java12-fundamentals-off-heap-allocation-workshop

• References
  • WJUG #166 - On-heap cache vs Off-heap cache w Javie - Radek Grębski
  • https://github.com/rgrebski/confitura2015/tree/master/offheap-test
  • https://dzone.com/articles/java-chroniclemap-part-1-go-off-heap
  • https://docs.oracle.com/javase/8/docs/technotes/guides/troubleshoot/tooldescr007.html

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