CODEWITHSUNDEEP
javagarbage-collectionjvmnewrelic
San Lee
San Lee

Reputation: 59

Why GC happens even there is lots of unused memory left

enter image description here (Committed and Max lines are the same)

I am looking at the memory usage for a Java application in newrelic. Here are several questions:

# 1

The committed PS Survivor Space Heap varied in past few days. But should it be a constant since it is configured by JVM?

# 2

From what I am understanding, the heap memory should decrease when there is a garbage collection. The memory of Eden could decrease when a major gc or a minor gc happens, while the memory of Old could decrease when a major gc happens.

But if you look at Old memory usage, some time between June 6th and 7th, the memory went up and then later it went down. This should represent that a major gc happend, right? However, there was still lots of unused memory left. It didn't seem it almost reach the limit. Then how did the major gc be triggered? Same for Eden memory usage, it never reached the limit but it still decreased.

The application fetches a file from other places. This file could be large and be processed in memory. Could this explain the issue above?

Upvotes: 1

Views: 1560

Answers (1)

Speakjava
Speakjava

Reputation: 3400

You need to provide more information about your configuration to answer this definitively, I will assume you are using the Hotspot JVM from Oracle and that you are using the G1 collector. Posting the flags you start the JVM with would also be useful.

  1. The key term here is 'committed'. This is memory reserved by the JVM, but not necessarily in use (or even mapped to physical pages, it's just a range of virtual memory that can be used by the JVM). There's a good description of this in the MemoryUsage class of the java.lang.management package (check the API docs). It says, "committed represents the amount of memory (in bytes) that is guaranteed to be available for use by the Java virtual machine. The amount of committed memory may change over time (increase or decrease). The Java virtual machine may release memory to the system..." This is why you see it change.

  2. Assuming you are using G1 then the collector performs incremental compaction. You are correct that if the collector could not keep up with allocation in the old gen and it was getting low on space it would perform a full compacting collection. This is not happening here as the last graph shows you are using nowhere near the allocated heap space. However, to avoid this G1 will collect and compact concurrently with your application. This is why you see usage go up (as you application instantiates more objects) and then go down (as the G1 collector reclaims space from no longer required objects). For a more detailed explanation of how G1 works there is a good read in the documentation, https://docs.oracle.com/javase/8/docs/technotes/guides/vm/gctuning/g1_gc.html.

Upvotes: 3

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