JVM memory management------Garbage collector parameters detailed explanation

This article is the last one related to GC. This time LZ just lists the key parameters related to the garbage collector in hotspot JVM, as well as the explanation of each parameter. Without further ado, let’s get started.

Garbage collector selection parameters

UseSerialGC: Enable this parameter to use serial & serial old collector (client mode default value).
UseParNewGC: Enable this parameter to use ParNew & serial old collector (not recommended).
UseConcMarkSweepGC: Turn on this parameter to use ParNew & CMS (serial old is the substitute) collector.
UseParallelGC: Turn on this parameter to use the parallel scavenge & parallel old collector (server mode default value).
UseParallelOldGC: Turn on this parameter to use the parallel old collector in the old generation (this parameter is no longer useful after JDK1.5).

Parameters related to the size of each JVM memory area

Xms: the initial value of the heap. The default is 1/64 of the physical memory, and the maximum does not exceed 1G.
Xmx: The maximum value of the heap. The default is 1/4 of the physical memory, and the maximum does not exceed 1G.
Xmn: The size of the new generation.
Xss: Thread stack size.
PermSize: Initial size of the permanent generation. The default is 1/64 of the physical memory, and the maximum does not exceed 1G.
MaxPermSize: Maximum permanent generation size. The default is 1/4 of the physical memory, and the maximum does not exceed 1G.
NewRatio: The ratio of the new generation to the old generation. For example, if it is 3, the new generation occupies 1/4 of the heap and the old generation occupies 3/4.
SurvivorRatio: Adjust the ratio of the eden area and the survivor area in the new generation. The default is 8, that is, the eden area is 80% of the size, and the two survivors are 10% of the size respectively. (Note: This parameter setting is a method to solve the problem of halving the memory of the replication algorithm in the chapter explaining the replication algorithm. The eden area is the 80% part in the chapter of the replication algorithm, and the survivor area is two 10% That part.)

Garbage collector performance general parameters

PretenureSizeThreshold: Increase the object size of the young generation. The default is 0. For example, if it is set to 10M, objects exceeding 10M will not be allocated in the eden area, but will directly enter the old generation.
MaxTenuringThreshold: The maximum age for promotion to the old generation. The default is 15. For example, if it is set to 10, the object will be placed in the old generation after 10 ordinary GC times.
DisableExplicitGC: Disable System.gc().

Parallel collector parameters

ParallelGCThreads: The number of threads opened during recycling. The default is equal to the number of CPUs.
GCTimeRatio: Set the throughput of the system. For example, if it is set to 99, the GC time ratio is 1/1+99=1%, which means the required throughput is 99%. If it cannot be satisfied, the size of the new generation will be reduced.
MaxGCPauseMillis: Set the maximum pause time for garbage collection. If the setting value cannot be met, the new generation size will be reduced first. If it is still not met, the throughput will be sacrificed.

Concurrent collector parameters

CMSInitiatingOccupancyFraction: The memory ratio that triggers the CMS collector. For example, 60% means that when the memory reaches 60%, CMS concurrent collection will start.
UseCMSCompactAtFullCollection: This has been mentioned before and is used to send memory deflation after each time the CMS collector cleans up garbage.
CMSFullGCsBeforeCompaction: Set to trigger a memory compaction after several CMS garbage collections.

Conclusion

The GC related series is basically over. This article is just a list. Then let’s enter the source code world of virtual machines.

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