Metaspace 與 Compressed Class Space Size

Class Metadata

Java classes have an internal representation within Java Hotspot VM and are referred to as class metadata. In previous releases of Java Hotspot VM, the class metadata was allocated in the so called permanent generation. In JDK 8, the permanent generation was removed and the class metadata is allocated in native memory. The amount of native memory that can be used for class metadata is by default unlimited. Use the option MaxMetaspaceSize to put an upper limit on the amount of native memory used for class metadata.

Java Hotspot VM explicitly manages the space used for metadata. Space is requested from the OS and then divided into chunks. A class loader allocates space for metadata from its chunks (a chunk is bound to a specific class loader). When classes are unloaded for a class loader, its chunks are recycled for reuse or returned to the OS. Metadata uses space allocated by mmap, not by malloc.

If UseCompressedOops is turned on and UseCompressedClassesPointers is used, then two logically different areas of native memory are used for class metadata. UseCompressedClassPointers uses a 32-bit offset to represent the class pointer in a 64-bit process as does UseCompressedOops for Java object references. A region is allocated for these compressed class pointers (the 32-bit offsets). The size of the region can be set with CompressedClassSpaceSize and is 1 gigabyte (GB) by default. The space for the compressed class pointers is reserved as space allocated by mmap at initialization and committed as needed. The MaxMetaspaceSize applies to the sum of the committed compressed class space and the space for the other class metadata.

Class metadata is deallocated when the corresponding Java class is unloaded. Java classes are unloaded as a result of garbage collection, and garbage collections may be induced in order to unload classes and deallocate class metadata. When the space committed for class metadata reaches a certain level (a high-water mark), a garbage collection is induced. After the garbage collection, the high-water mark may be raised or lowered depending on the amount of space freed from class metadata. The high-water mark would be raised so as not to induce another garbage collection too soon. The high-water mark is initially set to the value of the command-line option MetaspaceSize. It is raised or lowered based on the options MaxMetaspaceFreeRatio and MinMetaspaceFreeRatio. If the committed space available for class metadata as a percentage of the total committed space for class metadata is greater than MaxMetaspaceFreeRatio, then the high-water mark will be lowered. If it is less than MinMetaspaceFreeRatio, then the high-water mark will be raised.

Specify a higher value for the option MetaspaceSize to avoid early garbage collections induced for class metadata. The amount of class metadata allocated for an application is application-dependent and general guidelines do not exist for the selection of MetaspaceSize. The default size of MetaspaceSize is platform-dependent and ranges from 12 MB to about 20 MB.

Information about the space used for metadata is included in a printout of the heap. A typical output is shown in Example 11-1, “Typical Heap Printout”.

Example 11-1 Typical Heap Printout

Heap PSYoungGen total 10752K, used 4419K
[0xffffffff6ac00000, 0xffffffff6b800000, 0xffffffff6b800000)
eden space 9216K, 47% used
[0xffffffff6ac00000,0xffffffff6b050d68,0xffffffff6b500000)
from space 1536K, 0% used
[0xffffffff6b680000,0xffffffff6b680000,0xffffffff6b800000)
to space 1536K, 0% used
[0xffffffff6b500000,0xffffffff6b500000,0xffffffff6b680000) ParOldGen total 20480K, used 20011K
[0xffffffff69800000, 0xffffffff6ac00000, 0xffffffff6ac00000)
object space 20480K, 97% used
[0xffffffff69800000,0xffffffff6ab8add8,0xffffffff6ac00000) Metaspace used 2425K, capacity 4498K, committed 4864K, reserved
1056768K
class space used 262K, capacity 386K, committed 512K, reserved 1048576K

In the line beginning with Metaspace：

1. the used value is the amount of space used for loaded classes.
2. The capacity value is the space available for metadata in currently allocated chunks.
3. The committed value is the amount of space available for chunks.
4. The reserved value is the amount of space reserved (but not necessarily committed) for metadata.
   The line beginning with class space line contains the corresponding values for the metadata for compressed class pointers.
   原文地址：11 Other Considerations