爲什麼From/To space的大小几乎變成 0 了呢？

文章來源

https://hllvm-group.iteye.com/group/topic/39440

一、問題描述

Attaching to process ID 26424, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 25.231-b11

using thread-local object allocation.
Parallel GC with 15 thread(s)

Heap Configuration:
   MinHeapFreeRatio         = 40
   MaxHeapFreeRatio         = 70
   MaxHeapSize              = 1073741824 (1024.0MB)
   NewSize                  = 357564416 (341.0MB)
   MaxNewSize               = 357564416 (341.0MB)
   OldSize                  = 716177408 (683.0MB)
   NewRatio                 = 2
   SurvivorRatio            = 8
   MetaspaceSize            = 21807104 (20.796875MB)
   CompressedClassSpaceSize = 1073741824 (1024.0MB)
   MaxMetaspaceSize         = 17592186044415 MB
   G1HeapRegionSize         = 0 (0.0MB)

Heap Usage:
PS Young Generation
Eden Space:
   capacity = 268435456 (256.0MB)
   used     = 268435456 (256.0MB)
   free     = 0 (0.0MB)
   100.0% used
From Space:
   capacity = 44564480 (42.5MB)
   used     = 425984 (0.40625MB)
   free     = 44138496 (42.09375MB)
   0.9558823529411765% used
To Space:
   capacity = 44564480 (42.5MB)
   used     = 0 (0.0MB)
   free     = 44564480 (42.5MB)
   0.0% used
PS Old Generation
   capacity = 716177408 (683.0MB)
   used     = 577780552 (551.0144729614258MB)
   free     = 138396856 (131.98552703857422MB)
   80.67561829596278% used

41099 interned Strings occupying 4972688 bytes.

啓動參數是：

-Xms2048m -Xmx2048m -XX:NewSize=256m -XX:PermSize=256m -server -XX:+DisableExplicitGC -verbose:gc -XX:+PrintGCDateStamps -XX:+PrintGCDetails

SurvivorRatio是 8 ，這個時候Eden 也 600多M，爲啥From/To 幾乎是 0 呢？

Jdk version
java version "1.6.0_20"
Java(TM) SE Runtime Environment (build 1.6.0_20-b02)
Java HotSpot(TM) 64-Bit Server VM (build 16.3-b01, mixed mode)

二、解決方法

因爲ParallelScavenge GC會動態調整各個區的大小。這個行爲叫做adaptive size policy。要禁用的話請用 -XX:-UseAdaptiveSizePolicy 。可以通過 -XX:+PrintAdaptiveSizePolicy 來觀察它對各個區的大小的調整。

如果survivor space都幾乎變成0了，多半意味着tenuring threshold已經降到1了，也就是說所有eden裏活的對象在經歷第一次minor GC的時候就會被晉升到old generation。那樣的話survivor space就完全沒用了，所以會被設置到幾乎0。（無法真的完全設置到0大小是實現上的細節…有分區之間有對齊需求）

爲啥tenuring threshold會下降，最終降到1？有兩種可能：

1. minor GC耗時比期望的時間長。爲了降低minor GC的耗時，降低tenuring threshold來讓更多對象晉升，減少留在young generation裏的對象
2. survivor space有溢出

說到底還是創建對象太多太快了