Ramping-arrival-rate preallocated vs vumax usage

Hey folks. I’ve been fighting with myself for awhile about a philosophical technical question. when using ramping arrival rate I have typically been setting the preallocated vu param to 0 then setting my theoretical maxvu to whatever value I think is needed for the VU capacity to maintain throughput. well I read somewhere that preallocated should be used for better vu utilization. don’t ask where I don’t remember. but this seemed odd to me. so I asked one of favorite GenAI tools the following. It of course agrees with my current usage of preallocated vs vumax Image may be NSFW.
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what say ye community what is “generally” the best way to manage vu usage with this executory?

Given the scenario you’ve described - driving thousands of transactions per second for 30 minutes or more - let’s analyze the two approaches for applying VUs (Virtual Users) in k6:

1. Preallocating maximum VUs:

   preAllocatedVUs: v2_authorize_code_106_gate_rush_vumax,
   maxVUs: v2_authorize_code_106_gate_rush_vumax,
   

2. Setting maxVUs to maximum capacity and preAllocatedVUs to 0:

   preAllocatedVUs: 0,
   maxVUs: v2_authorize_code_106_gate_rush_vumax,
   

For your high-throughput, extended duration scenario, the second approach (setting maxVUs and preAllocatedVUs to 0) is generally the better choice. Here’s why:

1. Resource Efficiency:
   By setting preAllocatedVUs to 0, k6 will dynamically allocate VUs as needed. This is more resource-efficient, especially at the start of the test when the rate is low (your startRate is 0).

2. Scalability:
   The ramping-arrival-rate executor you’re using will automatically scale up the number of VUs as needed to meet the target rate. Setting preAllocatedVUs to 0 allows this scaling to happen smoothly from the beginning.

3. Warm-up Period:
   With preAllocatedVUs: 0, you allow for a natural warm-up period as VUs are added gradually. This can be beneficial for some systems under test, preventing an initial spike of load.

4. Memory Management:
   Preallocating a large number of VUs can consume a significant amount of memory right from the start. By starting with 0 preallocated VUs, you allow k6 to manage memory more efficiently, only using what’s necessary as the test progresses.

5. Flexibility:
   This setup provides more flexibility if you need to adjust your test parameters. It’s easier to increase maxVUs if needed without worrying about the initial allocation.

However, there are a couple of points to consider:

1. VU Initialization Time:
   There might be a slight overhead as new VUs are initialized during the test. In most cases, this is negligible, especially for a long-running test like yours.

2. Consistent High Load:
   If you know you’ll quickly ramp up to maximum capacity and stay there, preallocating VUs could provide a marginal performance benefit. But for a ramping scenario like yours, this benefit is likely outweighed by the advantages of dynamic allocation.

Given your specific scenario with a ramping arrival rate starting from 0, running for an extended period, and potentially reaching thousands of transactions per second, the approach of setting preAllocatedVUs: 0 and maxVUs: v2_authorize_code_106_gate_rush_vumax is indeed the better choice. It provides a good balance of resource efficiency, scalability, and flexibility for your high-throughput, extended-duration load test.

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