JVM-GC recovery mechanism

JVM garbage collection mechanism

The JVM uses different garbage collection mechanisms for the Cenozoic and the old generation respectively.

New generation of GC:

The new generation usually has a shorter survival time, so the so-called copying algorithm is used to retrieve the surviving object based on the copying algorithm, and copy it into a completely unused space, corresponding to the Cenozoic, which is copy between Eden and Fromspace or Tospace. The Cenozoic uses a free pointer to control the GC trigger, the pointer keeps the last allocated object in the Cenozoic interval, and when a new object is allocated memory, it is used to check if the space is sufficient and not enough to trigger the GC. When objects are continuously allocated, the objects gradually go from Eden to Survivor, and finally to the old generation.

Using JAVAVISUALVM to view, can obviously observe the new generation full after the object will be transferred to the old generation, and then empty continue to load, when the old generation is full, will be reported OutOfMemory exception, as shown:

On the execution mechanism the JVM provides serial GC (SERIALGC), parallel reclaim GC (Parallelscavenge), and parallel GC (PARNEW).

1) Serial GC

The entire scanning and copying process is a single-threaded way, suitable for single CPU, the new generation of small space and the demand for pause time is not very high application, is the client level of the default GC mode, can be-XX:+USESERIALGC to enforce the specified

2) Parallel Recovery GC

In the entire scanning and replication process in a multi-threaded way, for multi-CPU, the time required for a short pause on the application, the server level is the default use of GC mode, can be-XX:+USEPARALLELGC to enforce the designation, with-XX: Parallelgcthreads=4 to specify the number of threads

3) Parallel GC

Used in conjunction with the old generation of concurrent GC.

GC for old generation:

The old generation and the new generation, the object survival time is longer, more stable, so the mark (Mark) algorithm for recycling, so-called Mark is to scan out the surviving objects, and then to reclaim unmarked objects, after recycling the empty space is either merged, or marked out for the next allocation, The bottom line is to reduce the loss of efficiency caused by memory fragmentation. The JVM provides a serial GC (SERIALMSC), parallel GC (PARALLELMSC), and concurrent GC (CMS) on the execution mechanism, and the details of the algorithm need to be further studied.

The various GC mechanisms above need to be combined, as specified in the following table:

Reference Blog: http://blog.163.com/guixl_001/blog/static/4176410420108296361891/

JVM-GC recovery mechanism