This document discusses Java garbage collection and its performance impact. It provides an overview of garbage collection, including that garbage collectors reclaim memory from objects no longer in use. It describes the different Java GC algorithms like serial, parallel, CMS, and G1 collectors and how to choose between them based on factors like heap size and CPU availability. It also gives guidance on basic GC tuning techniques like sizing the heap and generations as well as using adaptive sizing controls.

1. Java Garbage Collection:
A Performance Impact
Dnepropetrovsk
7 November 2014
Blynov Viacheslav

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Java GC: A Performance Impact Introduction
Agenda
 Garbage Collection Overview
 Java GC Algorithms
 Basic GC Tuning
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Java GC: A Performance Impact Garbage Collection Overview
GC Purpose
Objects that are referenced are said to be live
Objects that are no longer referenced are considered dead and termed garbage
Garbage collector is responsible for:
 allocating memory
 ensuring that any referenced objects remain in memory
 recovering memory used by objects that are no longer reachable from
references in executing code
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Java GC: A Performance Impact Garbage Collection Overview
GC Purpose
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Java GC: A Performance Impact Garbage Collection Overview
GC performance impact
 Collector needs computational resources (CPU cycles) to perform garbage
collection
 As garbage collection involves moving objects in memory a collector must
ensure that no thread is using these objects
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The pauses when all application threads are stopped are called
stop-the-world pauses
These pauses generally have the greatest impact on the performance of an
application, and minimizing those pauses is the key consideration when tuning
GC.

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Java GC: A Performance Impact Garbage Collection Overview
Generational model
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Java GC: A Performance Impact Garbage Collection Overview
Generational model
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Java GC: A Performance Impact Garbage Collection Overview
Generational model
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Java GC: A Performance Impact Garbage Collection Overview
Generational model
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Java GC: A Performance Impact Garbage Collection Overview
Generational model
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Java GC: A Performance Impact Garbage Collection Overview
Generational model
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Java GC: A Performance Impact Garbage Collection Overview
Summary
 all GC algorithms divide the heap into old and young generation
 all GC algorithm employ stop-the-world approach to clearing objects from young
generation, which is usually a very quick operation
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Java GC: A Performance Impact Java GC Algorithms
“Client” and “Server” JVM types
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Depending on underlying hardware platform and version JVM can act as
“client” of “server” VM. This affect the choice of JIT compiler and default GC
algorithm.
 “client” platform is usually 32-bit and has 1 CPU
 “server” platform is usually 64-bit (but 32-bit is also possible) and has several
CPUs

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Java GC: A Performance Impact Java GC Algorithms
GC Algorithms
 Serial garbage collector (-XX:+UseSerialGC)
 Throughput collector (-XX:+UseParallelGC , -XX:+UseParallelOldGC)
 CMS collector (-XX:+UseConcMarkSweepGC, -XX:+UseParNewGC)
 G1 collector (-XX:+UseG1GC)
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18.  default collector for client-class platforms (32-bit JVMs on Windows or single-processor
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Java GC: A Performance Impact Java GC Algorithms
Serial garbage collector
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machines)
 uses single thread to process heap
 stops all application threads for both minor and full GC
Usage cases:
 no low-pause requirements
 “client-style” single-CPU environment
 very small heap (few hundred MBs)
 several JVMs running on single platform (number of JVM > number of available
CPUs)

19.  default collector for server-class machines (multi-CPU Unix machines or any 64-
 utilizes multiple threads for garbage collection to gain speed and minimize
 stops all application threads for both minor and full GC
 -XX:+UseParallelGC enables multi-threaded collection of young generation and
 -XX:+UseParallelOldGC enables multi-threaded collection of young generation
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Java GC: A Performance Impact Java GC Algorithms
Throughput (parallel) garbage collector
bit JVM)
pauses
single-threaded old-generation collection/compaction
and multi-threaded old-generation collection/compaction
Usage cases:
 multi-CPU are available
 large heap size and many object created/discarded
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20.  designed to eliminate long pauses associated with full GC cycles
 stops all application threads during minor GC
 uses different algorithm to collect young generation (-XX:+UseParNewGC)
 uses one or more background threads to periodically scan through the old
 do not perform any compaction
 in case of CPU unavailability and/or heap fragmentation – fallback to serial
 low pause requirement and available CPU resources
 in case of single-CPU machine can be used with -XX:+CMSIncrementalMode
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Java GC: A Performance Impact Java GC Algorithms
CMS (Concurrent Mark Sweep) collector
generation and discard unused objects. This makes CMS a low-paused collector
collector
 by default does not collect permgen
Usage cases:
(deprecated in Java 8)
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21.  designed to process large heaps (more than 4 Gb) with minimal pauses
 divides the heap into separate regions
 performs incremental compaction of old generation by copying data between
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Java GC: A Performance Impact Java GC Algorithms
G1 (Garbage First) garbage collector
regions
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22.  Serial GC is best only for application with heap <= 100 Mb
 Batch jobs which consume all available CPUs will get better performance with
 Batch jobs which DON’T consume all CPUs could get better performance with
 When measuring response time the choice between throughput and concurrent
 Most of the time CMS should overperform G1 for heaps < 4 Gb
 For large heaps G1 is better because of the way it can divide work between
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Java GC: A Performance Impact Java GC Algorithms
Summary (choosing GC algorithm)
concurrent collector
throughput collector
collectors depends on CPU availability
different threads and heap regions
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24.  Too small heap -> too much time spent in GC
 Main rule – never to specify heap more than the amount of available physical
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Java GC: A Performance Impact Basic GC Tuning
Sizing the heap
memory
 -Xms – initial heap size
 -Xmx – maximum heap size
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Java GC: A Performance Impact Basic GC Tuning
Sizing the Generations
 -XX:NewRatio=N – sets the ration of young generation to old
 -XX:NewSize=N – sets the size of young generaion
 -XX:MaxNewSize=N – maximum size for young generation
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Java GC: A Performance Impact Basic GC Tuning
Sizing Permgen and Metaspace
Java 7:
 -XX:PermSize=N
 -XX:MaxPermSize=N
Java 8:
 -XX:MetaspaceSize=N
 -XX:MaxMetaspaceSize=N
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27.  Adaptive sizing controls how the JVM alters the ratio of young generation to old
 Adjusting generation sizes is base on GC algorithms attempts to meet their
 Adaptive tuning can be disabled for small performance boost (usually not
 Command line argument differs for different GC algorithms. For, example for
-XX:GCTimeRatio=nnn - hint to the virtual machine that it's desirable that not more
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Java GC: A Performance Impact Basic GC Tuning
Adaptive Sizing
JVM can try to find optimal performance according to its policies and configuration
pause goals
recommended)
throughput collector:
-XX:+UseAdaptiveSizePolicy – whether to use adaptive policy (true by default)
-XX:MaxGCPauseMillis=nnn – maximal GC pause we can tolerate
than 1 / (1 + nnn) of the application execution time be spent in the collector
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28. Questions?
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