Introduction to Qiskit Runtime execution modes

There are several ways to run workloads, depending on your needs. Execution modes determine how your jobs are scheduled, and choosing the right execution mode allows your jobs to run efficiently within your time budget.

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 for examples.

Batch mode : 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 for examples.

Session mode : 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, and see Run jobs in a session for examples.

Note

The mode option requires qiskit-ibm-runtime 0.24 or later.

How long do workloads run?

The amount of time an execution runs depends on the values for maximum timeout (or time to live (TTL)), interactive timeout (or interactive time to live (ITTL)). Additionally, the time your workload actively runs is probably different from the amount of wall clock time a workload takes.

Maximum timeout (TTL) - Determines how long (in quantum time) a session or batch can run. Quantum time is the time spent by the QPU complex to process the workload. This timer starts when the first job in the workload starts being processed and continues running until the value is reached. After this value is reached, the workload is terminated, any jobs that are already running continue running, and any queued jobs that remain in the session or batch are put into a failed state. You can set this value with the max_time parameter for batches or sessions .

Interactive timeout (ITTL) - This value cannot be configured. If no jobs are queued for the active session or batch within that window, the workload is temporarily deactivated. A job submitted to the session or batch reactivates the deactivated workload if the maximum timeout value has not been reached.

For full details about these values, including how to determine the ITTL value, review the Maximum execution time guide.

Basic workflow

The basic workflow for batches and sessions is similar:

The first job in a batch or session enters the normal queue. Note: for batches, the entire batch of jobs is scheduled together.
When the first job starts running, the TTL clock starts.
The ITTL timer starts after each job is completed. If there are no workload jobs ready within the ITTL window, the workload is temporarily deactivated and normal job selection resumes. A job can reactivate the deactivated workload if the batch or session has not reached its TTL value.
Note
$The job must go through the normal queue to reactivate the workload.$
If the TTL value is reached, the workload ends and any remaining queued jobs fail. However, any jobs already running will run to completion.

The following video illustrates the session basic workflow, using sessions as an example:

Choose the right execution mode

Utility-scale workloads can take many hours to complete, so it is important that both the classical and quantum resources are scheduled efficiently to streamline the execution. Execution modes provide flexibility in balancing the cost and time tradeoff to use resources optimally for your workloads. There are several aspects to consider when choosing which execution mode to use, such as usage, overall execution time, ITTL requirement, and time between jobs.

The following table summarizes the benefits of each:

Mode	Benefits
Batch	● The entire batch of jobs is scheduled together and there is no additional queuing time for each. ● The jobs' classical computation, such as compilation, is run in parallel. Thus, running multiple jobs in a batch is significantly faster than running them serially. There is usually minimal delay between jobs, which can help avoid drift. ● If you partition your workload into multiple jobs and run them in batch mode, you can get results from individual jobs, which makes them more flexible to work with.
Session	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.
Job	Easiest to use when running a small experiment. ● Might run sooner than batch mode.

Recommendations and best practices

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

Use batch mode to submit multiple primitive jobs simultaneously to shorten processing time.
Use session mode for iterative workloads, or if you need dedicated access to the QPU (quantum processing unit).
Use job mode to submit a single primitive request for quick testing.

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

$There is a fixed overhead associated with running a job. In general, if each of your jobs uses less than one minute of QPU time, consider combining several into one larger job (this applies to all execution modes). "QPU time" refers to time spent by the QPU complex to process your job.$ $A job's QPU time is listed in the Usage column on the IBM Quantum Platform Workloads page, or you can query it by running this command: job.metrics()["usage"]["quantum_seconds"] in qiskit-ibm-runtime .$
$If each of your jobs consumes more than one minute of QPU time, or if combining jobs is not practical, you can still run multiple jobs in parallel. Every job goes through both classical and quantum processing. While a QPU can process only one job at a time, up to five classical jobs can be processed in parallel. You can take advantage of this by submitting multiple jobs in batch or session execution mode.$
$Do not run a session with a single job in it.$

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.

Usage

Usage is an important consideration when choosing which execution mode to use. It is a measurement of the amount of time the QPU is locked for your workload.

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.
Batch usage is the sum of quantum time of all jobs in the batch.
Single job usage is the quantum time the job uses in processing.

This image shows multiple sets of jobs. One set is being run in session mode and the other is being run in batch 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 on the QPU. There is no gap between jobs where the QPU is idle. — Sessions compared to batch

Usage differences between execution modes

The differences between job, batch, and session mode usage are summarized in the following table:

Mode	Usage
Batch mode	Quantum computation only.
Session mode	Both classical and quantum computation.
Job mode	Quantum computation only.

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 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. — Batch compared to sessions

Usage for failed and canceled jobs

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

$Job or batch mode: the reported usage is the time the QPU was locked for executing your workload until the time it failed or was canceled. Therefore, if the failure or cancellation occurred before the lock, the reported usage is zero. Otherwise, the workload's reported usage is the value that Qiskit Runtime returns as consumed . Thus, some failed jobs do not appear in your reported usage and others do.$
$Session mode: the reported usage is the wall-clock time from the when the first job started executing in the session until the session terminates, regardless of the number of jobs that fail or are canceled.$

Examples

Give some examples - e.g. what should you expect when running an iterative workload using session vs batch vs job

Next steps

Recommendations