Big disclaimer: I am not a GC expert/writer; all the things that are written bellow are subject to changes and some of them might be way too simplistic. Please take this with a grain of salt. I will only speak about Shenandoah, as I think I understand it; which is not a generational GC. There … Read more
HotSpot JIT inlining policy is rather complicated. It involves many heuristics like caller method size, callee method size, IR node count, inlining depth, invocation count, call site count, throw count, method signatures etc. Some limits are skipped for accessor methods (getters/setters) and for trivial methods (bytecode count less than 6). The related source code is … Read more
I have done some experimentation in order to see when Hotspot is able to stack allocate. It turns out that its stack allocation is quite a bit more limited than what you might expect based on the available documentation. The referenced paper by Choi “Escape Analysis for Java” suggests that an object that is only … Read more
-XX:PermSize specifies the initial size that will be allocated during startup of the JVM. If necessary, the JVM will allocate up to -XX:MaxPermSize.
There is a corner case which allows to check within a Java application whether a string existed in the pool prior to the test, but it can be done only once per string. Together with string literals of the same content, the lazy loading can be detected: public class Test { public static void main(String[] … Read more
ReservedCodeCacheSize (and InitialCodeCacheSize) is an option for the (just-in-time) compiler of the Java Hotspot VM. Basically it sets the maximum size for the compiler’s code cache. The cache can become full, which results in warnings like the following: Java HotSpot(TM) 64-Bit Server VM warning: CodeCache is full. Compiler has been disabled. Java HotSpot(TM) 64-Bit Server … Read more
Yes, HotSpot JVM is kind of “cheating”, because it has a special version of some BigInteger methods that you won’t find in Java code. These methods are called JVM intrinsics. In particular, BigInteger.multiplyToLen is an intrinsic method in HotSpot. There is a special hand-coded assembly implementation in JVM source base, but only for x86-64 architecture. … Read more
You could try specifying -XX:MinHeapFreeRatio and -XX:MaxHeapFreeRatio to control heap expansion and shrinking: -XX:MinHeapFreeRatio – when the percentage of free space in a generation falls below this value the generation will be expanded to meet this percentage. Default is 40. -XX:MaxHeapFreeRatio – when the percentage of free space in a generation exceeded this value the … Read more
Try: import java.lang.management.ManagementFactory; import java.lang.management.RuntimeMXBean; import java.util.List; public void runtimeParameters() { RuntimeMXBean bean = ManagementFactory.getRuntimeMXBean(); List<String> aList = bean.getInputArguments(); for (int i = 0; i < aList.size(); i++) { System.out.println( aList.get( i ) ); } } That should show all JVM parameters. Note: we do not have JVM parameter in VCS either, but in a … Read more
The definition of what exactly is a Java Virtual Machine is stated in the Java Virtual Machine Specification The JVM is by definition a virtual machine, i.e. a software machine that simulates what a real machine does. Like a real machine, it has an instruction set, a virtual computer architecture and an execution model. It … Read more