The JVM implementation needs space outside defined heap, let’s name it “ Native code area”. Larger heaps are not very common, though possible. Usually customers choose heap sizes (-Xms and –Xmx) between 2 GB and 8 GB per Java Server node. ➕ SAP recommends that the values for JVM parameters pairs -Xms and –Xmx, –XX:PermSize and –XX:MaxPermSize, –XX:NewSize and –XX:MaxNewSize should be set to same value – in this way heap areas will not resize at runtime, because the resize is associated with worst performance. ➕ SAP recommends to use the JVM parameters -XX:+UseConcMarkSweepGC and -XX:+UseParNewGC to enable parallel garbage collection algorithms. With the major (full) garbage collection the entire heap must be examined for unnecessary objects, that is why it is much slower compared to small garbage collection. On CPU 2.66GHz, -Xms=4096M –Xmx=4096M –XX:PermSize=512M –XX:MaxPermSize=512M –XX:NewSize=820M –XX:MaxNewSize=820M, the major (full) garbage collection takes about 9-10 seconds.
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ℹ Example for duration of full garbage collection: On CPU 2.66GHz, -Xms=4096M –Xmx=4096M –XX:PermSize=512M –XX:MaxPermSize=512M –XX:NewSize=820M –XX:MaxNewSize=820M, the minor (small) garbage collection takes about 0.8-0.9 seconds. ℹ Example for duration of small garbage collection Memory paging increases the duration of the garbage collection to unacceptably long execution times, and thus it is absolutely essential to have the complete java heap inside the main memory of the machine. With bigger heaps or with less powerful CPUs, the duration of both small and major garbage collection is longer. The garbage collection duration depends on the garbage collection algorithm, the size of the young, tenured and permanent generation area, the CPU processing speed, some specific influencing factors like implementation of finalize() method of java objects, and so on. performs the so-called “garbage collection”) using varies garbage collection algorithms, which improved over the years.īefore you proceed, if you are not familiar with the garbage collection mechanisms, please check. The Java Virtual Machine (JVM) manages objects de-allocation automatically (i.e.
To serve to this reality, Java Virtual Machine implements a concept of separate memory areas for different ages and types of objects/classes: young generation, old generation and permanent generation. – many objects and classes, once instantiated, remain in memory for a long time – most of the created objects become unused very soon JVM settings and number of Java server instances) based on defined application memory consumption metrics.īe aware that Java Memory Sizing is only relevant to memory-leaks-free applications – the java memory sizing approach helps to avoid only memory shortage situations, which are not leaks. This blog suggests a pragmatic approach to calculate the required memory for java applications (i.e. load test) for a concrete load profile, or even it is not calculated at all but later, in production, changes to JVM parameters are applied as reaction to already experienced java memory problems. Often the required java memory is calculated by applying some “guru-defined” ratio (e.g.”4GB per vCPU” or “4 GB memory per 1000 SAPS”) or is determined in experimental way (e.g.
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