Ramp-up is simply the time between when the first user accesses your application and when the target load of your test is reached. See the graphs below.
Since we are interested in the performance of the system only during the time between the start and end points, no performance data from outside of that time will be collected and since a user obviously can?t start in the middle of their activity on the site, all of the green users need to be included. The ramp-up time becomes the time between the beginning of the first user activity and the start of the model
, and the ramp-down time becomes the time between the end of the model and when the last user completes their activity. See the chart below.
The total ramp-up time can also be interpreted as "a few seconds shorter than the time it takes your longest script to run to completion.", which we can determine by running our script individually and noting the run time.
Ramp-down is actually quite a simple concept. Because the actual time when a user started was staggered over time, they will also complete their task and different, staggered, times. This accomplishes ramp-down.
"Stepping up" users periodically during the test may yield interesting information for analysis, but is not ramping, and will yield unrealistic results.