JAVA Garbage Collector Overview

JAVA Garbage Collector Overview

1. Combination of garbage collectors

There are a total of 7 JAVA garbage collectors, minus G1, which has not yet been officially used in a large scale. There are also 6, 3 in the new generation and 3 in the old generation.

Because the garbage collector is a group of tasks, these six collectors constitute a total of 5 usage modes.

Parameters	Description
-XX: + UseSerialGC	Default value for Jvm running in Client mode. After this switch is enabled, memory is recycled using the collector combination of Serial + Serial Old.
-XX: + UseParNewGC	Turn on this switch and use the ParNew + Serial Old collector for garbage collection
-XX: + UseConcMarkSweepGC	Use the collector combination of ParNew + CMS + Serial Old for memory recovery, and use Serial Old as the backup collector for CMS after "Concurrent Mode Failure" fails.
-XX: + UseParallelGC	Default value for Jvm running in Server mode. After this switch is enabled, use the collector combination of Parallel Scavenge + Serial Old for collection.
-XX: + UseParallelOldGC	Use the collector combination of Parallel Scavenge + Parallel Old for collection

I have read Mr. Zhou Zhiming's book "deep understanding of Java virtual machines". Section 3.4 (page 55) contains a garbage collector diagram. Among them, the new generation of serial collectors and the old generation of CMS collectors do not directly call the method. Therefore, I drew a picture again to help myself understand the content.

2. Legend of the spam collector combination relationship

The six common garbage collectors are scheduled and used by these five methods.

3. Some JVM Parameters

Some other Java virtual machine parameters can be viewed as needed.

-XX: Invalid vorratio	Capacity Ratio of the Eden region to the region vor region in the new generation. The default value is 8, which indicates Eden: Subrvivor = 8.
-XX: PretenureSizeThreshold	The size of objects directly promoted to the old age. After this parameter is set, objects larger than this parameter will be directly allocated in the old age.
-XX: MaxTenuringThreshold	The age of the object promoted to the old age. After each Minor GC operation, the age is added with 1. When the value of this parameter is exceeded, the old age is entered.
-XX: UseAdaptiveSizePolicy	Dynamically adjust the size of each region in the java heap and the age of the old age
-XX: + HandlePromotionFailure	Whether to allow the new generation of collection guarantees. After one minor gc, when the other shard VOR has insufficient space, it will be directly retained in the old age.
-XX: ParallelGCThreads	Set the number of threads for memory collection by parallel GC.
-XX: GCTimeRatio	The percentage of GC time to total time. The default value is 99, that is, 1% of GC time is allowed. It is only valid when the Parallel Scavenge collector is used.
-XX: MaxGCPauseMillis	Sets the maximum pause time for GC, which is valid under the Parallel Scavenge collector.
-XX: CMSInitiatingOccupancyFraction	Sets the size of the space used by the CMS collector to start garbage collection. The default value is 68%, which is only valid when the CMS collector is used.-XX: CMSInitiatingOccupancyFraction = 70
-XX: + UseCMSCompactAtFullCollection	Because the CMS collector generates fragments, this parameter sets whether a memory fragment process is required after the Garbage Collector, which is only valid when the CMS collector
-XX: + CMSFullGCBeforeCompaction	Sets the CMS collector to perform a memory fragment process after several spam collections, which is usually used together with the UseCMSCompactAtFullCollection parameter.
-XX: + UseFastAccessorMethods	Original Type optimization
-XX: + DisableExplicitGC	Disable manual System. gc?
-XX: + CMSParallelRemarkEnabled	Lower mark pause
-XX: LargePageSizeInBytes	The memory page size cannot be too large, which will affect the Perm size.-XX: LargePageSizeInBytes = 128 m

4. Features of jdk gc combination

	New Generation GC	GC in old age and persistent generation
-XX: + UseSerialGC	Serial GC	Serial Old Serial GC
-XX: + UseParallelGC	Parallel Scavenge Parallel GC collection	Serial Old Serial GC
-XX: + UseConcMarkSweepGC	ParNew parallel GC	CMS concurrent GC When "Concurrent Mode Failure" appears Serial GC using Serial Old
-XX: + UseParNewGC	ParNew parallel GC	Serial Old Serial GC
-XX: + UseParallelOldGC	Parallel Scavenge Parallel GC collection	Parallel Old Parallel GC
-XX: + UseConcMarkSweepGC -XX: + UseParNewGC	Serial GC	CMS concurrent GC When "Concurrent Mode Failure" appears Serial GC using Serial Old