Why were GC pauses so long? What caused it?

Long GC pauses in Java are usually related to the garbage collector stopping the application (stop-the-world) to clean up memory. Main reasons:

• Large volume of live objects that need to be scanned and moved.
• Use of old or inefficient garbage collection algorithms (e.g., Serial GC for large heaps).
• Memory fragmentation, which causes the collector to spend a lot of time on compaction.
• Frequent full GCs, which require stopping all threads.

To reduce pauses, modern GCs (G1, ZGC, Shenandoah) are used, which operate in parallel and incrementally, reducing application stop time.