MiddlewareBox: Types of GC , analyze GC logging and REDHAT JVM Tool.

Tuesday 28 May 2019

1. Serial Garbage Collector.
2. Parallel Garbage Collector.
3. CMS Garbage Collector.
4. G1 Garbage Collector.

1. Serial Garbage Collector (-XX:+UseSerialGC):-

The serial GC works on a single thread to perform Garbage Collection.
It is best for single core processor machine.
While performing GC, it's stop/freezes all the threads of application(Stop The World).
The (-XX:+UseSerialGC) argument have to be used on JVM configuration, to enable Serial Garbage Collector.

2. Parallel Garbage Collector (-XX:+UseParallelGC):-

The Parallel GC works on multiple threads to perform Garbage Collection also known as the throughput collector.
Similar to serial, it also freezes all the threads of application(Stop The World).
The Parallel collector uses multiple CPU to perform Garbage Collection.
This is default GC collector on most of JVM machines.
Parallel GC can be enabled using (-XX:+UseParallelGC) argument on JVM.
Only minor collections are executed in parallel using (-XX:+UseParallelGC) argument.
For minor and major collections in parallel GC, we have to put (-XX:+UseParallelOldGC) argument on JVM.
The number of GC threads can be controlled with the command line option (-XX: ParallelGCThreads=<N>) argument on JVM.

3. CMS Garbage Collector (-XX:+UseConcMarkSweepGC):-

The CMS stands for Concurrent Mark Sweep (CMS) Garbage Collector.
The CMS uses multiple threads to perform Garbage Collection.
Here the Garbage Collection is done concurrently with the application threads. hence it reduces the pause time.
This garbage collector is entered stop the world mode only in two cases,
1. During marking the referenced objects in the old generation space.
2. Any change in heap memory in parallel with doing the garbage collection.
So, (Stop The World) STW time of CMS garbage collector is very short.

4. G1 Garbage Collector (–XX:+UseG1GC):-

It is the new GC, experimental start on JDK 6update 14 then it is being introduced and supported in JDK 7 update 4.
The G1 uses multiple threads to scan the heap and divides it into the regions.
The heap is split into approximately 2000 regions.
The regions are spanning from minimum size 1MB to maximum size 32MB. (-XX:G1HeapRegionSize).
The G1 collector is a parallel, concurrent and compacting low-pause garbage collector.
To enable the G1 collector use (-XX:+UseG1GC) argument on JVM.

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Logging in Garbage Collection:

Below is the listed flags, which enable the logging.

-XX:+PrintGC (or the alias -verbose:gc) Enable the simple logging mode which prints the logs on every minor or major GC.
-XX:+PrintGCDateStamps would print the absolute timestamp in the log statement.
-XX:+PrintGCDetails property would print the details of how much memory is reclaimed in each generation.
-XX:+PrintGCTimeStamps would Print timestamps at garbage collection.
-Xloggc:<filename> makes Log GC verbose output to the specified file.