應用:shark-美團點評移動端長連代理,每日接受移動端請求約150億
應用特點:
-
QPS比較高,新生代增長飛快
-
用戶的連接需要維持一段時間
-
單機需要維持海量連接,大概在幾十萬的級別
以上三個特點導致有大量小對象聚集在old區,高峯期old區域增長非常快,對象在一段時間內必然消亡
初始的線上gc的情況如下(備註:相關截圖來自點評的APM軟件CAT:https://github.com/dianping/cat):
對應的jvm參數爲
-Xms10g -Xmx10g -Xss512k -XX:PermSize=384m -XX:MaxPermSize=384m -XX:NewSize=7g -XX:MaxNewSize=7g
-XX:SurvivorRatio=8 -XX:MaxDirectMemorySize=4g -XX:+UseParNewGC -XX:ParallelGCThreads=4
-XX:MaxTenuringThreshold=9 -XX:+UseConcMarkSweepGC -XX:+DisableExplicitGC
-XX:+UseCMSInitiatingOccupancyOnly -XX:+ScavengeBeforeFullGC -XX:+UseCMSCompactAtFullCollection
-XX:+CMSParallelRemarkEnabled -XX:CMSFullGCsBeforeCompaction=9 -XX:CMSInitiatingOccupancyFraction=70
可以看到新生代爲7G(其中Survivor爲7G*1/10=700M),老年代爲3G,對象最大的年齡居然只有9(-XX:MaxTenuringThreshold=9,導致new 區域進入到old區域的速度太快,old區域很快被撐滿,頻繁old gc),考慮到我這邊的對象在一段時間內(幾分鐘)肯定會消亡,於是先進行一次調優嘗試。
第一次調優
將年齡調到非常大,並且調大整個堆和young區,對應的jvm參數爲
-Xms14g -Xmx14g -Xss512k -XX:PermSize=384m -XX:MaxPermSize=384m -XX:NewSize=12g -XX:MaxNewSize=12g
-XX:SurvivorRatio=18 -XX:MaxDirectMemorySize=2g -XX:+UseParNewGC -XX:ParallelGCThreads=4
-XX:MaxTenuringThreshold=30 -XX:+UseConcMarkSweepGC -XX:+DisableExplicitGC-XX:+UseCMSInitiatingOccupancyOnly
-XX:+ScavengeBeforeFullGC -XX:+UseCMSCompactAtFullCollection -XX:+CMSParallelRemarkEnabled
-XX:CMSFullGCsBeforeCompaction=9 -XX:CMSInitiatingOccupancyFraction=70 -XX:+CMSClassUnloadingEnabled
-XX:SoftRefLRUPolicyMSPerMB=0 -XX:-ReduceInitialCardMarks -XX:+CMSPermGenSweepingEnabled
-XX:CMSInitiatingPermOccupancyFraction=70
結果old區域一直爲空,但是yong gc時間拉長許多,平均每次都要0.2s的時間,比之前的young gc多了整整三倍,這是無法接受的:
關於參數-XX:MaxTenuringThreshold:
在jdk1.7某個版本之前設置大於15表示是無窮大,即對象長命百歲,之後不管設置成多少,都是15,jdk1.8之後大於15直接報錯 見 http://mail.openjdk.java.net/pipermail/hotspot-gc-dev/2008-May/000309.html
第二次調優
將XX:MaxTenuringThreshold調整成15,由於之前cms 在old gc花的時間比較多,所以這裏嘗試serial old gc,對應的jvm參數爲:
-Xms14g -Xmx14g -Xss512k -XX:PermSize=384m -XX:MaxPermSize=384m -XX:NewSize=12g -XX:MaxNewSize=12g
-XX:SurvivorRatio=18 -XX:MaxDirectMemorySize=2g -XX:+UseParNewGC -XX:ParallelGCThreads=4
-XX:MaxTenuringThreshold=15
發現效果確實比較明顯,new gc的時間縮小了兩倍左右,並且old gc的時間也從原來的15000ms 縮小到1500ms
第三次調優
仔細研究了一下第二種調優方式的組合,yong區域使用parNew 的方式,old區域使用serial old 的方式,如果在其他條件都相同的條件下使用parNew+cms的方式,old gc的時間會不會大幅度縮短?畢竟cms還是比較先進的收集器,於是分析了一下cms的幾個階段,有兩個階段是stop the world的,一個是初始化標記(intial mark),一個是重複標記(remark),重複標記的原因是併發標記階段可能很多對象的狀態都產生了變化,所以這個時候需要再次暫停用戶所有的線程。
查看gc日誌,發現remark的時間比較長,日誌上下文如下
{Heap before GC invocations=23679 (full 18):
par new generation total 6606080K, used 5902124K [0x0000000568000000, 0x0000000728000000, 0x0000000728000000)
eden space 5872128K, 99% used [0x0000000568000000, 0x00000006ce54f988, 0x00000006ce680000)
from space 733952K, 4% used [0x00000006fb340000, 0x00000006fd1bb758, 0x0000000728000000)
to space 733952K, 0% used [0x00000006ce680000, 0x00000006ce680000, 0x00000006fb340000)
concurrent mark-sweep generation total 3145728K, used 2200878K [0x0000000728000000, 0x00000007e8000000, 0x00000007e8000000)
concurrent-mark-sweep perm gen total 393216K, used 37361K [0x00000007e8000000, 0x0000000800000000, 0x0000000800000000)
2016-08-27T17:26:00.058+0800: 261980.413: [GC2016-08-27T17:26:00.058+0800: 261980.413: [ParNew: 5902124K->27858K(6606080K), 0.0464930 secs] 8103002K->2230601K(9751808K), 0.0468010 secs] [Times: user=0.18 sys=0.00, real=0.05 secs]
Heap after GC invocations=23680 (full 18):
par new generation total 6606080K, used 27858K [0x0000000568000000, 0x0000000728000000, 0x0000000728000000)
eden space 5872128K, 0% used [0x0000000568000000, 0x0000000568000000, 0x00000006ce680000)
from space 733952K, 3% used [0x00000006ce680000, 0x00000006d01b4910, 0x00000006fb340000)
to space 733952K, 0% used [0x00000006fb340000, 0x00000006fb340000, 0x0000000728000000)
concurrent mark-sweep generation total 3145728K, used 2202742K [0x0000000728000000, 0x00000007e8000000, 0x00000007e8000000)
concurrent-mark-sweep perm gen total 393216K, used 37361K [0x00000007e8000000, 0x0000000800000000, 0x0000000800000000)
}
2016-08-27T17:26:00.107+0800: 261980.462: Application time: 0.0014750 seconds
2016-08-27T17:26:00.108+0800: 261980.463: [GC [1 CMS-initial-mark: 2202742K(3145728K)] 2235571K(9751808K), 0.0561400 secs] [Times: user=0.05 sys=0.00, real=0.05 secs]
2016-08-27T17:26:00.165+0800: 261980.520: [CMS-concurrent-mark-start]
2016-08-27T17:26:00.918+0800: 261981.273: [CMS-concurrent-mark: 0.753/0.754 secs] [Times: user=1.27 sys=0.12, real=0.76 secs]
2016-08-27T17:26:00.918+0800: 261981.273: [CMS-concurrent-preclean-start]
2016-08-27T17:26:00.949+0800: 261981.304: [CMS-concurrent-preclean: 0.028/0.031 secs] [Times: user=0.04 sys=0.01, real=0.03 secs]
2016-08-27T17:26:00.949+0800: 261981.304: [CMS-concurrent-abortable-preclean-start]
CMS: abort preclean due to time 2016-08-27T17:26:06.159+0800: 261986.514: [CMS-concurrent-abortable-preclean: 5.197/5.210 secs] [Times: user=8.31 sys=0.66, real=5.21 secs]
2016-08-27T17:26:06.160+0800: 261986.515: Application time: 5.9951640 seconds
2016-08-27T17:26:06.161+0800: 261986.516: [GC[YG occupancy: 4756927 K (6606080 K)]
2016-08-27T17:26:06.161+0800: 261986.516: [Rescan (parallel) , 18.1621480 secs]
2016-08-27T17:26:24.323+0800: 262004.678: [weak refs processing, 0.0000750 secs
2016-08-27T17:26:24.323+0800: 262004.678: [class unloading, 0.0069760 secs
2016-08-27T17:26:24.330+0800: 262004.685: [scrub symbol table, 0.0040020 secs
2016-08-27T17:26:24.334+0800: 262004.689: [scrub string table, 0.0009240 secs] [1 CMS-remark: 2202742K(3145728K)] 6959670K(9751808K), 18.1769610 secs] [Times: user=71.37 sys=0.06, real=18.18 secs]
2016-08-27T17:26:24.338+08002016-08-27T17:26:24.338+0800: : 262004.693: Application time: 0.0002080 seconds
262004.693: [CMS-concurrent-sweep-start]
2016-08-27T17:26:24.347+0800: 262004.702: Application time: 0.0079820 seconds
2016-08-27T17:26:24.868+0800: 262005.223: Application time: 0.5186580 seconds
{Heap before GC invocations=23680 (full 19):
par new generation total 6606080K, used 5899299K [0x0000000568000000, 0x0000000728000000, 0x0000000728000000)
eden space 5872128K, 99% used [0x0000000568000000, 0x00000006ce5d44b8, 0x00000006ce680000)
from space 733952K, 3% used [0x00000006ce680000, 0x00000006d01b4910, 0x00000006fb340000)
to space 733952K, 0% used [0x00000006fb340000, 0x00000006fb340000, 0x0000000728000000)
concurrent mark-sweep generation total 3145728K, used 1891716K [0x0000000728000000, 0x00000007e8000000, 0x00000007e8000000)
concurrent-mark-sweep perm gen total 393216K, used 37361K [0x00000007e8000000, 0x0000000800000000, 0x0000000800000000)
2016-08-27T17:26:24.870+0800: 262005.225: [GC2016-08-27T17:26:24.870+0800: 262005.225: [ParNew: 5899299K->56108K(6606080K), 0.0580910 secs] 7791015K->1949708K(9751808K), 0.0584000 secs] [Times: user=0.22 sys=0.01, real=0.05 secs]
Heap after GC invocations=23681 (full 19):
par new generation total 6606080K, used 56108K [0x0000000568000000, 0x0000000728000000, 0x0000000728000000)
eden space 5872128K, 0% used [0x0000000568000000, 0x0000000568000000, 0x00000006ce680000)
from space 733952K, 7% used [0x00000006fb340000, 0x00000006fea0b1f8, 0x0000000728000000)
to space 733952K, 0% used [0x00000006ce680000, 0x00000006ce680000, 0x00000006fb340000)
concurrent mark-sweep generation total 3145728K, used 1893599K [0x0000000728000000, 0x00000007e8000000, 0x00000007e8000000)
concurrent-mark-sweep perm gen total 393216K, used 37361K [0x00000007e8000000, 0x0000000800000000, 0x0000000800000000)
}
重新標記居然用了18s(對應這一段GC日誌:[1 CMS-remark: 2202742K(3145728K)] 6959670K(9751808K), 18.1769610 secs],另外,由於是多核心CPU的服務器,所以Times的real要遠小於user的值,這個remark階段是多線程並行的)!重新標記的時候,old區域的大小是固定的(這裏設置成old區域的70%),按理說每次remark的時間應該都差不多才對,但是查了很多cms old gc日誌,發現高峯期和低峯期remark的時間相差太大,兩者的區別也只有eden區域,因爲我這裏eden的設置是比較大的,高峯期的時候,一次cms old從初始化標記開始,到remark之間,即併發標記階段,用戶程序會和gc線程是併發執行的。另外,由於我們程序的TPS比較高,我們再看一下GC日誌,整個併發階段的時間跨度爲從2016-08-27T17:26:00.165+0800到2016-08-27T17:26:24.330+0800,總計24秒左右,那麼,這個過程中肯定會新產生大量的新的對象。
如果remark之前能把eden區域的對象都清理一遍,那remark時,需要掃描的對象將會少很多很多對吧?cms正好有這麼個參數-XX:+CMSScavengeBeforeRemark,這玩意的意思是在remark之前,來一次yong gc,恰好滿足我們的要求,加了這個參數之後,對應的jvm參數爲:
-Xms14g -Xmx14g -Xss512k -XX:PermSize=384m -XX:MaxPermSize=384m -XX:NewSize=12g -XX:MaxNewSize=12g
-XX:SurvivorRatio=18 -XX:MaxDirectMemorySize=2g -XX:+UseParNewGC -XX:ParallelGCThreads=4
-XX:MaxTenuringThreshold=15 -XX:+CMSScavengeBeforeRemark -XX:+UseConcMarkSweepGC -XX:+DisableExplicitGC
-XX:+UseCMSInitiatingOccupancyOnly -XX:+ScavengeBeforeFullGC -XX:+UseCMSCompactAtFullCollection
-XX:+CMSParallelRemarkEnabled -XX:CMSFullGCsBeforeCompaction=9 -XX:CMSInitiatingOccupancyFraction=70
效果如下:
發現old gc的時間縮小到原來cms的1/100,竊喜。接下來分析gc日誌,來驗證我的猜想
{Heap before GC invocations=4644 (full 6):
par new generation total 11953792K, used 11384636K [0x0000000468000000, 0x0000000768000000, 0x0000000768000000)
eden space 11324672K, 99% used [0x0000000468000000, 0x000000071b30eb48, 0x000000071b340000)
from space 629120K, 9% used [0x000000071b340000, 0x000000071ee004e0, 0x00000007419a0000)
to space 629120K, 0% used [0x00000007419a0000, 0x00000007419a0000, 0x0000000768000000)
concurrent mark-sweep generation total 2097152K, used 1464467K [0x0000000768000000, 0x00000007e8000000, 0x00000007e8000000)
concurrent-mark-sweep perm gen total 393216K, used 37291K [0x00000007e8000000, 0x0000000800000000, 0x0000000800000000)
2016-08-28T10:46:09.846+0800: 68434.688: [GC2016-08-28T10:46:09.846+0800: 68434.688: [ParNew: 11384636K->61763K(11953792K), 0.0716150 secs] 12849103K->1528727K(14050944K), 0.0719060 secs] [Times: user=0.28 sys=0.00, real=0.07 secs]
Heap after GC invocations=4645 (full 6):
par new generation total 11953792K, used 61763K [0x0000000468000000, 0x0000000768000000, 0x0000000768000000)
eden space 11324672K, 0% used [0x0000000468000000, 0x0000000468000000, 0x000000071b340000)
from space 629120K, 9% used [0x00000007419a0000, 0x00000007455f0dd8, 0x0000000768000000)
to space 629120K, 0% used [0x000000071b340000, 0x000000071b340000, 0x00000007419a0000)
concurrent mark-sweep generation total 2097152K, used 1466964K [0x0000000768000000, 0x00000007e8000000, 0x00000007e8000000)
concurrent-mark-sweep perm gen total 393216K, used 37291K [0x00000007e8000000, 0x0000000800000000, 0x0000000800000000)
}
2016-08-28T10:46:19.590+0800: 68444.431: Application time: 9.6715460 seconds
{Heap before GC invocations=4645 (full 6):
par new generation total 11953792K, used 11384705K [0x0000000468000000, 0x0000000768000000, 0x0000000768000000)
eden space 11324672K, 99% used [0x0000000468000000, 0x000000071b18f768, 0x000000071b340000)
from space 629120K, 9% used [0x00000007419a0000, 0x00000007455f0dd8, 0x0000000768000000)
to space 629120K, 0% used [0x000000071b340000, 0x000000071b340000, 0x00000007419a0000)
concurrent mark-sweep generation total 2097152K, used 1466964K [0x0000000768000000, 0x00000007e8000000, 0x00000007e8000000)
concurrent-mark-sweep perm gen total 393216K, used 37291K [0x00000007e8000000, 0x0000000800000000, 0x0000000800000000)
2016-08-28T10:46:19.591+0800: 68444.433: [GC2016-08-28T10:46:19.591+0800: 68444.433: [ParNew: 11384705K->69180K(11953792K), 0.0728020 secs] 12851669K->1538700K(14050944K), 0.0730170 secs] [Times: user=0.27 sys=0.01, real=0.07 secs]
Heap after GC invocations=4646 (full 6):
par new generation total 11953792K, used 69180K [0x0000000468000000, 0x0000000768000000, 0x0000000768000000)
eden space 11324672K, 0% used [0x0000000468000000, 0x0000000468000000, 0x000000071b340000)
from space 629120K, 10% used [0x000000071b340000, 0x000000071f6cf378, 0x00000007419a0000)
to space 629120K, 0% used [0x00000007419a0000, 0x00000007419a0000, 0x0000000768000000)
concurrent mark-sweep generation total 2097152K, used 1469519K [0x0000000768000000, 0x00000007e8000000, 0x00000007e8000000)
concurrent-mark-sweep perm gen total 393216K, used 37291K [0x00000007e8000000, 0x0000000800000000, 0x0000000800000000)
}
2016-08-28T10:46:19.666+0800: 68444.508: Application time: 0.0019110 seconds
2016-08-28T10:46:19.667+0800: 68444.509: [GC [1 CMS-initial-mark: 1469519K(2097152K)] 1545525K(14050944K), 0.0869600 secs] [Times: user=0.09 sys=0.00, real=0.08 secs]
2016-08-28T10:46:19.755+0800: 68444.597: [CMS-concurrent-mark-start]
2016-08-28T10:46:20.418+0800: 68445.260: [CMS-concurrent-mark: 0.663/0.663 secs] [Times: user=1.47 sys=0.24, real=0.66 secs]
2016-08-28T10:46:20.418+0800: 68445.260: [CMS-concurrent-preclean-start]
2016-08-28T10:46:20.438+0800: 68445.280: [CMS-concurrent-preclean: 0.018/0.020 secs] [Times: user=0.04 sys=0.01, real=0.02 secs]
2016-08-28T10:46:20.438+0800: 68445.280: [CMS-concurrent-abortable-preclean-start]
2016-08-28T10:46:24.542+0800: 68449.384: [CMS-concurrent-abortable-preclean: 4.090/4.104 secs] [Times: user=8.60 sys=1.40, real=4.10 secs]
2016-08-28T10:46:24.543+0800: 68449.385: Application time: 4.7880220 seconds
// 這裏在remark之前進行一次young gc
=====================yong gc開始=====================
2016-08-28T10:46:24.544+0800: 68449.386: [GC[YG occupancy: 5803756 K (11953792 K)]{Heap before GC invocations=4646 (full 7):
par new generation total 11953792K, used 5803756K [0x0000000468000000, 0x0000000768000000, 0x0000000768000000)
eden space 11324672K, 50% used [0x0000000468000000, 0x00000005c602bd88, 0x000000071b340000)
from space 629120K, 10% used [0x000000071b340000, 0x000000071f6cf378, 0x00000007419a0000)
to space 629120K, 0% used [0x00000007419a0000, 0x00000007419a0000, 0x0000000768000000)
concurrent mark-sweep generation total 2097152K, used 1469519K [0x0000000768000000, 0x00000007e8000000, 0x00000007e8000000)
concurrent-mark-sweep perm gen total 393216K, used 37291K [0x00000007e8000000, 0x0000000800000000, 0x0000000800000000)
2016-08-28T10:46:24.544+0800: 68449.386: [GC2016-08-28T10:46:24.544+0800: 68449.386: [ParNew: 5803756K->70533K(11953792K), 0.0668770 secs] 7273276K->1542365K(14050944K), 0.0669610 secs] [Times: user=0.25 sys=0.01, real=0.07 secs]
Heap after GC invocations=4647 (full 7):
par new generation total 11953792K, used 70533K [0x0000000468000000, 0x0000000768000000, 0x0000000768000000)
eden space 11324672K, 0% used [0x0000000468000000, 0x0000000468000000, 0x000000071b340000)
from space 629120K, 11% used [0x00000007419a0000, 0x0000000745e81738, 0x0000000768000000)
to space 629120K, 0% used [0x000000071b340000, 0x000000071b340000, 0x00000007419a0000)
concurrent mark-sweep generation total 2097152K, used 1471831K [0x0000000768000000, 0x00000007e8000000, 0x00000007e8000000)
concurrent-mark-sweep perm gen total 393216K, used 37291K [0x00000007e8000000, 0x0000000800000000, 0x0000000800000000)
}
=====================yong gc結束,開始remark=================
2016-08-28T10:46:24.611+0800: 68449.453: [Rescan (parallel) , 0.0392690 secs]
2016-08-28T10:46:24.650+0800: 68449.492: [weak refs processing, 0.0001190 secs
2016-08-28T10:46:24.650+0800: 68449.492: [class unloading, 0.0072200 secs]
2016-08-28T10:46:24.658+0800: 68449.500: [scrub symbol table, 0.0083430 secs]
2016-08-28T10:46:24.666+0800: 68449.508: [scrub string table, 0.0011760 secs] [1 CMS-remark: 1471831K(2097152K)] 1542365K(14050944K), 0.1264420 secs] [Times: user=0.42 sys=0.01, real=0.13 secs]
2016-08-28T10:46:24.671+0800: 68449.513: [CMS-concurrent-sweep-start]
2016-08-28T10:46:24.672+0800: 68449.514: Application time: 0.0018070 seconds
2016-08-28T10:46:26.388+0800: 68451.230: [CMS-concurrent-sweep: 1.714/1.717 secs] [Times: user=3.70 sys=0.58, real=1.72 secs]
2016-08-28T10:46:26.388+0800: 68451.230: [CMS-concurrent-reset-start]
2016-08-28T10:46:26.396+0800: 68451.238: [CMS-concurrent-reset: 0.007/0.007 secs] [Times: user=0.02 sys=0.00, real=0.01 secs]
2016-08-28T10:46:34.434+0800: 68459.276: Application time: 9.7600810 seconds
可以看到,在remark之前,來一次yong gc,eden區域從5803756k降到70533k,給這個清理效果點個贊。這些空間如果不消除,那麼cms將會在這些空間上進行非常耗時的標記,最後再看看remark的時間([1 CMS-remark: 1471831K(2097152K)] 1542365K(14050944K), 0.1264420 secs]),降到0.1264420s,和原來相比,整整一百倍的提高。
總結:
最後,對於長連接,push一類的海量服務端應用,16G內存8核心,推薦的JVM參數如下(僅供參考):
jdk1.7
-Xms13g -Xmx13g -Xss512k -XX:PermSize=384m -XX:MaxPermSize=384m -XX:NewSize=12g -XX:MaxNewSize=12g
-XX:SurvivorRatio=18 -XX:MaxDirectMemorySize=2g -XX:+UseParNewGC -XX:ParallelGCThreads=4
-XX:MaxTenuringThreshold=15 -XX:+CMSParallelRemarkEnabled -XX:+CMSScavengeBeforeRemark
-XX:+UseConcMarkSweepGC -XX:+DisableExplicitGC -XX:+UseCMSInitiatingOccupancyOnly
-XX:CMSInitiatingOccupancyFraction=70 -XX:+ScavengeBeforeFullGC -XX:+UseCMSCompactAtFullCollection
-XX:CMSFullGCsBeforeCompaction=9 -XX:+CMSClassUnloadingEnabled -XX:CMSInitiatingPermOccupancyFraction=70
-XX:+ExplicitGCInvokesConcurrent -XX:+PrintGCDetails -XX:+PrintGCDateStamps -XX:+PrintGCApplicationConcurrentTime
-XX:+PrintHeapAtGC -Xloggc:/data/applogs/heap_trace.txt -XX:-HeapDumpOnOutOfMemoryError
-XX:HeapDumpPath=/data/applogs/HeapDumpOnOutOfMemoryError
jdk1.8
-Xms13g -Xmx13g -Xss512k -XX:MetaspaceSize=384m -XX:MaxMetaspaceSize=384m -XX:NewSize=11g -XX:MaxNewSize=11g
-XX:SurvivorRatio=18 -XX:MaxDirectMemorySize=2g -XX:+UseParNewGC -XX:ParallelGCThreads=4
-XX:MaxTenuringThreshold=15 -XX:+UseConcMarkSweepGC -XX:+DisableExplicitGC -XX:+UseCMSInitiatingOccupancyOnly
-XX:+ScavengeBeforeFullGC -XX:+CMSScavengeBeforeRemark -XX:+CMSParallelRemarkEnabled
-XX:CMSInitiatingOccupancyFraction=70 -XX:+CMSClassUnloadingEnabled -XX:SoftRefLRUPolicyMSPerMB=0
-XX:-ReduceInitialCardMarks -XX:+CMSClassUnloadingEnabled -XX:+ExplicitGCInvokesConcurrent -XX:+PrintGCDetails
-XX:+PrintGCDateStamps -XX:+PrintGCApplicationConcurrentTime -XX:+PrintHeapAtGC -Xloggc:/data/applogs/heap_trace.txt
-XX:-HeapDumpOnOutOfMemoryError -XX:HeapDumpPath=/data/applogs/HeapDumpOnOutOfMemoryError
這樣可以保證大多數對象在young區域就能被銷燬,即使一些對象到了old區,也會被在remark之前yong gc清理掉,從而儘可能降低cms gc的remark這個需要完全STW的階段的時間消耗。