Comment by pron

> Is it possible to detect cyclic references perfectly?

Yes. The GCs in Java, .NET, V8, and Go do it.

> If we have 7 and 0.1% of the time you need one of the 6 that is non-default, how do we choose?

Java's GC are optimised for different workloads and environments, and when the choice matters, they're easy to choose among:

1. Parallel GC: Maximal throughput when latency doesn't matter (batch processing).

2. Serial GC: Very small machines.

3. ZGC: low latency (<<1ms maximal pause, i.e. effectively pauseless)

4. G1 (the current default): A balanced mix of throughput and latency.

These are all the standard GCs (the seven you mentioned include a GC similar to Go's that was removed years ago, an "no op" GC for benchmarking hidden behind a development flag, and alternative implementations by different companies to some of the ones above).

It's possible that either Serial or Parallel will be removed when G1 is able to fully replace them.

Now, why do users need options? Because Java runs most of the world's finance, manufacturing, shipping and logistics, telecommunication, travel, healthcare, retail, defence, and government. We're talking large, complex software that handles huge workloads, and the needs vary. What works well enough for a CLI dev tool or a simple website is often not good enough to handle the world's credit card transaction processing or mobile phone networks.

> If not, what about Python's impl makes it so lackluster to any of 7 of Java's?

Java's GCs are moving collectors, which offer advantages not just compared to Python's GC but to all memory management strategies. Memory management (even in C) imposes a CPU/RAM tradeoff. Moving collectors (used in Java, .NET, and V8) give you a knob for controlling the tradeoff, i.e. they're able to convert RAM to CPU (i.e. use RAM chips as a hardware accelerator) and vice-versa.