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JAVA虚拟机-G1 Heap Structure(四)

G1 
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1.The G1 Garbage Collector(G1垃圾回收器定义)

The Garbage-First (G1) collector is a server-style garbage collector, targeted for multi-processor machines with large memories. It meets garbage collection (GC) pause time goals with a high probability, while achieving high throughput. The G1 garbage collector is fully supported in Oracle JDK 7 update 4 and later releases. The G1 collector is designed for applications that:

  • Can operate concurrently with applications threads like the CMS collector.
  • Compact free space without lengthy GC induced pause times.
  • Need more predictable GC pause durations.
  • Do not want to sacrifice a lot of throughput performance.
  • Do not require a much larger Java heap.

G1 is planned as the long term replacement for the Concurrent Mark-Sweep Collector (CMS). Comparing G1 with CMS, there are differences that make G1 a better solution. One difference is that G1 is a compacting collector. G1 compacts sufficiently to completely avoid the use of fine-grained free lists for allocation, and instead relies on regions. This considerably simplifies parts of the collector, and mostly eliminates potential fragmentation issues. Also, G1 offers more predictable garbage collection pauses than the CMS collector, and allows users to specify desired pause targets.

 

释义：G1使用并发和并行阶段实现其目标暂停时间，并保持良好的吞吐量,与CMS收集器不同的是，G1回收算法采取的并发-整理 ；它可以解决传统的parNew+CMS垃圾收集算法中,有很多晋代失败(promotion failed)/并发失败(concurrent mode failure),然后做Full gc的问题；G1提供更可预测的垃圾收集停顿比CMS收集器，并允许用户指定所需的停顿的目标。

 

2.G1 heap(G1堆)

2.1 G1 Heap Structure 

Region size is chosen by the JVM at startup. The JVM generally targets around 2000 regions varying in size from 1 to 32Mb.

 

2.2 G1 Heap Allocation

 

The heap is partitioned into a set of equal-sized heap regions, each a contiguous range of virtual memory. Certain region sets are assigned the same roles (eden, survivor, old) as in the older collectors, but there is not a fixed size for them. This provides greater flexibility in memory usage.

 

2.2 G1 Footprint

If you migrate from the ParallelOldGC or CMS collector to G1, you will likely see a larger JVM process size. This is largely related to "accounting" data structures such as Remembered Sets and Collection Sets.

Remembered Sets or RSets track object references into a given region. There is one RSet per region in the heap. The RSet enables the parallel and independent collection of a region. The overall footprint impact of RSets is less than 5%.

Collection Sets or CSets the set of regions that will be collected in a GC. All live data in a CSet is evacuated (copied/moved) during a GC. Sets of regions can be Eden, survivor, and/or old generation. CSets have a less than 1% impact on the size of the JVM.

 

2.3 Recommended Use Cases for G1 （推荐使用G1场景）

The first focus of G1 is to provide a solution for users running applications that require large heaps with limited GC latency. This means heap sizes of around 6GB or larger, and stable and predictable pause time below 0.5 seconds.

 3.G1 Collection Phases

The G1 collector performs the following phases on the old generation of the heap. Note that some phases are part of a young generation collection.

Phase Description 释义
(1) Initial Mark
(Stop the World Event) 		This is a stop the world event. With G1, it is piggybacked on a normal young GC. Mark survivor regions (root regions) which may have references to objects in old generation. 初始标记：此阶段,G1GC对根进行标记。该阶段与年轻代垃圾(STW)回收密切相关
(2) Root Region Scanning Scan survivor regions for references into the old generation. This happens while the application continues to run. The phase must be completed before a young GC can occur. 根区域扫描：G1GC 在初始标记的存活区扫描对老年代的引用,并标记被引用的对象.该阶段与应用程序同时运行,该阶段完成后,才能开始下一次年轻代垃圾回收。
(3) Concurrent Marking Find live objects over the entire heap. This happens while the application is running. This phase can be interrupted by young generation garbage collections. 并发标记阶段：G1GC 在整个堆中查找存活对象。该阶段与应用程序同时运行，可以被年轻代垃圾回收中断
(4) Remark
(Stop the World Event) 		Completes the marking of live object in the heap. Uses an algorithm called snapshot-at-the-beginning (SATB) which is much faster than what was used in the CMS collector. 重新标记阶段：该阶段是 STW 回收，帮助完成标记周期。G1 GC 清空 SATB 缓冲区，跟踪未被访问的存活对象，并执行引用处理。
(5) Cleanup
(Stop the World Event and Concurrent)
  • Performs accounting on live objects and completely free regions. (Stop the world)
  • Scrubs the Remembered Sets. (Stop the world)
  • Reset the empty regions and return them to the free list. (Concurrent)
 清理阶段：在这个最后阶段，G1 GC 执行统计和 RSet 清理的STW操作。在统计期间，G1GC 会识别完全空闲的区域和可供进行混合垃圾回收的区域。
(*) Copying
(Stop the World Event) 		These are the stop the world pauses to evacuate or copy live objects to new unused regions. This can be done with young generation regions which are logged as [GC pause (young)]. Or both young and old generation regions which are logged as [GC Pause (mixed)].

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回收过程按回收区域集合(CollectionSets/CSets)划分.CSets可以包含(Eden,survivor/old)中的region,CSets占用内存一般小于整个jvm的1%;

年轻代垃圾回收:

G1 Young GC同时回收eden区域和上次垃圾回收的存活区域。Eden 和survivor的存活对象被疏散(复制/移动)到新的区域集。
被疏散对象的目标区域取决于对象的年龄,达到晋升年龄的对象疏散到老年代区域,
否则疏散到存活区,并将它包含在下一次年轻代或混合垃圾回收的CSet中。

混合垃圾回收:

G1 Mixd GC同时回收Eden,survivor,old的存活区域。当成功完成并发标记周期后，G1 GC从执行年轻代垃圾回收切换为执行混合垃圾回收。
在混合垃圾回收期间,G1GC 将一些旧的区域添加到 eden 和存活区供将来回收。G1GC 回收了足够的旧区域后（经过多次混合垃圾回收），
G1将恢复执行年轻代垃圾回收,直到下一次标记周期完成。

 

 

4.G1 youngGC

 4.1 The heap is split into approximately 2000 regions. Minimum size is 1Mb and maximum size is 32Mb. Blue regions hold old generation objects and green regions hold young generation objects.

 

Note that the regions are not required to be contiguous like the older garbage collectors.

 

4.2 Live objects are evacuated (i.e., copied or moved) to one or more survivor regions. If the aging threshold is met, some of the objects are promoted to old generation regions.

 

This is a stop the world (STW) pause. Eden size and survivor size is calculated for the next young GC. Accounting information is kept to help calculate the size. Things like the pause time goal are taken into consideration.

This approach makes it very easy to resize regions, making them bigger or smaller as needed.

 

4.3 

End of a Young GC with G1

Live objects have been evacuated to survivor regions or to old generation regions.

 

Recently promoted objects are shown in dark blue. Survivor regions in green.

In summary, the following can be said about the young generation in G1:

  • The heap is a single memory space split into regions.
  • Young generation memory is composed of a set of non-contiguous regions. This makes it easy to resize when needed.
  • Young generation garbage collections, or young GCs, are stop the world events. All application threads are stopped for the operation.
  • The young GC is done in parallel using multiple threads.
  • Live objects are copied to new survivor or old generation regions.

 

5.g1 oldGC

 5.1 Initial Marking Phase

    Initial marking of live object is piggybacked on a young generation garbage collection. In the logs this is noted as GC pause (young)(inital-mark).
5.2 Concurrent Marking Phase

If empty regions are found (as denoted by the "X"), they are removed immediately in the Remark phase. Also, "accounting" information that determines liveness is calculated.

 

 
5.3 Remark Phase
Empty regions are removed and reclaimed. Region liveness is now calculated for all regions.
5.4 Copying/Cleanup Phase
G1 selects the regions with the lowest "liveness", those regions which can be collected the fastest. Then those regions are collected at the same time as a young GC. This is denoted in the logs as [GC pause (mixed)]. So both young and old generations are collected at the same time.
5.5 After Copying/Cleanup Phase

The regions selected have been collected and compacted into the dark blue region and the dark green region shown in the diagram.

 

 

5.6 Summary of Old Generation GC

In summary, there are a few key points we can make about the G1 garbage collection on the old generation.

  • Concurrent Marking Phase
    • Liveness information is calculated concurrently while the application is running.
    • This liveness information identifies which regions will be best to reclaim during an evacuation pause.
    • There is no sweeping phase like in CMS.
  • Remark Phase
    • Uses the Snapshot-at-the-Beginning (SATB) algorithm which is much faster then what was used with CMS.
    • Completely empty regions are reclaimed.
  • Copying/Cleanup Phase
    • Young generation and old generation are reclaimed at the same time.
    • Old generation regions are selected based on their liveness.

6.G1相关参数

 

 

 

 

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