When to Garbage Collect

Question

I have a piece of code that load a very big image in memory. So it seemed like a reasonable thing to call

System.gc();

before loading the

Answer 1

Typically, you should not interfere with the garbage collector. If it's necessary to free some memory before loading the image, then the garbage collector will do it for you.

Regardless, if you're only doing it once, it's probably not going to affect your performance drastically. Things done in loops are far more important.

Answer 2

Generally, no. It isn't appropriate to call System.gc(). However, I have had a few cases where it made sense.

In the software I write, there is a built-in performance tracking layer. It is mostly used during automated testing, but can be used in the field for diagnostic purposes. Between tests or after specific runs we will call System.gc a few times and then record the memory still present. It provides us with a basic memory foot print benchmark for watching memory consumption trend lines over time. While this can be done with some of the external JVM interfaces, it was easier to do it in place and exact numbers were not required.

On a much older system, we could have upwards of 72 separate JVMs (yeah, 72, there was a good reason for it at the time of construction). In that system, leaving the heap to free float on all 72 JVMs could yield some excessive (well beyond physical memory) total memory consumption. System.gc() was called between heavy data exercises to try and keep the JVM closer to average to keep the heap from growing (capping the heap could have been another direction, but then it would have required the implementers to configure more per site and be more aware of what was happening under the hood to get it right and not have the system fail under load).