A use case for a manual GC invocation?

Question

I\'ve read why is it bad practice to call System.gc(), and many others, e.g. this one describing a really disastrous misuse of System.gc(). However, there are c

Answer 1

However, there are cases when the GC takes too long and avoiding long pauses

You have to distinguish between pauses caused by young-only, mixed/concurrent phase and full GCs.

In most cases it's the full GCs that you want to avoid while the other ones are acceptable, which can often be achieved with some GC-tuning and optimizing the code to avoid large allocation bursts.

In principle G1 should be able to run forever on young/mixed cycles and a full GC could be considered a soft failure. CMS can at least do so for many days with careful tuning, but may eventually succumb to fragmentation and require a full GC for compacting.

In cases where even the young GC pauses are not acceptable or garbage piles up too fast for the concurrent phase to handle with acceptable pause times then the strategy you outline may be a viable workaround.

Note that there also other use-cases for manually triggering GCs, such as GC-managed native resources, e.g. direct byte buffers, although those are fairly troublesome in general.

Also note that not all System.gc() calls are created equal, there is the ExplicitGCInvokesConcurrent option too.