GC Out of memory Error: GC overhead limit exceeded-蒲公英云

Problem

In the T/D project , I occurred a problem that “Out of memory Error: GC overhead limit exceeded”, when I request data on the browser.

Out of memory Error: GC overhead limit exceeded 预测OOM,在应用挂掉之前做最后挣扎，如保存现场保存数据。

Create gc.log

Firstly, I need read the gabage collection (GC) log file.But where is it?

We use the eclipse. right-click the java project , choose properties —> Run/Debug Settings —> run_7thonline —> Edit.. If there is not application, we can new an application, then edit it.

in the VM arguments we can config the VM includes GC like that:

-Dcatalina.home="D:\Tomcat6.0"  
-Dcatalina.base="D:\Tomcat6.0"  
-Djava.io.tmpdir="D:\Tomcat6.0\temp"  
-XX:-UseGCOverheadLimit
-Xmx1024m  -Xms564m 
-XX:+PrintGCDetails -Xloggc:E:GCLOG.log
-Dfile.encoding=UTF-8 
-Dsystem.rootLocation=E:\7thOnline\ 
-DefsnPackage.EfsnClass.languageLocale=en_US 
-DefsnPackage.EfsnClass.sendEmail=false 
-DefsnPackage.EfsnClass.cachePages=false  
-DefsnPackage.EfsnClass.reportReplaceError=true 
-DefsnPackage.EfsnClass.hideLanguageTag=true

Now we run the project , we can find the GCLOG.log at E:GCLOG.log.

We can see these data in the file.

1434.182: [Full GC [PSYoungGen: 170752K->18288K(339200K)] [PSOldGen: 1023999K->1023999K(1024000K)] 1194751K->1042288K(1363200K) [PSPermGen: 57173K->57173K(57472K)], 2.1048891 secs] [Times: user=2.09 sys=0.00, real=2.11 secs] 
1437.589: [Full GC [PSYoungGen: 170752K->22251K(339200K)] [PSOldGen: 1023999K->1023999K(1024000K)] 1194751K->1046251K(1363200K) [PSPermGen: 57173K->57173K(57472K)], 2.0801601 secs] [Times: user=2.06 sys=0.00, real=2.08 secs] 
1441.242: [Full GC [PSYoungGen: 170752K->0K(339200K)] [PSOldGen: 1023999K->1020329K(1024000K)] 1194751K->1020329K(1363200K) [PSPermGen: 57173K->57173K(57472K)], 2.5077113 secs] [Times: user=2.51 sys=0.00, real=2.51 secs] 
1445.061: [Full GC [PSYoungGen: 170752K->407K(339200K)] [PSOldGen: 1020329K->1023999K(1024000K)] 1191081K->1024407K(1363200K) [PSPermGen: 57173K->57173K(57472K)], 2.0983663 secs] [Times: user=2.09 sys=0.00, real=2.10 secs] 
1448.933: [Full GC [PSYoungGen: 170752K->12820K(339200K)] [PSOldGen: 1023999K->1023999K(1024000K)] 1194751K->1036820K(1363200K) [PSPermGen: 57173K->57173K(57472K)], 2.1792817 secs]

GC和Full GC代表gc的停顿类型，Full GC代表stop-the-world。箭头两边是gc前后的区空间大小，分别是young区、tenured区和perm区，括号里是该区的总大小。冒号前面是gc发生的时间，单位是秒，从jvm启动开始计算。DefNew代表Serial收集器，为Default New Generation的缩写，类似的还有PSYoungGen，代表Parallel Scavenge收集器。这样可以通过分析日志找到导致GC overhead limit exceeded的原因，通过调节相应的参数解决问题。

root cause

这里要查看是否有使用大内存的代码或死循环。

The parameter configuration of GC

Where can we find the parameter information of GC?

http://docs.oracle.com/cd/E22289\_01/html/821-1274/configuring-the-default-jvm-and-java-arguments.html

Configuring JVM Options

The following table summarizes the Java options that can have an impact onserver performance. Note that some of these options apply only to theSun JVM.

Condition	Option	Description
	-server	Selects server application runtime optimizations. The directory server will take longer to startand “warm up” but will be more aggressively optimized to produce higher throughput.
	-d64	For64-bit machines only. By default, the directory server selects a 32-bit JVM regardlessof the architecture. This options should be specified when a large JVM heapis required (greater than 4 Gytes) and the architecture is 64-bit.
	-Xms2G -Xmx2G	Selects the initialand maximum memory sizes available to the JVM, respectively. These values are usedfor the JVM heap, which reserves memory for the directory server and itsdatabase (DB) cache (or caches if more than one). Increasing the amount ofmemory available can improve performance, but increasing it to too high a valuecan have a detrimental effect in the form of longer pauses for fullgarbage collection runs. Therefore, the initial and maximum sizes should be set tothe same value. As a general guideline, take a look atthe size of the Oracle Berkeley Java Edition (JE) database folders ( instance-dir/OUD/db/userRoot). Basedon the folders’ combined size, determine how much memory you want to reservefor the DB cache. After determining this value, tune the local DB back-endproperties, db-cache-percent or db-cache-size and other JVM options appropriately. Be careful to allowadditional memory for the server runtime. For example, if you have a singledatabase of 1 Gbyte, which you want to store entirely in memory, thena 2 Gbyte heap with 60% reserved for the DB cache shouldbe sufficient for efficient directory server performance. You can test this setup by preloadingthe database with the local database back end by using the preload-time-limitproperty. JVM heaps greater than 4 Gbytes require a 64-bit JVM.
	DisableExplicitGC	Prevents externalapplications from forcing expensive garbage collections. If you are using jstatd or otherRMI-based applications to monitor Oracle Unified Directory, you should consider using this optionin order to avoid unexpected pauses.
	-XX:NewSize=512M	In heavy throughput environments, you should consider usingthis option to increase the size of the JVM young generation. By default,the young generation is quite small, and high throughput scenarios can result ina large amount of generated garbage. This garbage collection, in turn, causes theJVM to inadvertently promote short-lived objects into the old generation.
Server Only	-XX:+UseConcMarkSweepGC	Selects the CMSgarbage collector. This garbage collector is set for low pause time. It will resultin a Java application that has a lower average throughput, but much shorterCPU-intensive garbage collections. This option is required in environments that have response timeconstraints.
	-XX:CMSInitiatingOccupancyFraction=70	Selects the level at which the collection is started. The default value is68%.
Offline Import Only	-XX:+UseParallelOldGC	Selects the parallel old generational garbage collector. This garbage collectoris set for high throughput. It will maximize the average throughput of the import-ldifutility at the cost of an occasional stop-the-world garbage collection, which is notas critical to imports.
	-XX:+PrintGCDetails	Prints the garbage collection details.
	-XX:+PrintGCTimeStamps	Prints the garbage collection time stampsto help with debugging.
Other Applications (for example, dsconfig)	-client	Selects client application run-time optimizations.The application will be faster to start and more responsive due to lowercompilation overheads.
	-Xms8m	Selects a low initial JVM heap size for an application.