java12-fundamentals-off-heap-allocation-workshop

References

preface

goal of this workshop:
- get accustomed with rudimentary java’s diagnostic tools:
  visualVM, jmap, jcmd
- learn how to allocate objects in the native java memory (off-heap)
exemplary solution is in answers package

introduction

java 9 memory model (assumption: g1)
weak generational hypothesise: most objects die young
- cache and http session don’t follow it
Off-Heap memory allows your cache data outside of main Java Heap
space, but still in RAM
off-heap is far more bigger than heap
off-heap - no gc
jmap -histo <pid>
- prints the number of objects, memory size in bytes, and fully
  qualified class name for each class
jcmd <pid> VM.native_memory | grep Internal
- if JVM was started with the flag -XX:NativeMemoryTracking=summary
  shows the amount off-heap memory being used
  project description
we show how to allocate off-heap short array (using Unsafe)
- public long allocateMemory(long bytes)
  - allocates a new block of native memory, of the given size in
    bytes
  - contents of the memory are uninitialized
  - the resulting native pointer will never be zero, and will be
    aligned for all value types
- public void setMemory(long address, long bytes, byte value)
  - sets all bytes in a given block of memory to a fixed value
- public void putShort(long address, short x)
  - stores a value into a given memory address
  - if the address is zero, or does not point into a block
    obtained from allocateMemory, the results are undefined
- public short getShort(long address)
  - fetches a value from a given memory address
  - if the address is zero, or does not point into a block
    obtained from allocateMemory, the results are undefined
- public void freeMemory(long address)
  - disposes of a block of native memory, as obtained from
    allocateMemory or reallocateMemory

java12-fundamentals-off-heap-allocation-workshop

preface

introduction

project description