Garbage Collection and Efficient Memory System for Java

Who is collecting your java Garbage By: Chia-tien, W Srisa and J MorrisIEEE Computer Society Press, Vol 5 ,Apr 2003

An Efficient Memory System for Java By: Li, Richard C.L. and Fong, Anthony S.S.IJCSNS , Vol 7 June2007

Presented By:Rohit Deshpande

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Table Of Contents

1. Introduction2. Garbage Collection(GC) Techniques3. Dynamic Memory Usage in Java4. Hardware/Software Memory Management5. Results Analysis6. Conclusion

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1. Introduction

One of the significant issues that hinder the performance of java program execution is dynamic memory usage.

15.58% of the CPU time is used in handling memory allocation request and 28.08% of the CPU time in garbage collection.

JVM performance often depends on the efficiency of its GC process.

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2. Garbage Collection Techniques

There are 6 types of Techniques/Strategies1. Mark-sweep2. Generational collection3. Reference counting4. Copying5. Mark-compact6. Stop-the-world

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2.1 Mark-sweep

Mark-sweep involves two phases: Marking and sweeping.

Marking phase marks reachable objects through either a depth-or breadth-first search on the object reference graph.

The sweep phase consists of reclaiming garbage back to system.

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2.2 Generational collection

It make use of Generational collectors.This collectors divide objects into groups(young ,

old and infant).Mostly Infant objects die quickly so become

garbage quickly.Young generation objects involves minimal

marking efforts.Whereas old generation objects could still need

full marking.04/12/2023

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2.3 Reference Counting

This technique associates each object with a reference count, which records the number of objects that can access the object. Once the reference reaches 0, the corresponding object is garbage.

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2.4 Copying

In this copying collector trace garbage(performs garbage detection) but do not mark it.

Once it identifies live objects, it copies those objects to another memory location and free rest of the memory.

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2.5 Mark-compact

It involves two phases: marking and compacting.Marking phase is same as in Mark-sweep. Once

the marking phase is finished, live objects are moved side by side in memory this is known as compaction.

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2.6 Stop-the-world

Stop-the-world garbage collector suspends all program threads when its threads are running. Then it search for free objects and destroy them.

This technique is not used for time critical applications as it suspends the current running threads to free the memory.

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3. Dynamic Memory Usage in Java

In order to obtain the statistics on the memory allocation and deallocation behavior of java programs, a tracing tool was developed.

This tracing tool is a profiler agent developed according to the specification of the Java Virtual Machine Profiler Interface (JVMPI).

Different Java applications were chosen for evaluation as follows

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The different results obtained were passed to an analyzer to analyze the data in 2 different dimensions:

Size of block requested Life time of a blockFollowing are the 3-dimensional plots of the

analysis

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Fig.1 Dynamic Memory Usage of JVM-98 Benchmark

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Fig.2 Dynamic Memory Usage of Java2D Demo

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Fig.3 Dynamic Memory Usage of J2EE PetStore Demo

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On the basis of above study it was stated that Java applications show similar locality, and that memory allocation requests are concentrated on small block sizes(<1K bytes) and blocks allocated usually have short life times.

Based on this findings a Hardware/Software solution was proposed.

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4. Hardware/Software Memory Management

Allocation requests are first classified into small size and large size types.

To provide effective memory management in hardware, the main memory is divided into fixed sized regions.

The Small Object Heap (SOH) is used for purpose of allocating small blocks.

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Fig.4 Memory Management Cache

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5. Result Analysis

Using above Memory Management Cache Scheme it was found that around 98.2% of all memory allocation and deallocation requests can be handled by the hardware in a single cycle.

1.6% of the requests will have a penalty on the miss in the memory management cache and 0.2% of the request will rely on software method to fulfill the request.

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Effective time for allocation= 98.2% X 1cycle + 1.6% X 1014cycles + 0.2% X 400

cycles=18cycles

Comparing with the 400 cycles required by best malloc software algo, the performance of the proposed solution give a gain of

400/18=22 times.Thus the overall speed up is around 17%. 04/12/2023

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6. Conclusion

With the given algo for garbage collection and proposed hardware/software combined memory system that is responsible for handling memory allocation/deallocation requests of small size(<=1Kbytes)blocks, while traditional software malloc routine is used for handling large block(>1kbytes). An overall 17% gain is possible while executing Java program.

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Thank You

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