©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

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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

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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

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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 9Designed 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 9Designed 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 ReadingY 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 ReadingY 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 ReadingY 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 ReadingY 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 LocationOffset

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

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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 ObjectLooking 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 longTaking 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 AllocationElement 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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©2015 GlobalLogic Inc.