Interview_JVM, GC

✅ Explain the structure of the JVM

• JVM: runtime engine that enables Java bytecode to be executed on any platform
• ClassLoader: load .class file into JVM
• Runtime data Area: memory
• Execution Engine: interprete, compile

✅ 클래스 로더에 대해 설명해 주세요.

• load .class file into JVM

• save class metadata and static into method area in memory

• loading: load into memory(메모리에 로드)
• linking:
  • verification: ensure class file format follows JVM style
  • preparation: allocate meory for static variables(필요로 하는 메모리 할당)
  • resolution: convert symnolic reference into direct reference
• initialization: execute static blocks, assign static field
  • static field를 설정한 값으로 초기화

✅ 클래스 로더의 종류

• bootstrap ClassLoader: loads java core classes java.lang.*
• etension ClassLoader: loads classes from lib/ext 확장 라이브러리 로드
• application ClassLoader: loads classes from application classpath 사용자 정의 클래스 로드

✅ Explain the JVM Memory Structure in detail

• method area: class metadata, static, shared by thread
  • class field, method
  • has Runtime Constant Pool
• heap area: new instances, array, GC area, shared by thread
• stack area: local variables, parameters, each thread has own stack
• PC register: point to next command to be run, each thread has own
• native method stack: cade that is not java, each thread has own

✅ Why is the Heap divided into Young and Old Generations?

• Young Generation: store newly created objects, for Minor GC
  • fast GC cycles
  • use COPY as GC algorithm

• Old Generation: store long-living objects, cleaned by Major GC

  • less frequent, more expensive
  • STW
  • mark-sweep-compact, G!
• most objects in Java die young
• Minor GC is cheaper than Major GC

✅ GC(Garbage Collection)란 무엇인가요?

• automatic process of reclaiming memory by removing objects that are no longer referenced

• 👍🏻 prevent memory leak
• 👍🏻 manage heap memory

✅ GC의 장단점을 설명해 주세요.

• 👍🏻 automatically manage memory

• 👍🏻 prevent memory leak

• 👎🏻 application pause, Stop-The-World
• 👎🏻 cannot predict GC timing
• 👎🏻 Poor GC tuning can severly impact performance

✅ GC를 모니터링해야 하는 이유

• monitor how many times, for how long GC run to detect performance degradation

• too frequent full GCs can freeze your app: STW
• too frequent minor GCs can reduce throughput
• by monitoring GC, can analyze heap size ➡️ detect OOM(out of Memory error), memory leak

✅ how would you tune GC?

• monitor GC logs and heap dump
• tune heap size
• tune GC algorithm
• adjust GC thresholds: point that GC is triggered

✅ 자바로 jvm 응용 프로그램을 개발한 후에 운영하다보면 out of memory가 나올 수 있는데 이 에러가 발생했을때 어떻게 대처할 것 같으세요?

• 1. check GC logs and heap dumps
• 1. tune GC algorithm
• 1. tune heap size
• 1. check for large collections

✅ What is a memory leak?

• no longer referenced object not being removed by GC
• and keep accumulating ➡️ memory leak

✅ What causes memory leaks?

• creating meaningless instances with Wrapper class
  • Long sum = 0L; 만들고 1씩 만번 더하기 ➡️ Long 객체를 만번 새로 만들게 됨
• unreleased reference in static fields
• poorly managed large collections Map, Set, List that are not removed when no longer needed
• inner class that holds implicit references
• incorrect use of equals/hashCode() can end up saving same object many times

✅ How to detect memory leaks?

• analyze heap dump
• monitor GC logs

✅ GC에서 사용하는 알고리즘은 무엇이 있나요?

• Mark and Sweep: Check if object is referenced Mark and if not, remove Sweep
  • 👎🏻 can cause fragmentation
• Copying: move referenced objects to new space, compact memory. Used in Young Gen
• Mark-Compact: like mark and sweep, but compact memory to avoid fragmentation

✅ How does Java implement GC?

• Serial GC: Mark-Sweep, Mark-Compact, single threaded, for small heap
• Parallel GC: Copying(young), Mark-Compact(olf), default in Java 8, multiple threads, best throughput
• G1 GC: Copying(young), Mark-Compact(olf), default in Java 11+, divide heap into regions, prioritize area with most garbage.
  • 👍🏻 reduce STW time(Stop The World)

✅ Java 8 기준으로, GC는 어떤 방식으로 수행되나요?

• Parallel GC: for multi thread, focus on throughput
• multiple thread can run GC in parallel
• young gen: copy
• old gen: mark-compact, full heap pause

✅ Java 8과 Java 11의 디폴트 GC 실행 방식은 어떤 것인가요?

• Java 8: Parallel GC
• Java 11: G1 GC

✅ G1 GC에 대해 설명해 주세요.

• split heap into fixed size regions
• prioritize cleaning region with more garbage
• Prioritizes regions with the most garbage (“Garbage-First”)
• 👍🏻 efficient in big heap memory, can manage better pause time and performance
• 👍🏻 short STW

✅ G1 GC의 Heap 구조를 설명해주세요

• Divides heap into uniform regions.
• different region for young generation, old generation
• young gen: remove short lived objects
• old gen: save long lived objects

✅ 왜 Java 11은 디폴트 GC를 G1 GC로 변경하였을까요?

• shorter STW
• less fragmentation of memory, as operated with regions

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