G1 important configuration parameters and their default values in the garbage collector

-XX:MaxGCPauseMillis

Expected maximum GC Pause time , The default is ：200ms.

Be careful ：G1 The default policy for is to expect a balance between throughput and latency , So if you want to get higher throughput , Then you can reduce GC The frequency of pauses , And less GC The main way to increase the pause frequency is to increase the maximum GC Pause time .

-XX:GCPauseTimeInterval

The target of the maximum pause interval .

By default ,G1 Don't set any goals , allow G1 In extreme cases, garbage collection is performed continuously .

-XX:ParallelGCThreads

Maximum number of threads used for parallel work during garbage collection pauses .

By default, it runs according to JVM The number of threads available on the computer determines , Calculation method ： When the process is available CPU The number of threads is less than or equal to 8 when , Then use this number directly , otherwise , Set to ：8 + (n - 8) * (5/8) .

At each pause , The maximum number of threads used also needs to consider the maximum heap limit , The parameter ：-XX:HeapSizePerGCThread.

-XX:ConcGCThreads

Maximum number of threads for concurrent work , By default , This value is ：-XX:ParallelGCThreads Divide 4.

If you want to be able to complete concurrent markup faster , You can appropriately increase the number of threads marked concurrently .

-XX:+G1UseAdaptiveIHOP

Use adaptive IHOP.

Initiating Heap Occupancy Percent（ Threshold to trigger concurrent start collection ）,G1 Automatically determine the best by observing how long the tag takes and how much memory is usually allocated to older generations during the tag cycle IHOP, This feature is called adaptation IHOP.

If this function is enabled , be G1 Will use -XX:InitiatingHeapOccupancyPercent=45 The value of is used as the threshold for concurrent startup collection .

If G1 There is no good prediction , The age occupancy rate that triggers the concurrency flag , It is recommended to disable ：-XX:-G1UseAdaptiveIHOP, And self adjust -XX:InitiatingHeapOccupancyPercent The threshold of .

-XX:G1HeapRegionSize

G1 Each one of them Region Size .

The default is based on the maximum heap size , Divided by 2048 individual region To calculate each region Size , The maximum value determined according to ergonomics is 32M, The user-defined range is 1~512M, And it must be 2 The power of .

-XX:G1NewSizePercent、-XX:G1MaxNewSizePercent

The size of heap memory occupied by the new generation , For the initial -XX:G1NewSizePercent Set the value of the , The default is ：5%,G1 The proportion will be adjusted according to the value satisfying the pause time , But not more than -XX:G1MaxNewSizePercent Set the value of the , The default is ：60%.

-XX:G1HeapWastePercent

For more efficient garbage collection ,G1 From Collection Set Choose to free some memory space gain greater region, One of the references is that the recoverable space should be larger than XX:G1HeapWastePercent Set the value of the , The default is ：5%, Represents the current heap space 5%.

-XX:G1MixedGCCountTarget

In the mixed recycling phase ,G1 Expect to maximize the recycling , But it also needs to be considered XX:MaxGCPauseTimeMillis, Therefore, a large mixture is usually recycled , Split into multiple , This number is determined by XX:G1MixedGCCountTarget decision , The default is ：8 Time , This reduces the pause time for each hybrid recycle , In order to achieve XX:MaxGCPauseTimeMillis The target value .

-XX:G1MixedGCLiveThresholdPercent

In the mixed recycling phase , Avoid recycling things that take a lot of time to process region, So if the identification needs a lot of time to process ？ So in most cases , High occupancy region It takes more time to deal with ,XX:G1MixedGCLiveThresholdPercent This is the threshold value set for the occupancy rate of living objects , The default is ：85%, That is, if one region The proportion of surviving objects in the total population reaches this re-XX:GCPauseTimeInterval= < ERG >gion Of 85%, Then this will not be recycled region.

Be careful ： The following three parameters , By default, it is determined in an ergonomic way according to the environment

-XX:GCPauseTimeInterval
-XX:ParallelGCThreads
-XX:ConcGCThreads

Ergonomics ： Ergonomics is Java virtual machine (JVM) And garbage collection heuristics （ Such as behavior based heuristics ） The process of improving application performance .