Use separate user home for each version under performance test Until now, each performance test was using a separate test directory, but the different versions under test were all using the same directory.
This meant that therer was a consistent bias depending on the order in which the versions were executed.
Simplify performance measurements The many measurements that we injected into the build under test were skewing our measurements to the point of making them unreliable or unrealistic. We now only measure end to end build time. Here's a breakdown with the rationale for removing each other measurement:
- configuration time: can be done by having a `gradle help` scenario instead - execution time: the user does not care whether a long build is stuck in execution or configuration - setup/teardown: was ill-defined anyway, basically total - configuration - execution - JIT compile time: this is nothing we can influence and thus pointless to measure - Memory usage: Was only measured at one point in the build, which doesn't tell us anything about any problems at any other point in the build - GC CPU time: If this increases we'd see it in total execution time
Generally, looking at the graphs has never pointed us directly at the problem, we always need to profile anyway. So instead of skewing our measurements with lots of profling code, we should instead use a dedicated profiling job to measure if we actually see a regression.
Memory usage can be tested indirectly by giving each scenario a reasonable amount of memory. If memory usage rises above that reasonable limit, we'd see execution time rise, telling us about the regression. Generally, we do not optimize for smallest memory usage, but for fastest execution with reasonable memory overhead.
This change also removes all JVM tweaking and wait periods which we introduced in an attempt to make tests more predictable and stable. These tweaks have not really helped us achieve more stable tests and have often done the opposite. They also add lots of complexity and make our tests more unrealistic. A real users will not add all these JVM options to Gradle.