The JVM is a virtual machine that runs Java bytecode. It provides memory management, code optimization through just-in-time compilation, and allows Java programs to run on different platforms. The JVM optimizes code at runtime by techniques like loop unrolling, reducing unnecessary data loading and method calls. It has two compilation modes - client mode for less optimization and server mode for more aggressive optimization based on runtime profiling.

1. JVM: a brief introduction
by Artem Shubovych

2. What is JVM?
JVM (Java Virtual Machine) - is an environment, designed
to run Java programs.

3. What is JVM?

4. What is JVM?

5. What is JVM?
Bytecode

6. What is JVM?
JVM is a environment to run Java bytecode..?

7. What is JVM?

8. What the hell is JVM?

9. What is JVM?

10. What is JVM?
Java APIs
Class loader
Runtime
JIT compiler
GC
OS + Hardware
Application
JVM
JRE

11. Optimizes code?

12. How do computers run programs?
Programming
language
Compiler
Processor
commands

14. Let’s check out the example...
#include <stdio.h>
int main() {
int *a = new int[10];
printf("size(a) = %lun", sizeof(a) / sizeof(int));
return 0;
}

15. Well, screw it! It’s 2016 already!

16. Managed languages

17. JVM puts it the level higher
Programming
language
Compiler
Virtual
Machine
commands

18. What’s the purpose of having Virtual
Machine?

19. It can handle many things for us

20. Memory management
MyClass *obj = new MyClass();
// ...
delete obj;
MyClass **objs = new (MyClass*)[100];
for (int i = 0; i < 100; ++i) {
objs[i] = new MyClass();
}
delete objs;
In C++ you were responsible for freeing the memory:

21. Memory management
Java will free the memory for you. If it can’t do this - it just
kills your process.

22. Code optimization
JIT can optimize your code in many different ways.

23. Short example
public class Main {
public static final String[] opts = { "Eggs", "Chickens", "Philosophers" };
public static void main(String[] args) {
for (String option : opts) {
process(option);
}
}
public static void process(String option) {
System.out.printf("%sn", option);
}
}

24. Loops unrolling
public class Main {
public static final String[] opts = { "Eggs", "Chickens", "Philosophers" };
public static void main(String[] args) {
process(opts[0]);
process(opts[1]);
process(opts[2]);
}
public static void process(String option) {
System.out.printf("%sn", option);
}
}

26. Reducing unneeded data loading
public class Main {
public static void main(String[] args) {
System.out.printf("%sn", "Eggs");
System.out.printf("%sn", "Chickens");
System.out.printf("%sn", "Philosophers");
}
}

28. Reducing unneeded data loading
public class Main {
public static void main(String[] args) {
System.out.printf("%sn", "Eggs");
System.out.printf("%sn", "Chickens");
System.out.printf("%sn", "Philosophers");
}
}
And unneeded method calls!

30. Reducing unneeded data loading
public class Main {
public static void main(String[] args) {
System.out.printf("%sn", "Eggs");
System.out.printf("%sn", "Chickens");
System.out.printf("%sn", "Philosophers");
}
}
Reduced memory cost here

32. How can we call those?

33. OpenJDK’s JIT (Hotspot) has two modes

34. Client mode (C1)
● Less aggressive inlines
● Fewer optimization possibilities

35. Server mode (C2)
● Aggressive inlines
● Based on rich runtime profiling

36. Why inlines?

37. Because they’re fast!
● Inlining is cheap (in means of time)
● Inlining gives great speed-up

38. How can we call those?

39. How can we call those?
● C1 tier: 1000 calls to the same method
● C2 tier: 100_000 calls to the same method