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Memory allocation and recycling strategy

Priority is given to Eden Distribute

Most of the time , The object is the new generation Eden Regional distribution . When Eden When there is not enough space for allocation , The virtual machine will launch once Minor GC.

• Minor GC： Refers to the garbage collection in the new generation , because Java Most of the objects have the characteristics of living and dying , therefore Minor GC Very often , Generally, the recycling speed is also relatively fast .
• Full GC： Refers to what happened in the old days GC, There is Full GC, Often accompanied at least once Minor GC,Full GC It's usually faster than Minor GC slow 10 More than times .

Large objects go directly into the old generation

Large objects are those that require a large amount of continuous memory space Java object , Such as long strings or data .

A large object can be stored in Eden The probability of a zone is relatively small , The probability of distribution guarantee is relatively high , And distributing guarantees involves a lot of duplication , It's inefficient .

The virtual machine provides one -XX:PretenureSizeThreshold Parameters , Make objects larger than this setting directly allocated in the elderly generation , The purpose of this is to avoid Eden District and two Survivor There are a lot of memory copies between regions .

Long term survivors will enter the old age

JVM An object age counter is defined for each object . When the Cenozoic happens once Minor GC after , The age of the survivors +1, When the age is over a certain value , All objects exceeding this value will be transferred to the old age .

Use -XXMaxTenuringThreshold Set the maximum age of the new generation , As long as the new generation object exceeds this parameter, it will be transferred to the old age .

Dynamic object age determination

If the current new generation Survivor in , The sum of all object sizes of the same age is greater than Survivor Half of the space , Age >= People of that age can directly enter the old generation , Don't wait for MaxTenuringThreshold The age required in .

Space allocation guarantee

JDK 6 Update 24 The previous rule was this ：

In the event of a Minor GC Before , Virtual opportunity first check Is the maximum available continuous space in the old age greater than the total space of all objects in the new generation , If this condition holds ,Minor GC To make sure it's safe ; If not , Then virtual opportunity view HandlePromotionFailure Value is set to allow guarantee failure , If it is , Then it continues to check whether the maximum available contiguous space in the old age is greater than the average size of the objects that have been promoted to the old age , If it is greater than , There will be an attempt to Minor GC, Even though this time Minor GC There are risks ; If it is less than , perhaps HandlePromotionFailure Set not to take risks , At this time, we have to do it again Full GC.

JDK 6 Update 24 And then the rule became ：

As long as the continuous space of the old age is larger than the total size of the new generation of objects or the average size of previous promotions , It will Minor GC, Otherwise, it will be Full GC.

Expand the free space of the older generation by removing the discarded data of the older generation , In order to guarantee the new generation .

The process is to distribute guarantees .

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