A summary of Java garbage collector

1 , a combination of garbage collectors

JAVA a total of garbage collectors 7 , minus those that have not yet been formally used on a large scale G1 , there are 6 One, the Cenozoic 3 One, Laosheng generation 3 A.

Because the garbage collector is a group of work, this 6 Collectors constitute a total of 5

parameter	description
-XX:+USESERIALGC	JVM run the default value in client mode, when this switch is turned on, Memory reclamation using Serial + Serial old collector combination
-XX:+USEPARNEWGC	After this switch is turned on, garbage collection using Parnew + Serial old collector
-XX:+USECONCMARKSWEEPGC td>	using parnew + CMS +  serial old collector combination for memory recycling, Serial old as a CMS "Concurrent Mode Failure" After the failure of the backup Used by the collector.
-XX:+USEPARALLELGC	JVM run the default value in Server mode, After you turn on this switch, use the parallel scavenge +  serial old collector combination to recycle
-xx:+ USEPARALLELOLDGC

I read Mr. Zhou Zhiming's book "In-depth understanding of JAVA virtual machines", where the 3.4 Chapter Page) has a picture of the garbage collector. The new generation of serial collectors and laosheng generations of CMS collectors, and there is no direct method of invocation, so refer to this piece of content, I re-draw a picture in order to let themselves understand.

2 , garbage collector combination relationship legend

The 6 commonly used Garbage collectors are dispatched and used by these five methods.

3 , part JVM Parameters

some of the other JAVA The parameters of the virtual machine so that it is consulted when needed.

-xx:survivorratio	The capacity ratio of the Eden region to the survivor region in the Cenozoic, the default is 8, which represents eden:subrvivor = 8:1
-xx:pretenuresizethreshold	Directly promoted to the size of the old age object, after setting this parameter, objects larger than this parameter will be allocated directly in the old age
-xx:maxtenuringthreshold	Promotion to the age of the old, each time after minor GC, the age of 1, when the value of this parameter to enter the old age
-xx:useadaptivesizepolicy	Dynamically adjusts the size of each area in the Java heap and the age of entry into the old age
-xx:+handlepromotionfailure	Whether to allow the new generation to collect the guarantee, once minor GC, another piece of survivor space is not enough, will be directly in the old age to retain
-xx:parallelgcthreads	Set the number of threads in a parallel GC for memory reclamation
-xx:gctimeratio	GC time is the column of the total time, the default value is 99, which is to allow 1% GC time, only valid when using the parallel scavenge collector
-xx:maxgcpausemillis	Set maximum pause time for GC, valid under parallel scavenge collector
-xx:cmsinitiatingoccupancyfraction	Set the CMS collector to start garbage collection after the old age space is used, the default value is 68%, only valid in the CMS collector,-xx:cmsinitiatingoccupancyfraction=70
-xx:+usecmscompactatfullcollection	Because the CMS collector is fragmented, this parameter sets whether a memory defragmentation process is required after the garbage collector, only valid when the CMS collector
-xx:+cmsfullgcbeforecompaction	Set up a CMS collector to perform a memory defragmentation process after several garbage collections, usually with the usecmscompactatfullcollection parameter
-xx:+usefastaccessormethods	Original type optimization
-xx:+disableexplicitgc	Whether to turn off manual System.GC
-xx:+cmsparallelremarkenabled	Reduce Mark pauses
-xx:largepagesizeinbytes	The size of the memory page cannot be set too large, affecting the size of the perm,-xx:largepagesizeinbytes=128m

4 , JDK GC some characteristics of the combination method

	New Generation GC Mode	Old age and persistent generation GC mode
-xx:+useserialgc	Serial Serial GC	Serial Old serial GC
-xx:+useparallelgc	Parallel Scavenge Parallel Recovery GC	Serial Old serial GC
-xx:+useconcmarksweepgc	Parnew Parallel GC	CMS concurrency GC When "Concurrent Mode Failure" appears Using the serial old serial GC
-xx:+useparnewgc	Parnew Parallel GC	Serial Old serial GC
-xx:+useparalleloldgc	Parallel Scavenge Parallel Recovery GC	Parallel Old parallel GC
-xx:+useconcmarksweepgc -xx:+useparnewgc	Serial Serial GC	CMS concurrency GC When "Concurrent Mode Failure" appears Using the serial old serial GC

A summary of Java garbage collector