Detailed garbage collector

Introduction

Effect is shown in FIG seven different generational collectors, if there is the connection between the two collectors, suggests that they may be used with. When JDK8 the Serial + CMS, ParNew + Serial Old combination of these two statements is abandoned, and completely abolished support for these combinations in JDK9

First clear a concept

Parallel (Parallel) : refers to the number of threads in parallel garbage collection, but this time the user thread is still in a wait state
concurrent (Concurrent ): refers to the user threads and threads execute simultaneously garbage collection

Serial Collector

The new generation, mark - replication algorithm, single-threaded. When garbage collection, threads must suspend all other work until it is collected by the end of

ParNew collector

It is a multi-threaded parallel version Serial collector on ParNew nature

Parallel Scavenge collector

The new generation, mark replication algorithm, multi-threaded, focuses on the throughput
throughput = user code running time / (run time + user code to run garbage collection time)

Serial Old collectors

Years old, Mark - sorting algorithm, single-threaded, old collector's edition Serial

Use the following two

1. In JDK5 and earlier in the Parallel Scavenge collector with the use of
2. , Used as a back-up plan when failure occurs when the CMS collector Concurrent Mode Failure concurrent collection happen

Parallel Old collectors

Years old, Mark - sorting algorithms, multi-threaded, the old version's Parallel Scavenge collector

Focus on throughput or processor resources are more scarce occasion, it can give priority to add Parallel Old Parallel Scavenge collector this combination

CMS collector

Years old, mark - sweep algorithm, multi-threaded, the main concern delay

The operation process is divided into four steps

1. The initial mark (CMS initial mark)
2. Concurrent mark (CMS concurrent mark)
3. Relabeled (CMS remark)
4. Concurrent Clear (CMS concurrent sweep)
   
5. The initial mark: mark what objects can be linked directly to the GC Roots, very fast
6. Concurrent mark: direct associated objects GC Roots from the beginning of the entire object graph traversal, this process takes a long time but do not need to stop user threads can run concurrently with the garbage collection
7. Relabeled: To amend during concurrent mark, due to the continued operation of the user program which led to marked record that part of the object changes from label
8. Concurrent clear: clean up the deleted objects marking phase judgment have died, because no moving live objects, so you can also look at this stage concurrent user threads

Garbage First collector (G1)

to sum up

collector	Collect objects and algorithms	Collector type	Explanation	Applicable scene
Serial	The new generation, replication algorithm	Single-threaded		Simple and efficient; not suitable memory situation
ParNew	The new generation, replication algorithm	Parallel multi-threaded Collector	ParNew garbage collector is multi-threaded version of the Serial collector	With CMS garbage collector's choice
Parallel Scavenge collector certain priority	The new generation, replication algorithm	Parallel multi-threaded Collector	Similarly ParNew, more attention throughput, throughput reaches a controlled	GC itself is the default mode on Server-level multi-CPU machines, the main task for background operation does not require much interaction

collector	Collect objects and algorithms	Collector type	Explanation	Applicable scene
Serial Old	Years old, mark Collation Algorithm	Single-threaded		Under Client mode virtual machine
Parallel Old	Years old, mark Collation Algorithm	Parallel multi-threaded Collector	Paraller Scavenge collector years old version, in order to configure the characteristics of the throughput Parallel Svavenge corresponding compositions developed for	In focusing on the use of CPU resources and throughput sensitive applications
CMS	Years old, mark sweep algorithm	Parallel and concurrent collector	User thread to stop garbage collection time shortened as much as possible; the disadvantage is that memory fragmentation 1, 2 requires more CPU resources, 3 floating garbage problem that requires more heap space.	The corresponding emphasis on service speed, dwell time system and user experience of Internet sites or B / S system. Internet backend cms is currently the mainstream garbage collector
G1		Across the old and the new generation's; tags to organize decentralized +	Parallel and concurrent collector	JDK1.7 was formally introduced, using the partition recovery of thinking, memory recovery complete standstill of low basic premise without sacrificing throughput; predictable pause is its greatest strength

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