Introduction to Qiskit Runtime execution modes

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

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.

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:

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 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.

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.

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 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.

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.

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