![]() ![]() To use them, they must be scaled by a factor of 8 and added to the Java heap base address to find the object to which they refer. Because they're object offsets rather than byte offsets, oops can be used to address up to four billion objects (not bytes), or a heap size of up to about 32 gigabytes. Compressed oops represent managed pointers (in many but not all places in the Java Virtual Machine (JVM) software) as 32-bit object offsets from the 64-bit Java heap base address. Managed pointers in the Java heap point to objects that are aligned on 8-byte address boundaries. Memory is inexpensive, but these days bandwidth and cache are in short supply, so significantly increasing the size of the heap and only getting just over the 4 gigabyte limit is undesirable. ![]() This requirement is due to the expanded size of managed pointers. On an LP64 system, the heap used by a given program might have to be around 1.5 times larger than when it is run on an ILP32 system. On an ILP32 system, maximum heap size is less than 4 gigabytes, which is insufficient for many applications. Typically, an oop is the same size as a native machine pointer, which is 64-bit on an LP64 system. To organize and manage the larger space effectively, segmented code cache is used.Īn ordinary object pointer (oop) in Java Hotspot parlance, is a managed pointer to an object. To accommodate the additional profiling code that is generated with tiered compilation, the default size of code cache is multiplied by 5x. You can disable tiered compilation by using the -XX:-TieredCompilation flag with the java command. The 64-bit mode and Compressed Ordinary Object Pointer are supported. Tiered compilation is enabled by default for the server VM. Tiered compilation can also achieve better peak performance than a regular server VM, because, the faster profiling phase allows a longer period of profiling, which can yield better optimization. Often, startup is faster than the client VM startup speed because the final code produced by the server compiler might be available during the early stages of application initialization. The compiled code is substantially faster than the interpreter, and the program executes with greater performance during the profiling phase. With tiered compilation, the server VM also uses the client compiler to generate compiled versions of methods that collect profiling information about themselves. ![]() Without tired compilation, a server VM uses the interpreter to collect profiling information about methods that is sent to the compiler. Tiered compilation, introduced in Java SE 7, brings client VM startup speeds to the server VM. When the feature is disabled, the class stores characters as two bytes, encoded as UTF-16, and the HotSpot VM string intrinsics to use UTF-16 encoding. You can disable the compact strings feature by using the -XX:-CompactStrings flag with the java command line. The HotSpot VM string intrinsics are updated and optimized to support the internal representation. The additional character encoding field indicates the encoding that is used. Therefore, the class stores characters as one byte for each character, encoded as Latin-1. In Java SE 9, the compact strings feature is enabled by default. Other string-related classes, such as AbstractStringBuilder, StringBuilder, and StringBuffer are updated to use a similar internal representation. It modifies the internal representation of the class from a UTF-16 (two bytes) character array to a byte array with an additional field to identify character encoding. The compact strings feature does not introduce new public APIs or interfaces. ![]()
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