[jrockway]

Prior to GOMEMLIMIT being a thing, people would get themselves into this situation occasionally. Their steady state working set size is something like 2.1GB, but they want to fit into a 4GB container limit, so GOGC=100 led to OOMs occasionally. They would bring GOGC down to 10 or something (why not 90!?), and then they are running GC every time they allocated 10% or 200MB, and then their CPU profiles would show they were burning a lot on GC, because it's pretty easy for an RPC server to generate 200MB of garbage.

GOMEMLIMIT definitely improves things, taking the very blunt instrument GOGC out of people's hands, and does what they actually want instead, which is to prioritize GC only when you're about to run out of memory.

People dislike tunables like this, and rightfully so, but a similar problem exists somewhat insidiously in non-garbage collected languages. Calling malloc() and free() is not a zero-cost operation, and you can certainly waste CPU by calling free() when nothing actually needs the memory you are freeing. GC lets cases like "this is a CLI app, the system has 128GB of RAM, the working set size is 2GB, and it's over in 1 second" basically never do any memory freeing. You allocate as much as you need, GC never runs, and the app exits. That kind of optimization is actually pretty neat. But, for steady state servers that run for an unbounded period of time, it is inevitable that you are going to spend time trying to reuse memory that you no longer need, because the system only has a finite amount of memory. Then the compromises start to balance throughput with efficiency. gc + GOMEMLIMIT is a very nice compromise that is excellent for many applications, but no heuristic is perfect for every possible computer program.