Visual Studio Cloud Load Test Average Test Time Seems Long

Question

I have a WebAPI service that I put together to test throughput hosted in Azure. I have it set up to call Task.Delay with a configurable number (IE webservice/api/endpoint?delay=500). When I run against the endpoint via Fiddler, everything works as expected, delays, etc.

I created a Load Test using VS Enterprise and used some of my free cloud load testing minutes to slam it with 500 concurrent users over 2 minutes. After multiple runs of the load test, it says the average test time is roughly 1.64 seconds. I have turned off think times for the test.

When I run my request in Fiddler concurrently with the Load test, I am seeing sub-second responses, even when spamming the execute button. My load test is doing effectively the same thing and getting 1.64 second response times.

What am I missing?

Code running in my unit test (which is then called for my load test):

var client = new HttpClient { BaseAddress = new Uri(CloudServiceUrl) };

var response = client.GetAsync($"{AuthAsyncTestUri}&bankSimTime={bankDelay}&databaseSimTime={databaseDelay}");

AuthAsyncTestUri is the endpoint for my cloud-hosted service.

Answer 1

There are several delay(), sleep(), pause(), etc methods available to a process. These methods cause the thread (or possible the program or process for some of them) to pause execution. Calling them from code used in a load test is not recommended, see the bottom of page 187 of the Visual Studio Performance Testing Quick Reference Guide (Version 3.6).

Visual Studio load tests do not have one thread per virtual user. Each operating system thread runs many virtual users. On a four-core computer I have seen a load test using four threads for the virtual users.

Suppose a load test is running on a four-core computer and Visual Studio starts four threads to execute the test cases. Suppose one virtual user calls sleep() or similar. That will suspend that thread, leaving three threads available to execute other virtual user activity. Suppose that four virtual users call sleep() or similar at approximately the same time. That will stop all four threads and no virtual users will be able to execute.

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Responding to the following comment that was added to the question

I did try running it with a 5 user load, and saw average test times of less than 500 ms, which match what I see in my Fiddler requests. I'm still trying to figure out why the time goes up dramatically for the 500 user test while staying the same for Fiddler requests run in the middle of the 500 user test.

I think that this comment highlights the problem. At a low user load, the Visual Studio load test and the Fiddler test give similar times. At higher loads something between the load test and the server is limiting throughput and causing the slowdown. It would be worth examining the network route between the computer running the tests and the system being tested. Are there any slow segments on that path? Are there any segments that might see the load test as a denial of service attack and hence might slow down the traffic?

Running a test for as little as 2 minutes does not really show how the test runs. The details in the question do net tell how many tests started, how many finished and how many were abandoned at the end of the two minute run. It is possible that many tests cases were abandoned and that the average time of those that completed was 1.6 second.

If you have the results of the problem run then look at the "details" section of the results. Expand the slider below the image to include the whole run. Tick the option (top left corner) to highlight failing tests. I would expect to see a lot of red at the two minute mark for failing tests. However, the two minute run may be too short compared to the sampling interval (in the run settings) to see much.

Running a first test at 500 users tells you very little. It tells you either that the system copes with that load or that it does not. You need to run the test at several different user loads. Then you start to learn where the boundary between working and not working lies. Hence I recommend using a stepped load.

I believe you need at least one more test run to understand what is happening. I suggest doing a run as follows. Set a one minute cool-down period. Set a stepped load: start at 5 users as you know that that works. Increment by 1 user every two seconds until 100 users. That will take 190 seconds. Run for about another minute at that 100 user load. Total of 4 minutes 10 seconds. Call it 4 minutes. Adding in the one minute cool down makes (5 minutes) x (100 VU) = 500 VUM, which is a small portion of the free minutes per month. After the run look at the graphs of average test times. If all is OK on that test then you could try another that ramps up more quickly to say 500 users.