1. Runtime Analysis of Hotspot Java Virtual Machine
Progress Report Meeting
December 7, 2017
Houssem Daoud Michel Dagenais
École Polytechnique de Montréal
Laboratoire DORSAL

2. Agenda Introduction Thread Management JIT compilation
Object Allocation
Garbage Collection
Java Flame Chart
Conclusion
POLYTECHNIQUE MONTREAL – Houssem Daoud

3. Introduction Features of Java: Object Oriented Platform independent
Dynamic
Interpreted/Compiled
Hotspot is an open source virtual machine by Oracle
POLYTECHNIQUE MONTREAL – Houssem Daoud

4. Introduction Runtime Environment Source .java Class Loader Compiler
Java Interpreter
JIT Compiler
Bytecode
.class
Execution Engine
Operating System
Hardware
POLYTECHNIQUE MONTREAL – Houssem Daoud

5. Introduction Hotspot VM provides:
Class loader
Java bytecode interpreter
JIT compiler
Automatic memory management mechanisms
Several garbage collectors
The behavior of programs is sometimes unpredictable.
Runtime analysis is required
POLYTECHNIQUE MONTREAL – Houssem Daoud

6. Introduction
Most available tools use userspace tracing or JMX technology to collect runtime information.
Some important events are not visible from the userspace
POLYTECHNIQUE MONTREAL – Houssem Daoud

7. Introduction
Kernel tracing provides more information about the system:
System calls, disk access, network access, interrupts, etc.
But..
POLYTECHNIQUE MONTREAL – Houssem Daoud

8. Our goal is Introduction
To provide a JVM analysis tool that bridges the gap between userspace and kernel space traces
POLYTECHNIQUE MONTREAL – Houssem Daoud

9. Agenda Introduction Thread Management JIT compilation
Object Allocation
Garbage Collection
Java Flame Chart
Conclusion
POLYTECHNIQUE MONTREAL – Houssem Daoud

10. Thread Management Hotspot VM uses a 1:1 threading model
Each Java thread (JavaThread) is mapped to a native operating system thread (OSThread)
The main types of threads are: Compiler threads, GC threads, VM ops threads, etc..
JIT Compiler
Garbage collector
JIT 1
JIT 2
GC 1
GC 2
Main thread
GC 4
GC 3
Reference
handler
Finalizer
Signal
Dispatcher
POLYTECHNIQUE MONTREAL – Houssem Daoud
A simple java program can generate a dozen or more threads in the system

11. Thread Management (name, java_threadid, os_threadid, deamon)
thread_start
thread_stop
thread_sleep_start
thread_sleep_end
(name, java_threadid, os_threadid, deamon)
We use this information to associate Java thread to Kernel threads
POLYTECHNIQUE MONTREAL – Houssem Daoud

12. Thread Management Unified Kernel/User-space view for Java threads
POLYTECHNIQUE MONTREAL – Houssem Daoud

13. Thread Management We can see low-level details
syscall_entry_futex
Java threads can be considered RUNNING from the user-space point of view, but in reality they are interrupted by the operating system.
POLYTECHNIQUE MONTREAL – Houssem Daoud

14. Agenda Introduction Thread Management JIT compilation
Object Allocation
Garbage Collection
Java Flame Chart
Conclusion
POLYTECHNIQUE MONTREAL – Houssem Daoud

15. JIT Compilation
Hotspot VM is equipped with an advanced just-in-time (JIT) compiler
C1 provides fast compilation.
C2 is more aggressive and generates more optimized code
Tiered compilation: Methods are first compiled by C1; as they become hot, they are recompiled by C2.
POLYTECHNIQUE MONTREAL – Houssem Daoud

16. JIT Compilation
We added the following tracepoints: method_compile_begin, method_compile_end and method_load
POLYTECHNIQUE MONTREAL – Houssem Daoud

17. JIT Compilation
The view shows that there is one C1 thread and three C2 threads.
We can see the methods that are currently being compiled.
POLYTECHNIQUE MONTREAL – Houssem Daoud

18. Agenda Introduction Thread Management JIT Compilation
Object Allocation
Garbage Collection
Java Flame Chart
Conclusion
POLYTECHNIQUE MONTREAL – Houssem Daoud

19. Object Allocation
Each Java thread has its own TLAB (Thread Local Allocation Buffer). Allocating an object inside TLAB doesn't require synchronization mechanisms.
Java objects are preferably allocated inside TLABs. Large objects are allocated outside
We instrumented the memory management subsystem to track object allocation: alloc_new_tlab and alloc_outside_tlab
POLYTECHNIQUE MONTREAL – Houssem Daoud

20. Object Allocation
POLYTECHNIQUE MONTREAL – Houssem Daoud

21. Agenda Introduction Thread Management JIT compilation
Object Allocation
Garbage Collection
Java Flame Chart
Conclusion
POLYTECHNIQUE MONTREAL – Houssem Daoud

22. Garbage Collection
The garbage collector reclaims unused memory by automatically removing unreachable objects
Characteristics of garbage collectors
Serial / Parallel
Concurrent / Stop-the-world
Compacting / Non-Compacting
Young generation
Promotion
Old generation
POLYTECHNIQUE MONTREAL – Houssem Daoud

23. Garbage Collection
report_gc_start and report_gc_end provide information about the garbage collection (start, end, major/minor, type)
alloc_requiring_gc indicates the thread that caused the garbage collection
POLYTECHNIQUE MONTREAL – Houssem Daoud

24. Garbage Collection
By adding the userspace callstack context to garbage collector events, it is possible know the reason and the type of triggered GCs
POLYTECHNIQUE MONTREAL – Houssem Daoud

25. Agenda Introduction Thread Management JIT compilation
Object Allocation
Garbage Collection
Java Flame Chart
Conclusion
POLYTECHNIQUE MONTREAL – Houssem Daoud

26. Java Flame Chart
We are able to get the flame chart of a java program by instrumenting methods entry/exit (high frequency event)
POLYTECHNIQUE MONTREAL – Houssem Daoud

27. Conclusion We instrumented Hotspot JVM
We used Kernel/User-space traces to provide an advanced performance analysis tool for Java applications
The analysis covers JIT compilation, object allocation, garbage collection and method invocation.
POLYTECHNIQUE MONTREAL – Houssem Daoud

28. Thank You !
POLYTECHNIQUE MONTREAL – Houssem Daoud