How is java code executed

JVM converts Java code into machine code

JVM converts Java code into machine code

Code conversion is not one time process

Its Just-In-Time

Code compilation at run time is Slow

Do it only if needed

Store results of compilation for future use

Premature optimization is the root of all evil.

JIT Optimizations

JIT optimizes hotspots

Heavily used methods and loops

How does JIT find hotspots

JIT is composed of tiers

level 0 - interpreter

level 1 - C1 with full optimization (no profiling)

level 2 - C1 with invocation and backedge counters

level 3 - C1 with full profiling

level 4 - C2

							
private static final int LOOP_COUNT = 5000;

public static void main(String[] args) {
	for (int i = 0; i < 1000; ++i) {
		long startTime = System.nanoTime();

		for (int j = 0; j < LOOP_COUNT; ++j) {
			new Object();
		}

		long endTime = System.nanoTime();
		System.out.printf("%d\t%d%n", i, endTime - startTime);
	}
}
								

					

						
javac test/HotLoops.java && java test.HotLoops
0	224194
1	197060

19	66723
20	78074

59	1737
60	1747

996	2699
997	3261
998	2465
999	3111
						
					

JIT gets rid of the inner loop

Few extra things

• Ultimate goal is to either reach tier 1 or tier 4
• Made not entrant flag used to invalidate using compiled code from cache

Throwing JIT for a loop

Introduce branch condition which is false most of the times

						
private static final int LOOP_COUNT = 10000;

public static void main(String[] args) {
	for (int i = 0; i < 1000; ++i) {
		long startTime = System.nanoTime();

		for (int j = 0; j < LOOP_COUNT; ++j) {
			Object x = new Object();
			if (i == 900) {
				x = null;
			}
		}

		long endTime = System.nanoTime();
		System.out.printf("%d\t%d%n", i, endTime - startTime);
	}
}
						
					

						
69	128783
70	133282
71	139054
72	162974
73	154268
74	52
75	50
76	49
77	47
897	39
898	40
899	40
900	571824
901	322669
902	233286
903	162727							
						
					

How to handle nulls

What if value is null?

						
static final void hotMethod(final Object value) {
	value.hashCode();
}
						
					

JIT assumes absence of nulls

						
public static void main(String[] args) throws InterruptedException {
	for (int i = 0; i < 20_000; ++i) {
		hotMethod("hello");
	}

	Thread.sleep(5_000);
	int count = 0;

	for (int i = 0; i < 1000; ++i) {
		System.out.printf("tempting fate %d%n", i);
		try {
			hotMethod(null);
		} catch (NullPointerException e) {
			// ignore
		}
	}
}

static final void hotMethod(final Object value) {
	value.hashCode();
}
						
					

Output

						
tempting fate 0
tempting fate 1
tempting fate 2
tempting fate 3
	5178  111       4       test.NullJIT::hotMethod (6 bytes)   made not entrant
tempting fate 4
	5178  193       3       java.util.Arrays::copyOfRange (63 bytes)
tempting fate 5
tempting fate 6
						
					

We usually see this in adserv logs, exceptions without stack traces

						
java.lang.NullPointerException
java.lang.NullPointerException
java.lang.NullPointerException
java.lang.NullPointerException
java.lang.NullPointerException
java.lang.NullPointerException
						
					

This is JIT optmizing for throwing exceptions.