3/21/2023 0 Comments Songkong java heap![]() This is when tuning becomes extremely vendor specific (HP,IBM,Hotspot etc). You will see poor application response times, longer GC pauses and in some case ‘OutOfMemory’ errors. Heap fragmentation becomes an issue ONLY when your application requires to allocate memory for a large object (which will need contiguous blocks of memory). When heap is fragmented, GC will try to compact the heap which can result in a longer GC pause time for a heavily fragmented heapī. ![]() ![]() But there is couple of crucial things to consider.Ī. To some extent, you cannot avoid fragmentation – over time, heap will get fragmented. Heap gets fragmented when small and large objects are allocated in a mixed fashion that have various lifetimes. Profiling your application using tools like Jprobe will shed light. This can also happen due to a library you are using that is outside of your code. When you plot the heap graph, you would see a ‘staircase’ pattern, indicating a gradual leak (rather than a sudden spike). This results in the gory ‘OutOfMemory’ error. What this means is, after certain period, the heap will be filled with objects are being used by the application and GC will NOT be able to reclaim those. Memory leak means the application is allocating memory and holding on to them unnecessarily. Some JVMs like IBM have an option to specify a GC policy such as ‘optgcpause’ which optimizes GC for smaller pause time. This issue can also be code related so you need to analyze the code in parallel to tuning the heap. A minor GC works on new generation and a Major GC (Full GC) works on the entire heap. The heap also has a ‘tenured’ or ‘old generation’ ara where long living objects are stored. The idea is GC is will be quicker in ‘new generation’ since the entire heap does not have to be scanned. With generational heap, the heap has a special area called ‘nursery’ or ‘new generation’ that is used for short lived objects. Make sure you have ‘generational’ heap configured. In some cases, the CPU utilization of the Server can go up significantly as well. You will also hear from your customers about poor performance. When you plot the graph for GC Time (How to do this ? Answer at the end of this article), you will see longer duration (several seconds). Ideally each GC pause should be less than 500ms depending upon how often the GC runs. When GC takes a long time to complete the JVM pause time also becomes long resulting in very poor end user experience. In most cases, when GC is running, the entire JVM is paused. This can be a resource intensive process especially when the heap is big and it is filled to the brink. Poor application response time due to long Garbage Collection PausesĪs mentioned earlier GC is responsible for scanning the heap and clearing unused objects so that the memory can be reclaimed. May be you are processing something unnecessarily.Ģ. May be you are trying to pull millions of records from DB and process them at once. But some times, the code needs to be revisited to see why the demand is high in first place. You could try increasing the Heap size to see if you can live through the heavy weight transaction. ![]() This will be in contrast to a ‘gradual increase in heap. When you plot the heap usage graph (how to do this? Answer at the end of this article), you will most probably see a sudden spike in memory utilization, indicating a heavy weight transaction has just started. For example, if you are doing heavy weight activities like processing images/video or crunching big numbers. It is possible that you indeed are using all the objects i.e there is real demand for memory. This error simply means that the Heap is filled with objects that are being used/referenced by your application. This is when JVM runs out of Heap and GC is unable to reclaim memory to meet the demand of the application. ‘ OutOfMemory’ Error due to insufficient Heap Let me walk you through four of the most common Java Heap related issues and how to fix them.ġ. JVM periodically runs a process called ‘ Garbage collection’ (GC) which will scan the heap and clear objects that are no longer referenced by your application.One cannot request Garbage Collection on demand. As your application runs, it will gradually fill up Heap. You define the maximum memory that can be for the Heap by specifying ‘-Xmx’ java command line option (Example: ‘ -Xmx1024m‘, where m stands for Mega bytes). ![]() Java Heap is the memory used by your application to create and store objects. Out of the box default parameters are not enough. Care must be taken to tune the Heap related parameters that suit your application. Java heap related issues can cause severe damage to your application that will directly result in poor end user experience. ![]()
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