The application server is a Java based process and requires a Java virtual machine (JVM) environment to run and support the Java applications running on the application server. You can configure the Java runtime environment to tune performance and system resource usage. This topic applies to the IBM Technology for Java Virtual Machine. Refer to the topic Tuning the Classic JVM if you are using the IBM Developer Kit for Java that is provided with the i5/OS product.
Use the enablejvm command, if you want to enable your application server profile to use a different JVM
A Java runtime environment provides the execution environment for Java based applications and servers such as WebSphere Application Server. Therefore the Java configuration plays a significant role in determining performance and system resource consumption for the product, and the applications that you are running.
The IBM Java 5.0 and newer versions include major improvements in virtual machine technology to provide significant performance and serviceability enhancements over IBM's earlier Java execution technology. Refer to the Web site http://www.ibm.com/software/webservers/appserv/was/performance.html for more information about this new technology.
Even though JVM tuning is dependent on the JVM provider you use, there are some general tuning concepts that apply to all JVMs. These general concepts include:
The following steps provide specific instructions on how to perform the following types of tuning for each JVM. The steps do not have to be performed in any specific order.
In some environments, such as a development environment, it is more important to optimize the startup performance of your application server rather than the runtime performance. In other environments, it is more important to optimize the runtime performance. By default, IBM virtual machines for Java are optimized for runtime performance, while HotSpot based JVMs are optimized for startup performance.
The Java Just-In-Time (JIT) compiler has a big impact on whether startup or runtime performance is optimized. The initial optimization level that the compiler uses influences the length of time it takes to compile a class method, and the length of time it takes to start the server. For faster startups, you should reduce the initial optimization level that the compiler uses. However if you reduce the initial optimization level, the runtime performance of your applications might be degraded because the class methods are now compiled at a lower optimization level.
This setting influences how the IBM virtual machine for Java uses a lower optimization level for class method compiles. A lower optimization level provides for faster server startups, but lowers runtime performance. If this parameter is not specified, the IBM virtual machine for Java defaults to starting with a high initial optimization level for compiles, which results in faster runtime performance, but slower server starts.
Default: | High initial compiler optimization level |
Recommended: | High initial compiler optimization level |
Usage: | -Xquickstart provides faster server startup. |
The heap size settings control garbage collection in the JVM that is provide with i5/OS. The initial heap size is a threshold that triggers new garbage collection cycles. For example, if the initial heap size is 10 MB, a new collection cycle is triggered as soon as the JVM detects that since the last collection cycle, 10 MB are allocated.
Smaller heap sizes result in more frequent garbage collections than larger heap sizes. If the maximum heap size is reached, the garbage collector stops operating asynchronously, and user threads are forced to wait for collection cycles to complete.
The maximum heap size can affect application performance. The maximum heap size specifies the maximum amount of object space the garbage collected heap can consume. If the maximum heap size is too small, performance might degrade significantly, or the application might receive out of memory errors when the maximum heap size is reached.
The IBM Developer Kit and Runtime Environment, Java2 Technology Edition, Version 5.0 Diagnostics Guide, that is available on the developerWorks Web site, provides additional information on tuning the heap size.
To use the administrative console to configure the heap size:
You can also specify values for both fields if you need to adjust both settings.
The Initial heap size setting specifies, in megabytes, the amount of storage that is allocated for the JVM heap when the JVM starts. The Maximum heap size setting specifies, in megabytes, the maximum amount of storage that can be allocated to the JVM heap. Both of these settings have a significant effect on performance.
The illustration represents three CPU profiles, each running a fixed workload with varying Java heap settings. In the middle profile, the initial and maximum heap sizes are set to 128MB. Four garbage collections occur. The total time in garbage collection is about 15% of the total run. When the heap parameters are doubled to 256MB, as in the top profile, the length of the work time increases between garbage collections. Only three garbage collections occur, but the length of each garbage collection is also increased. In the third profile, the heap size is reduced to 64MB and exhibits the opposite effect. With a smaller heap size, both the time between garbage collections and the time for each garbage collection are shorter. For all three configurations, the total time in garbage collection is approximately 15%. This example illustrates an important concept about the Java heap and its relationship to object utilization. There is always a cost for garbage collection in Java applications.
You can also use the following command line parameters to adjust these settings. These parameters apply to all supported JVMs and are used to adjust the minimum and maximum heap size for each application server or application server instance.
This setting controls the initial size of the Java heap. Properly tuning this parameter reduces the overhead of garbage collection, which improves server response time and throughput. For some applications, the default setting for this option might be too low, which causes a high number of minor garbage collections.
Default: | 50MB. This default value applies for both 31-bit and 64-bit configurations. |
Recommended: | Workload specific, but higher than the default. |
Usage: | -Xms256m sets the initial heap size to 256 megabytes. |
This setting controls the maximum size of the Java heap. Increasing this parameter increases the memory available to the application server, and reduces the frequency of garbage collection. Increasing this setting can improve server response time and throughput. However, increasing this setting also increases the duration of a garbage collection when it does occur. This setting should never be increased above the system memory available for the application server instance. Increasing the setting above the available system memory can cause system paging and a significant decrease in performance.
Default: | 256MB. This default value applies for both 31-bit and 64-bit configurations. |
Recommended: | Workload specific, but higher than the default, depending on the amount of available physical memory. |
Usage: | -Xmx512m sets the maximum heap size to 512 megabytes. |
This parameter can be used to allocate the heap using medium size pages, such as 64 KB. Using this virtual memory page size for the memory that an application requires can improve the performance and throughput of the application because of hardware efficiencies that are associated with a larger page size.
i5/OS and AIX® provide
rich support around 64 KB pages because 64 KB pages are intended to
be general purpose pages. 64 KB pages are easy to enable, and applications
might receive performance benefits when 64 KB pages are used. Starting
with Java 6 SR 7, the Java heap is allocated with 64K pages by default.
For Java 6 SR 6 or earlier, 4K pages is the default setting, This
setting can be changed without changing the operating system configuration.
However, it is recommended that you run your application servers in
a separate storage pool if you use of 64KB pages.
Recommended | Use 64 KB page size whenever possible. i5/OS POWER5+ systems, and i5/OS Version 6, Release 1, support a 64 KB page size. |
This parameter can be used to allocate the heap using 4 KB pages. Using this virtual memory page size for the memory that an application requires, instead of 64 KB, might negatively impact performance and throughput of the application because of hardware inefficiencies that are associated with a smaller page size.
Starting
with Java 6 SR 7, the Java heap is allocated with 64K pages by default.
For Java 6 SR 6 or earlier, 4K pages is the default setting, This
setting can be changed without changing the operating system configuration.
However, it is recommended that you run your application servers in
a separate storage pool if you use of 64KB pages.
Recommended | Use -Xlp64k instead of -Xlp4k whenever possible. |
You can use the Tivoli Performance Viewer to check You can use the Tivoli Performance Viewer to observe the counters for the JVM runtime. This information indicates whether the application is overusing objects. Refer to the topic Enabling the Java virtual machine profiler data for more information JVMPI counters.
The best result for the average time between garbage collections is at least 5-6 times the average duration of a single garbage collection. If you do not achieve this number, the application is spending more than 15 percent of its time in garbage collection.
If the information indicates a garbage collection bottleneck, there are two ways to clear the bottleneck. The most cost-effective way to optimize the application is to implement object caches and pools. Use a Java profiler to determine which objects to target. If you can not optimize the application, adding memory, processors and clones might help. Additional memory allows each clone to maintain a reasonable heap size. Additional processors allow the clones to run in parallel.
Memory leaks in the Java language are a dangerous contributor to garbage collection bottlenecks. Memory leaks are more damaging than memory overuse, because a memory leak ultimately leads to system instability. Over time, garbage collection occurs more frequently until the heap is exhausted and the Java code fails with a fatal out-of-memory exception. Memory leaks occur when an unused object has references that are never freed. Memory leaks most commonly occur in collection classes, such as Hashtable because the table always has a reference to the object, even after real references are deleted.
High workload often causes applications to crash immediately after deployment in the production environment. This is especially true for leaking applications where the high workload accelerates the magnification of the leakage and a memory allocation failure occurs.
Memory leak problems can manifest only after a period of time, therefore, memory leaks are found easily during long-running tests. Short running tests can lead to false alarms. It is sometimes difficult to know when a memory leak is occurring in the Java language, especially when memory usage has seemingly increased either abruptly or monotonically in a given period of time. The reason it is hard to detect a memory leak is that these kinds of increases can be valid or might be the intention of the developer. You can learn how to differentiate the delayed use of objects from completely unused objects by running applications for a longer period of time. Long-running application testing gives you higher confidence for whether the delayed use of objects is actually occurring.
In many cases, memory leak problems occur by successive repetitions of the same test case. The goal of memory leak testing is to establish a big gap between unusable memory and used memory in terms of their relative sizes. By repeating the same scenario over and over again, the gap is multiplied in a very progressive way. This testing helps if the number of leaks caused by the execution of a test case is so minimal that it is hardly noticeable in one run.
You can use repetitive tests at the system level or module level. The advantage with modular testing is better control. When a module is designed to keep the private module without creating external side effects such as memory usage, testing for memory leaks is easier. First, the memory usage before running the module is recorded. Then, a fixed set of test cases are run repeatedly. At the end of the test run, the current memory usage is recorded and checked for significant changes. Remember, garbage collection must be suggested when recording the actual memory usage by inserting System.gc() in the module where you want garbage collection to occur, or using a profiling tool, to force the event to occur.
Some memory leak problems can occur only when there are several threads running in the application. Unfortunately, synchronization points are very susceptible to memory leaks because of the added complication in the program logic. Careless programming can lead to kept or unreleased references. The incident of memory leaks is often facilitated or accelerated by increased concurrency in the system. The most common way to increase concurrency is to increase the number of clients in the test driver.
Also, look at the difference between the number of objects allocated and the number of objects freed. If the gap between the two increases over time, there is a memory leak.
Heap consumption indicating a possible leak during a heavy workload (the application server is consistently near 100% CPU utilization), yet appearing to recover during a subsequent lighter or near-idle workload, is an indication of heap fragmentation. Heap fragmentation can occur when the JVM can free sufficient objects to satisfy memory allocation requests during garbage collection cycles, but the JVM does not have the time to compact small free memory areas in the heap to larger contiguous spaces.
Another form of heap fragmentation occurs when small objects (less than 512 bytes) are freed. The objects are freed, but the storage is not recovered, resulting in memory fragmentation until a heap compaction has been run.
Examining Java garbage collection gives insight to how the application is utilizing memory. Garbage collection is a Java strength. By taking the burden of memory management away from the application writer, Java applications are more robust than applications written in languages that do not provide garbage collection. This robustness applies as long as the application is not abusing objects. Garbage collection normally consumes from 5% to 20% of total execution time of a properly functioning application. If not managed, garbage collection is one of the biggest bottlenecks for an application.
Monitoring garbage collection during the execution of a fixed workload, enables you to gain insight as to whether the application is over-utilizing objects. Garbage collection can even detect the presence of memory leaks.
You can use JVM settings to configure the type and behavior of garbage collection. When the JVM cannot allocate an object from the current heap because of lack of contiguous space, the garbage collector is invoked to reclaim memory from Java objects that are no longer being used. Each JVM vendor provides unique garbage collector policies and tuning parameters.
You can use the Verbose garbage collection setting in the administrative console to enable garbage collection monitoring. The output from this setting includes class garbage collection statistics. The format of the generated report is not standardized between different JVMs or release levels.
To ensure meaningful statistics, run a fixed workload until the application state is steady. It usually takes several minutes to reach a steady state.
You can also use object statistics in the Tivoli Performance Viewer to monitor garbage collection statistics.
To adjust your JVM garbage collection settings:
For more information about the –X options for the different JVM garbage collectors refer to the following:
By default, the JVM unloads a class from memory whenever there are no live instances of that class left. The overhead of loading and unloading the same class multiple times, can decrease performance.
When this option is used, if you have to redeploy an application, you should always restart the application server to clear the classes and static data from the pervious version of the application.
Default: | Class garbage collection is enabled. |
Recommended: | Do not disable class garbage collection. |
Usage: | Xnoclassgc disables class garbage collection. |
Information | Value |
---|---|
Default | com.ibm.cacheLocalHost = false |
Recommended | com.ibm.cacheLocalHost = true (see description) |
Usage | ![]() ![]() sep2012 |
If you are using the wsadmin command wsadmin -conntype none in local mode, you must set the config_consistency_check property to false before issuing this command.
If you use DB2, consider disabling SafepointPolling technology in the HP virtual machine for Java for HP-UX. Developed to ensure safepoints for Java threads, SafepointPolling technology generates a signal that can interfere with the signal between WebSphere Application Server and a DB2 database. When this interference occurs, database deadlocks often result. Prevent the interference by starting the JVM with the -XX:-SafepointPolling option, which disables SafepointPolling during runtime.
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