# Java Memory Model

• Program Counter Register
• VM Stack (StackFrame)
  • Local Variables
  • Operands Stack
  • Frame Data
    • Method’s name
    • class
    • return address
• Native Method Stack
• Method Area
• Heap

# GC

How to:

• GC Root enumeration
• Safe Point
  • Preepmtive Suspension
  • Voluntary Suspension - make every thread check a markword periodically
• Safe Region (much longer safe-point)
• RememberedSet & Card Table
  • to solve cross generation reference problem
• Writer Barrier
• Concurrent Reachable Analysis

Three Metrics:

• footprint (memory)
• throughput
• latency

# G1GC

Steps

• Initial Marking (STW)
• Concurrent Marking
• Final Marking (STW)
• Live Data Counting and Evacuation (STW)

# how to choose one garbage collector

• -Xlog[:[selector][:[output][:[decorators][:output-options]]]]
  • time
  • uptime
  • timemillis
  • up timemillis
  • up timenanos
  • pid
  • tid
  • level
  • tags

# Performance Monitoring

• jmc
• jfr
• jps
• jstat -gcutil 2874
• jinfo
• jmap
• jhat
• jstack