GC Heap/heap Size

180 pts.
Tags:
AS/400
Heap Size
iSeries applications
Java
WebSphere Application Server
Does anyone know what this message is and how to fix?

 /QEJBSVR/METALWEB JAVA USED 85% OF THE GC HEAP.

USED HEAP SIZE(KB) AND MAX HEAP(KB):222890 262144.

 

 



Software/Hardware used:
WebSphere Application Server V6.1 Express Iseries V6R1

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Discuss This Question: 3  Replies

 
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  • carlosdl
    GC is Java Garbage Collection. WebSphere includes a Heap monitor which issues a message on certain conditions, one of which was met in your server. Take a look at this: Tuning Java virtual machines
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  • Karens6
    Looked at link, not sure how to fix
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  • carlosdl
    From the Heap Monitor documentation: "...For the maximum Garbage Collection heap size, the default operation is to monitor the size of the JVM Garbage Collection heap so that it does not exceed 85, 90, or 95 percent of the maximum..." And this is from the tuning guide suggested above: "It is recommended that you do not change the maximum heap size. When the maximum heap size triggers a garbage collection cycle, the i5/OS JVM's garbage collection stops operating asynchronously. When this happens, the application server cannot process user threads until the garbage collection cycle ends, which significantly lowers performance." In other words, garbage collecion is executed when the GC threshold (inigial heap size) is reached and it works asynchronously, but if you set a maximum heap size and that maximum is reached, the garbage collector runs synchronously and all processes need to wait until it ends to continue running, and that is why it is not recommended to set a maximum size, unless there is a good reason to do it. In your case the heap monitor is saying that you have set a maximum heap size, and that your server was reaching a 85% of that maximum size at the time that message was logged/sent. This is from this guide: Tuning Java Garbage Collection and WebSphere on iSeries (which is also referenced in the Java tuning guide previously suggested): "In cases where memory is especially limited, it may be useful to set the maximum heap size... ...(in these cases) it's preferred to use the maximum heap size as a safety net to handle times of unexpected heap growth and ensure that the heap doesn't grow larger than the available memory. The GC threshold should be set so that this maximum size is never actually reached under normal circumstances." So, in summary, it is recommended not to set a maximum heap size (0 indicates that there is no maximum), but in some circumstances it could be a good option to do it, but if you do it, you should carefully choose an appropriate maximum value. If the message you received from the heap monitor is happening frequently then you might consider changing the maximum heap size, or removing such limit, but if that was a one-time message, it is possible that you don't need to take additional actions. The above pdf guide has good information on analyzing the GC performance, and offers good advice on how to tune it.
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