Low Overhead Allocation Sampling with VMProf in PyPy's GC

There are many time-based statistical profilers around (like VMProf or py-spy just to name a few). They allow the user to pick a trade-off between profiling precision and runtime overhead. On the other hand there are memory profilers such as memray. They can be handy for finding leaks or for discovering functions that allocate a lot of memory. Memory profilers typlically save every single allocation a program does. This results in precise profiling, but larger overhead. In this post we describe our experimental approach to low overhead statistical memory profiling. Instead of saving every single allocation a program does, it only saves every nth allocated byte. We have tightly integrated VMProf and the PyPy Garbage Collector to achieve this. The main technical insight is that the check whether an allocation should be sampled can be made free. This is done by folding it into the bump pointer allocator check that the PyPy’s GC uses to find out if it should start a minor collection. In ... Read full article.