JVM architecture

Class file load – Verify – Prepare – Parse – Initialize – Use – unload one stack per thread, one stack frame per method

How to identify garbage

  • Reference counting
    • There are problems with circular references
  • Forward to
    • Count reachable objects from the roots object

Garbage collection algorithm

  • Tag cleanup (disadvantages: memory fragmentation, no contiguous memory)

  • Copying (disadvantage: waste memory commonly used in the new generation)

  • Mark compression (a bit slower than copying is commonly used in old times [tenured])

  • new

    • Few viable objects
    • Using copying takes up less memory space and is also more efficient

  • old

    • Less waste
    • Mark-compact is commonly used

Look at the last oneGarbage Collector

  • Serial Collector
    • XX:+UseSerialGC
    • Single thread
  • Parallel Collector
    • There is a lot of parallelism, but the JVM pauses every time garbage is collected
    • CMS Collector
    • Do this with concurrent partitions and short pause times
  • G1
    • Not only the pause is short, the colleague concurrency is large. It’s just a balance point
    • Someone suggested that Java9 use this

The JVM parameter

  • – Standard parameter, which should be supported by all JVMS
  • -x is non-standard, and each JVM implementation is different
  • -XX Indicates an unstable parameter that may be cancelled in the next version

Allocation of Java objects

No open escape analysis is allocated on the stack firstCopy the code

  • On the stack

    • Thread private small object
    • No escape (-xx :+DoEscapeAnalysis enable Escape analysis is enabled by default)
    • Support for scalar substitution
    • Don’t need to adjust

  • TLAB (Thread Locla Allacation Buffer)

    It is also Eden on the heap, so that a portion of the data can be unlocked

    • Occupy Eden, default 1%
    • Multi-threaded users can apply for space without competing for Eden, which improves efficiency
    • Small objects
    • Don’t need to adjust

  • The old s

    • The big object

  • eden

  • Heap: – XX: XX: HeapDumpOnOutOfMemoryError – HeapDumpPath = TMP/JVM dump – XX: PrintGCDetails Xms10M – Xmx10M augmenting the biggest directly, avoid midway through analysis and calculation of recycling

  • Stack +Xss128k large thread recursive calls can be very deep and the number of concurrent small threads can be particularly large

The last

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