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Monday, 3 February 2014

4 mains causes of Long Garbage Collection Pause?

So a long GC Pause can have a direct impact on response time and throughput. But there are some causes for long GC pause which you can check and avoid to increase performance. They are as follows:-

1)Insufficient heap size:-

If heap size is small then the size required then the allocation requests for new objects fail and the JVM needs to invoke garbage collections in an attempt to reclaim space for the allocations. So these frequent Full GCs cause long pauses in the application run.

To avoid long GC pause by this identify the average footprint of the application and then specify the heap size accordingly.

2)Fragmentation in the heap:-

GCs can occur more frequently due to fragmentation in the Java Heap and also sometimes causing long pauses in the GCs. This is more probable in the case of Concurrent Mark Sweep collector, also known as CMS, where the tenured generation space is not compacted with the concurrent collections. For more please go through the previous post about heap compaction

3)Explicit System Gcs:-

Full GCs could be coming from the System.gc() calls from some class in the application or from a some third party module. So check or disable by adding value in JVM parameter.

4)Other OS activities happening at the time of GC:-

Sometimes the OS activities such as the swap space or networking activity happening at the time when GC is taking place can make the GC pauses last much longer. These pauses can be of the order of few seconds to some minutes.

Apart from swapping, we should monitor if there is any i/o or network activity happening during the long GC pauses. These can be monitored using iostat and netstat tools. It is also helpful to see the CPU statistics with the mpstat tool to figure out if enough CPU resources were available during the GC pauses

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How to Reduce Garbage-Collection Pause Time?

Long GC Pause can have a direct impact on response time and throughput.There are two general ways

to reduce Long garbage-collection pause time and the impact it has on application performance:

1)The garbage collection can be done using multiple CPUs and be executed in parallel. Although the application threads remain fully suspended during this time, the garbage collection can be

done in a fraction of the time, effectively reducing the suspension time.

2)The second approach is leave the application running, and execute garbage collection

concurrently with the application execution.

These two logical solutions have led to the development of serial, parallel, and concurrent

garbage-collection strategies, which represent the foundation of all existing Java garbage-

collection implementations.

The serial collector suspends the application threads and executes the mark-and-sweep algorithm in a single thread. It is the simplest and oldest form of garbage collection in Java and is still the default in the Oracle HotSpot JVM.

The parallel collector uses multiple threads to do its work. It can therefore decrease the GC

pause time by leveraging multiple CPUs. It is often the best choice for throughput applications.

The concurrent collector does the majority of its work concurrent with the application execution.

It has to suspend the application for only very short amounts of time. This has a big benefit for

response-time–sensitive applications, but has its own drawbacks. Such as the algorithm is more

complicated than the other two and consequently requires more CPU cycles and also requires more

fine tuning and has more configuration options than the other approaches

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Some Amazing fact about memory (petabyte,exabyte,zettabyte) And How much data exists on the Web?

How Application Performance get impacted by Garbage Collection?

As we seen in our earlier posts that garbage collection is fully dependent on number of objects alive. Because according to mark and sweep algorithm first it will mark all the alive object and then it will remove all other from memory. Means if heap is completely filled with dead objects it will take fraction of second to complete GC cycle. Garbage collector must suspend the execution of the application to ensure the integrity of the object trees.

So if we have more live objects it will take more time for GC and the suspension time will increase. Which will give a direct impact on response time and throughput. And will slow your application. This suspension off application threads to run the garbage collector is called garbage-collection pause or GC pause time. An application that spends 1% of its execution time on garbage collection will loose more than 20% throughput on a 32-processor system. If we increase the GC time to 2%, the overall throughput will drop by another 20%.

So a long GC Pause can have a direct impact on response time and throughput.So Now this becomes important to check all long GC pause to make application quick and decrease its response time . This can be done by checking all the possible case of long gc and try to eliminate them . To learn more about Causes of Long GC pause check

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