This presentation is about Java performance and the most effective ways to work with Java memory, including memory saving techniques and overcoming of memory barriers. Moreover, it contains debunking of the most popular myths on speed boosting. This presentation by Andrii Antilikatorov (Consultant, GlobalLogic) was delivered at GlobalLogic Java Conference #2 in Krakow on April 23, 2016.

1. ©2015 GlobalLogic Inc.

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Java Memory Management
Tricks
By Andrii Antilikatorov

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Agenda

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When we start to think about memory-related things?

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Battle of Garbage Collectors
Serial Parallel CMS G1

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Battle of Garbage Collectors
Do I really need to think about this?

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Battle of Garbage Collectors

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Serial GC
Runs great on low-end computers
Long pauses during garbage collections
Recommended for single-core CPU and
heaps under 100Mb
…old, but not obsolete! ©

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Serial GC
Min & Max heap size - Xms/Xmx
Free space ratio in each generation:
MinHeapFreeRatio=?, MaxHeapFreeRatio=?
Young/Old generations ratio: NewRatio=?
Young generation min/max size: NewSize=?, MaxNewSize=?
Eden/Survivor ratio: SurvivorRatio=?
Set GC activity limit: UseGCOverheadLimit

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Parallel GC
Supports automatic self-tuning for optimal performance
Memory fragmentation
Perfectly consumes multi-core CPU power

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Parallel GC
Supports automatic self-tuning for optimal performance
Memory fragmentation
Perfectly consumes multi-core CPU power
All options of Serial GC are applicable
Number of GC threads: ParallelGCThreads=?
Disable compaction in Old Gen: UseParallelOldGC
Performance options: MaxGCPauseMillis=?, GCTimeRatio=?
Generation size increment: YoungGenerationSizeIncrement,
TenuredGenerationSizeIncrement
Generation size decrease: AdaptiveSizeDecrementScaleFactor

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Concurrent Mark-Sweep GC
Works as Parallel GC in case of minor GC
Minimizes pauses, but sacrifices CPU and throughput
Consumes more memory (+20%)
Long pauses in case of concurrency mode failures
Works great with big data with long-living objects

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Concurrent Mark-Sweep GC
Works as Parallel GC in case of minor GC
All options of Serial and Parallel GC are applicable
Major GC threshold:
CMSInitiatingOccupancyFraction=?

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Garbage First (G1) GC
JEP 248: Make G1 the Default Garbage Collector on 32- and
64-bit server configurations starting from Java 9
Designed for systems where limiting latency is more
important than maximizing throughput
More accurate pause prediction
No memory fragmentation
High CPU utilization

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Garbage First (G1) GC
JEP 248: Make G1 the Default Garbage Collector on 32- and
64-bit server configurations starting from Java 9
Designed for systems where limiting latency is more
important than maximizing throughput
More accurate pause prediction
No memory fragmentation
High CPU utilization
Number of GC threads and Marking threads:
ParallelGCThreads=?, ConcGCThreads=?
Heap region size: G1HeapRegionSize=?
Pause minimization: MaxGCPauseMillis=?
Heap memory allocation threshold:
InitiatingHeapOccupancyPercent=?
Options for real geeks: UnlockExperimentalVMOptions,
AggressiveOpts

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Memory Access
Heap
Direct Memory Access
(Off-Heap)
Non-Direct ByteBuffer
Direct ByteBuffer

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Memory Access
Heap
Direct Memory Access
(Off-Heap)
Non-Direct ByteBuffer
X Axis – No Of Reading
Y Axis – Op/Second in Millions
Direct ByteBuffer

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Memory Access
Heap
Direct Memory Access
(Off-Heap)
Non-Direct ByteBuffer
X Axis – No Of Reading
Y Axis – Op/Second in Millions
Direct ByteBuffer

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Memory Access
Heap
Direct Memory Access
(Off-Heap)
Non-Direct ByteBuffer
X Axis – No Of Reading
Y Axis – Op/Second in Millions
Direct ByteBuffer

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Memory Access
Heap
Direct Memory Access
(Off-Heap)
Non-Direct ByteBuffer
X Axis – No Of Reading
Y Axis – Op/Second in Millions
Direct ByteBuffer

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Direct Memory Alignment in Java
Type alignment Page size alignment
Cache line alignment
Memory Alignment

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Direct Memory Alignment in Java
Type alignment Page size alignment
Cache line alignment
Memory Alignment“= new SomeObj()” always type-aligned.
“Unsafe.allocateMemory” always 8-bytes aligned.
“ByteBuffer.allocateDirect” …
Memory is 0-ed out automatically
Memory is page-aligned in JDK ≤ 1.6 and ‘8-bytes’ aligned in JDK ≥ 1.7
Memory is is freed as part of the ByteBuffer object GC

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Comparing Aligned/Unaligned Access Performance
Type aligned access provides better performance than
unaligned access.
Memory access that spans 2 cache lines has far worse
performance than aligned mid-cache line access.
Cache line access performance changes based on
cache line location.

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Comparing Aligned/Unaligned Access Performance
Type aligned access provides better performance than
unaligned access.
Memory access that spans 2 cache lines has far worse
performance than aligned mid-cache line access.
Cache line access performance changes based on
cache line location.

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Type-aligned vs unaligned Access Test
Number of pages
Alignment
Relative cost

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Comparing Aligned/Unaligned Access Performance
Type aligned access provides better performance than
unaligned access.
Memory access that spans 2 cache lines has far worse
performance than aligned mid-cache line access.
Cache line access performance changes based on
cache line location.

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Cross-line cache access
Number of pages
Offset
Relative cost

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Comparing Aligned/Unaligned Access Performance
Type aligned access provides better performance than
unaligned access.
Memory access that spans 2 cache lines has far worse
performance than aligned mid-cache line access.
Cache line access performance changes based on
cache line location.

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Comparing Aligned/Unaligned Access Performance
Type aligned access provides better performance than
unaligned access.
Memory access that spans 2 cache lines has far worse
performance than aligned mid-cache line access.
Cache line access performance changes based on
cache line location.

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Cost of Access Based on Cache Line Location
Offset
Number of pagesRelative cost

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Comparing Aligned/Unaligned Access Performance
Type aligned access provides better performance than
unaligned access.
Memory access that spans 2 cache lines has far worse
performance than aligned mid-cache line access.
Cache line access performance changes based on
cache line location.

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sun.misc.Unsafe
Direct Object
Manipulations
Class Structure
Manipulations
Arrays
Manipulations
Synchronization
Primitives
Low-level
Memory Access
Low-level
Memory Info
sun.misc.Unsafe

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sun.misc.Unsafe – Obtain Unobtainable
Unsafe unsafe = Unsafe.getUnsafe();

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sun.misc.Unsafe – Obtain Unobtainable
Unsafe unsafe = Unsafe.getUnsafe();

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sun.misc.Unsafe – Obtain Unobtainable

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Create an Instance Without Calling a Constructor
Need a “hack” to create new instance of Singleton
Need to avoid execution of heavy constructor logic
Custom serialization/deserialization.

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Create an Instance Without Calling a Constructor

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Create an Instance Without Calling a Constructor

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Create an Instance Without Calling a Constructor
50
50
0

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Measure Shallow Size of an Object
Sizes of data structures are fixed for 32/64bit platforms
According to ‘sizeof’ is not required because…
Java VM’s GC does complete memory management
…and you still can ‘measure’ the object by serializing
to byte stream and looking at its length…

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Measure Shallow Size of an Object
Looking through all
non-static fields
Calculating offset of
the last field
Taking into account
memory alignment

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Measure Shallow Size of an Object
Getting data from
class struct
Converting signed
integer to long
Taking into account
header size

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Low-Level Memory Allocation
…for those who need extremely large arrays

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Low-Level Memory Allocation
Element size
Direct memory
allocation
Get/Put elements
to array

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How About a Kind of Multiple Inheritance?
No multiple inheritance
Really?

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How About a Kind of Multiple Inheritance?

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How About a Kind of Multiple Inheritance?

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51. ©2015 GlobalLogic Inc.