JVM內(nèi)存診斷方法和工具

現(xiàn)狀：系統(tǒng)運(yùn)行一段時(shí)間后，速度開(kāi)頭變慢，而且越來(lái)越嚴(yán)峻；FullGC過(guò)于頻繁，使得系統(tǒng)中斷次數(shù)越來(lái)越多；處理方法：在JVM中增加GC輸出，在tomcat/bin/catalina.sh中增加如下設(shè)置JAVA_OPTS=“$JAVA_OPTS -verbose:gc -Xloggc:/var/log/gc.log -XX:+PrintGCDetails-XX:+PrintGCTimeStamps-XX:+PrintHeapAtGC-XX:+PrintTenuringDistribution“，JVM會(huì)自動(dòng)gc日志，格式如下：{HeapbeforeGCinvocations=35(full2):PSYoungGen total 1047552K, used 1046528K [0xb42a0000, 0xf42a0000,0xf42a0000)edenspace1046528K,100%used[0xb42a0000,0xf40a0000,0xf40a0000)fromspace1024K,0%used[0xf41a0000,0xf41a0000,0xf42a0000)to space1024K,0%used[0xf40a0000,0xf40a0000,0xf41a0000)PSOldGen total2097152K, used 277656K [0x342a0000, 0xb42a0000,0xb42a0000)objectspace2097152K,13%used[0x342a0000,0x451c6000,0xb42a0000)PSPermGen total 65536K, used 21366K [0x142a0000, 0x182a0000,0x342a0000)objectspace65536K,32%used[0x142a0000,0x1577d920,0x182a0000)333342.537:[GCDesiredsurvivorsize1048576bytes,newthreshold1(max15)[PSYoungGen: 1046528K->1024K(1047552K)] 1324184K->400132K(3144704K),0.2424250secs][Times:user=1.92sys=0.00,real=0.24secs]HeapafterGCinvocations=35(full2):PSYoungGen total 1047552K, used 1024K [0xb42a0000, 0xf42a0000,0xf42a0000)edenspace1046528K,0%used[0xb42a0000,0xb42a0000,0xf40a0000)fromspace1024K,100%used[0xf40a0000,0xf41a0000,0xf41a0000)to space1024K,0%used[0xf41a0000,0xf41a0000,0xf42a0000)PSOldGen total2097152K, used 399108K [0x342a0000, 0xb42a0000,0xb42a0000)objectspace2097152K,19%used[0x342a0000,0x4c861010,0xb42a0000)PSPermGen total 65536K, used 21366K [0x142a0000, 0x182a0000,0x342a0000)objectspace65536K,32%used[0x142a0000,0x1577d920,0x182a0000)}JVMHEAP的構(gòu)造如下：GC有兩種：rC：對(duì)于生成的對(duì)象，都放在n中；當(dāng)n布滿時(shí)〔小孩太多了C將old空間中；fullGColdfullGC。fullGCold，young里面大局部垃圾block。jmapJVMheap使用狀況：jmap-heap4306AttachingtoprocessID4306,pleasewait...Debuggerattachedsuccessfully.Servercompilerdetected.JVMversionis16.3-b01usingthread-localobjectallocation.ParallelGCwith8thread(s)HeapConfiguration:MinHeapFreeRatio=40MaxHeapFreeRatio=70MaxHeapSize =3221225472(3072.0MB)NewSize =4194304(4.0MB)MaxNewSize =4294901760(4095.9375MB)OldSize =4194304(4.0MB)NewRatio =2SurvivorRatio =1000PermSize =67108864(64.0MB)MaxPermSize =536870912(512.0MB)HeapUsage:PSYoungGenerationEdenSpace:capacity=1071644672(1022.0MB)used =662329344(631.646484375MB)free =409315328(390.353515625MB)61.80493976272023%usedFromSpace:capacity=1048576(1.0MB)used =1048576(1.0MB)free =0(0.0MB)100.0%usedToSpace:capacity=1048576(1.0MB)used =0(0.0MB)free =1048576(1.0MB)0.0%usedPSOldGenerationcapacity=2147483648(2048.0MB)used =713975824(680.9004058837891MB)free =1433507824(1367.099594116211MB)33.24709013104439%usedPSPermGenerationcapacity=67108864(64.0MB)used =21879232(20.86566162109375MB)free =45229632(43.13433837890625MB)32.602596282958984%usedMinorGCNewRatio來(lái)把握new空間的大小。FullGC比較耗時(shí)，對(duì)系統(tǒng)影響較大。假設(shè)覺(jué)察幾次FullGC后，內(nèi)存使用沒(méi)有降下來(lái)，則有可能存在內(nèi)存泄露：catgc.log|grep“FullGC“2581.257: [Full GC [PSYoungGen: 640K->0K(589248K)] [PSOldGen:664345K->207093K(699072K)] 664985K->207093K(1288320K) [PSPermGen:30828K->30828K(131072K)],1.2023230secs][Times:user=1.19sys=0.01,real=1.20secs]2929.640: [Full GC [PSYoungGen: 64K->0K(645824K)] [PSOldGen:676059K->185422K(699072K)] 676123K->185422K(1344896K) [PSPermGen:30962K->30962K(131072K)],1.0784410secs][Times:user=1.08sys=0.00,real=1.07secs]3465.263: [Full GC [PSYoungGen: 640K->0K(564096K)] [PSOldGen:676009K->208685K(699072K)] 676649K->208685K(1263168K) [PSPermGen:30985K->30985K(131072K)],1.2440760secs][Times:user=1.24sys=0.00,real=1.24secs]3765.206:[FullGC[PSYoungGen:640K->0K(387648K)][PSOldGen:656238K->260889K(725952K)] 656878K->260889K(1113600K) [PSPermGen:31005K->30805K(131072K)],1.7598360secs][Times:user=1.76sys=0.00,real=1.76secs]4006.283: [Full GC [PSYoungGen: 640K->0K(549568K)] [PSOldGen:641960K->418960K(905152K)] 642600K->418960K(1454720K) [PSPermGen:30818K->30818K(131072K)],2.3026830secs][Times:user=2.30sys=0.00,real=2.30secs]4545.212: [Full GC [PSYoungGen: 640K->0K(472023K)] [PSOldGen:879051K->207236K(699072K)] 879691K->207236K(1171072K) [PSPermGen:30854K->30854K(131072K)],1.2098150secs][Times:user=1.19sys=0.02,real=1.21secs]6052.607: [Full GC [PSYoungGen: 64K->0K(573568K)] [PSOldGen:682860K->155937K(699072K)] 682924K->155937K(1272640K) [PSPermGen:30890K->30890K(131072K)],0.8968940secs][Times:user=0.89sys=0.00,real=0.90secs]HPDiagnostics是一個(gè)比較好用的工具，可以診斷出執(zhí)行時(shí)間較長(zhǎng)和頻繁的sql、執(zhí)行時(shí)間較長(zhǎng)的方法，還可以跟蹤內(nèi)存的使用狀況以及對(duì)象的數(shù)量〔Jprofilertomcat格外慢，不知道現(xiàn)在性能怎樣〕設(shè)置步驟如下：tomcatHpDiagnosticsDiagnostics分析器使用在 tomcat 中 加 入 如 下 設(shè) 置 ： JAVA_OPTS=“$JAVA_OPTS-javaagent:/opt/MercuryDiagnostics/JavaAgent/DiagnosticsAgent/lib/probeagent.jar-agentpath:/opt/MercuryDiagnostics/JavaAgent/DiagnosticsAgent/lib/x86-linux64/libjvmti.so“tomcat35000端口HPJavaAgentjavawebstart://server:35000即可連接FullGC后的對(duì)象數(shù)量，確定特別對(duì)象的數(shù)量。所以需要對(duì)內(nèi)部的規(guī)律有確定的了解，否則很難確定對(duì)象的數(shù)量存在特別。假設(shè)能確定到內(nèi)存泄露的對(duì)象，還需要確認(rèn)是哪個(gè)地方引用了這些對(duì)象，以至于無(wú)法釋放。通常需要留意的地方時(shí)一些Map、ThreadLocal、靜態(tài)變量等，web應(yīng)用中還需要留意sessionapplication中是否存在沒(méi)有去除的數(shù)據(jù)。分析代碼是一種方式，但是很多時(shí)候可能很難從代碼中定位。JVM自帶的工具jmap在這個(gè)時(shí)候可以派上大用場(chǎng)。使用如下命令可以生成dump信息：jmap-dump:live,format=b,file=heap20230114.bin3624Dumpingheapto/var/log/heap20230114.bin...Heapdumpfilecreatedjhatdump信息：jhat-J-mx1024mheap_snapshot.binReadingfromheap.bin...DumpfilecreatedFriDec3109:30:54CST2023Snapshotread,resolving...Resolving2474879objects...Chasing references, expect 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