A fixed number of iterations are started in a specified period of time. This executor will continue to start iterations at the given rate as long as there are VUs available to run them. Since iteration execution time can vary because of test logic or the system-under-test responding more slowly, this executor will try to compensate by running a variable number of VUs—including potentially initializing more in the middle of the test—to meet the configured iteration rate. This approach is useful for a more accurate representation of RPS, for example.
Iteration starts are spaced fractionally
Iterations do not start at exactly the same time. At a rate of 10 with a timeUnit of 1s, each iteration starts about every tenth of a second (that is, each 100ms).
Besides the common configuration options, this executor has the following options:
|duration(required)||string||Total scenario duration (excluding gracefulStop).||-|
|rate(required)||integer||Number of iterations to start during each timeUnit period.||-|
|preAllocatedVUs(required)||integer||Number of VUs to pre-allocate before test start to preserve runtime resources.||-|
|timeUnit||string||Period of time to apply the rate value.||"1s"|
|maxVUs||integer||Maximum number of VUs to allow during the test run.||If unset, same as preAllocatedVUs|
When you want to maintain a constant number of iterations without being affected by the performance of the system under test.
Don't put sleep at the end of an iteration
The arrival-rate executors already pace the iteration rate through the rate and timeUnit properties. So it's unnecessary to use a sleep() function at the end of the VU code.
In this example, we'll start a constant rate of 30 iterations per second for 30 seconds, allowing k6 to dynamically schedule up to 50 VUs.
The following graph depicts the performance of the example script:
Based upon our test scenario inputs and results:
- The desired rate of 30 iterations started every 1 second is achieved and maintained for the majority of the test.
- The test scenario runs for the specified 30 second duration.
- Having started with 2 VUs (as specified by the preAllocatedVUs option), k6 automatically adjusts the number of VUs to achieve the desired rate, up to the maxVUs; for our test, this ended up as 17 VUs.
- Exactly 900 iterations are started in total, 30s * 30 iters/s.
As in our example, using too low of a preAllocatedVUs setting will reduce the test duration at the desired rate, as resources need to continually be allocated to achieve the rate.