Java Garbage Collector

Java Garbage Collector

   June 25, 2021       Ravi Yasas

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Garbage Collectors in Java

Cleanup in Java

• GC can process only in the heap area
• GC is running its own thread when it feels memory is running low

Types of GCs

• Serial GC
• Parallel GC
• CMS (Concurrent Mark Swap) collector
• G1 GC (Garbage First)
• Z GC

Serial GC

• Uses a single thread
• Uses mark-copy for the young generation and mark-swap-compact for the old generation
• When executing it freezes all other threads until GC finishes.
• Efficient because no communication overhead between threads
• Suitable for small applications
• Use java -XX:+UseSerialGC to use this GC

Parallel GC

• Same as Serial GC, but this has multiple threads to speed up the process.
• Uses mark-copy for the young generation and mark-swap-compact for the old generation
• This was the default GC until Java 8.
• Suitable for multi-core machines and if you need better performance.
• Use java -XX:+UseParallelGC to use this GC

CMS (Concurrent Mark Swap) collector

• This is an improved and advanced version of parallel GC
• Many threads are being used to scan heap memory
• This is running using Mark and Swap algorithm
• Marking live objects
• Removing unreachable objects
• How it works
• Define GC roots. Ex. Active threads, local variables/ input variables of executing methods... etc.
• Traverse through the object graph in the memory
• The application thread must stop to traverse
• This is called Stop The World pause
• GC visited object marked as live
• Now free up the memory, releasing unused memory. 
• There are different ways to do this
• Normal deletion
• Deletion with compacting
• Deletion with coping
• This is deprecated in JDK 9
• Use java -XX:+UseConcMarkSweepGC to use this GC

G1 GC (Garbage First)

• Introduced in Java 7 to replace the CMS collector
• Mostly used in multi-processor machines with a large amount of data
• This divides the heap into several equal-sized heaps and collects the regions with lesser live data.
• Most garbage-contained sections will be cleaned first.
• Since Java 9, this is the default GC.
• Use java XX:+UseG1GC to use this GC

Z GC

• This is available since JDK 11
• This performs all expensive works concurrently without stopping the execution.
• Use java -XX:+UseZGC to use this GC

How to select a GC?

• It is better to allow the virtual machine to select the GC
• If necessary, adjust the heap size to improve performance.
• If that is also not enough, select a GC as follows ,according to the official document
• If the application has small data set (100MB) select Serial GC
• If the application runs on a single processor and no pause time requirement, then select Serial GC
• If the peak application performance is the first priority and there is no pause time then select Parallel GC
• If the response time is more important, then select G1 GC
• If the response time is the highest priority, then select Z1 GC

Possible causes for java.lang.OutOfMemoryError: Java heap space

• Configuration issues (heap size may be not enough)
• The application is holding objects without any usages
• Excessive use of finalize() methods.