Breakdown of Service Time¶
The service time can be broken into:
- Any time required by the worker before and after QPU access
- Wait time in queues before and after QPU access
- QPU access time
- Postprocessing (PP) time
Service time is defined as the difference between time-in and time-out for each QMI, as shown in the table.
|Keyword in SAPI||Meaning|
|time_received||When QMI arrives|
|time_solved||When bundled samples are available|
As mentioned in the introduction, service time for a single QMI depends on the system load; that is, how many other QMIs are present at a given time. During periods of heavy load, wait time in the two queues may contribute to increased service times. D-Wave has no control over system load under normal operating conditions. Therefore, it is not possible to guarantee that service time targets can be met. Service time measurements described in other D-Wave documents are intended only to give a rough idea of the range of experience that might be found under varying conditions.
Postprocessing optimization and sampling algorithms provide local improvements with minimal overhead to solutions obtained from the quantum processing unit (QPU).
Ocean software provides postprocessing tools, and you can optionally run postprocessing online on D-Wave 2000Q and earlier systems.
As shown in Fig. 96, online postprocessing (red) works in parallel with sampling (blue), so that the computation times overlap except for postprocessing the last batch of samples. In this diagram, the time consumed by gathering small batches of samples are marked by vertical blue lines. Within execution of a QMI, gathering the current set of samples takes place concurrently with postprocessing for the previous set of samples (red boxes), which is applied to batches of samples as they are returned by the QPU. As illustrated by Fig. 96, only the time for postprocessing the last set of samples (the rightmost red box) is not overlapped with sampling.
Postprocessing overhead is designed not to impose any delay to QPU access for the next QMI, because postprocessing of the last batch takes place concurrently with the next QMI’s programming time.
The system returns two associated timing values, as shown in the table below. Referring to Fig. 96, total_post_processing_time is the sum of all times in the red boxes, while post_processing_overhead is the extra time needed (a single red box) to process the last batch. This latter time together with qpu_access_time contributes to overall service time.
Even if no postprocessing is run on a QMI, the returned post_processing_overhead value is non-zero. This is because computing the final energies occurs after samples are returned and is accounted as postprocessing overhead.
|Keyword in SAPI||Meaning|
|total_post_processing_time||Total time for postprocessing|
|post_processing_overhead_time||Added for the last batch|
For more details about postprocessing and how it is handled in the timing structure, see Postprocessing Methods on D-Wave Systems.
“Total Time” Reported in Statistics (for Administrators)¶
One timing parameter, qpu_access_time, provides the raw data for the “Total Time” values reported as system statistics, available to administrators. Reported statistics are the sum of the qpu_access_time values for each QMI selected by the users, solvers, and time periods selected in the filter.
Reported statistics are in milliseconds, while SAPI inputs and ouputs are in microseconds. One millisecond is 1000 microseconds.