment that is comprised of bundles of independent jobs. Use batch mode to submit multiple primitive jobs simultaneously.   To run in batch mode, specify `mode=batch` when instantiating a primitive or run the job in a batch context manager. See [Run jobs in a batch](run-jobs-batch) for examples.

[**Session mode**](sessions): A dedicated window for running a multi-job workload. This allows users to experiment with variational algorithms in a more predictable way and even run multiple experiments simultaneously, taking advantage of parallelism in the stack. Use sessions for iterative workloads or experiments that require dedicated access.  To run in session mode, specify `mode=session` when instantiating a primitive, or run the job in a session context manager. Learn more about sessions in the [Introduction to sessions,](/guides/sessions) and see [Run jobs in a session](run-jobs-session) for examples.

<span id="sessions-versus-batch-usage" />

## Choose batch or sessions mode

Generally, you can use batch mode unless you have workloads that don’t have all inputs ready at the outset.

The differences are summarized in the following table:

|     Mode     |                  Usage                  |                                                                                                        Benefit                                                                                                        |
|------------|---------------------------------------|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| Job mode     | Quantum computation only.               | Easiest to use when running a small experiment. Might run sooner than batch mode.                                                                                                                                     |
| Batch mode   | Quantum computation only.               | The entire batch of jobs is scheduled together and there is no additional queuing time for each. Jobs in a batch usually run close together.                                                                     |
| Session mode | Both classical and quantum computation. | Dedicated and exclusive access to the QPU during the session active window, and no other users’ or QPU jobs can run. This is particularly useful for workloads that don’t have all inputs ready at the outset.  |

<span id="usage"></span>
## Batch and session usage calculation

The _usage_ reported on the dashboard or by using the API is the time a QPU is locked for your workload. Failed or canceled jobs count toward your usage in certain circumstances - see the [Failed and canceled jobs](#failed-job) section for details.

* Single job usage is the quantum time they use in processing.
* Batch usage is the amount of time all jobs spend on the QPU.
* Session usage is the time from when the first job starts until the session goes inactive, is closed, or when its last job completes, whichever happens **last**.

To find the usage time for your batch or session, in `qiskit-ibm-runtime` 0.30 or later, run `batch.usage()` or `session.usage()`. If using an older version of `qiskit-ibm-runtime` (>= 0.23 and < 0.30), the usage can be still be found in `session.details()["usage_time"]` and `batch.details()["usage_time"]`.

If you are calling the REST API directly, the usage time is the `elapsed_time` value returned by the `GET /sessions/{id}` endpoint, for both batch and session workloads.

![This image shows multiple sets of jobs.  One set is being run in batch mode and the other is being run in session mode.  For session mode, between each job is the interactive TTL (time to live).  The active window starts when the first job starts and ends after the last job is completed. After the final job of the first set of jobs completes, the active window ends and the session is paused (but not closed).  Another set of jobs then starts and jobs continue in a similar manner. The QPU is reserved for your use during the entire session.  For batch mode, the classical computation part of each job happens simultaneously, then all jobs are sent to the QPU.  The QPU is locked for your use from the time the first job reaches the QPU until the last job is done processing.  There is no gap between jobs where the QPU is idle.](/images/guides/execution-modes/SessionVsBatch.svg 'Batch compared to sessions')

<span id="failed-job"></span>
### Failed and canceled jobs

When a job is failed or canceled, the reported usage is as follows:

* Job or batch mode: the reported usage is the time the QPU was locked for executing your workload until the time it failed or was canceled. Therefore, if the failure or cancellation occurred before the lock, the reported usage is zero. Otherwise, the workload's reported usage is the value that Qiskit Runtime returns as `consumed`. Thus, some failed jobs do not appear in your reported usage and others do.

* Session mode: the reported usage is the wall-clock time from the when the first job started executing in the session until the session terminates, regardless of the number of jobs that fail or are canceled.

<span id="best-practices"></span>
## Prepare to use execution modes

To ensure the most efficient use of the execution modes, the following practices are recommended:

- There is a fixed overhead associated with running a job. In general, if each of your jobs uses less than one minute of QPU time, consider combining several into one larger job (this applies to all execution modes). "QPU time" refers to time spent by the QPU complex to process your job.

    A job's QPU time is listed in the **Usage** column on the IBM Quantum Platform [Workloads](https://quantum.ibm.com/workloads) page, or you can  query it by running this command: `job.metrics()["usage"]["quantum_seconds"]` in `qiskit-ibm-runtime`.

- If each of your jobs consumes more than one minute of QPU time, or if combining jobs is not practical, you can still run multiple jobs in parallel. Every job goes through both classical and quantum processing. While a QPU can process only one job at a time, up to five classical jobs can be processed in parallel. You can take advantage of this by submitting multiple jobs in [batch](run-jobs-batch#partition) or [session](run-jobs-session#two-vqe) execution mode.

The above are general guidelines, and you should tune your workload to find the optimal ratio, especially when using sessions. For example, if you are using a session to get exclusive access to a backend, consider breaking up large jobs into smaller ones and running them in parallel. This might be more cost-effective because it can reduce wall clock time.

## Next steps

<Admonition type="tip" title="Recommendations">
    - [Run a job in batch mode.](/guides/run-jobs-batch/)
    - Read the [introduction to sessions.](/guides/sessions)
    - [Run a job in session mode.](/guides/run-jobs-session)
</Admonition>