Java Performance Tuning: Garbage Collection Outline

This document is a summary or outline of Sun's document: Tuning Garbage collection with the 1.4.2 Hotspot JVM located here: http://java.sun.com/docs/hotspot/gc1.4.2/ (link no longer works)

1.0 Introduction

• For many applications garbage collection performance is not significant
• Default collector should be first choice

2.0 Generations

• Most straightforward GC will just iterate over every object in the heap and determine if any other objects reference it.
• This gets really slow as the number of objects in the heap increase
• GC's therefor make assumptions about how your application runs.
• Most common assumption is that an object is most likely to die shortly after it was created: called infant mortality
• This assumes that an object that has been around for a while, will likely stay around for a while.
• GC organizes objects into generations (young, tenured, and perm) This is important!

2.1 Performance Considerations

• Ways to measure GC Performance
• Throughput - % of time not spent in GC over a long period of time.
• Pauses - app unresponsive because of GC
• Footprint - overall memory a process takes to execute
• Promptness - time between object death, and time when memory becomes available
• There is no one right way to size generations, make the call based on your applications usage.

2.2 Measurement

• Throughput and footprint are best measured using metrics particular to the application.
• Command line argument -verbose:gc output
  [GC 325407K->83000K(776768K), 0.2300771 secs]
• GC - Indicates that it was a minor collection (young generation). If it had said Full GC then that indicates that it was a major collection (tenured generation).
• 325407K - The combined size of live objects before garbage collection.
• 83000K - The combined size of live objects after garbage collection.
• (776768K) - the total available space, not counting the space in the permanent generation, which is the total heap minus one of the survivor spaces.
• 0.2300771 secs - time it took for garbage collection to occur.
• You can get more detailed output using -XX:+PrintGCDetails and -XX:+PrintGCTimeStamps

3 Sizing the Generations

• The -Xmx value determines the size of the heap to reserve at JVM initialization.
• The -Xms value is the space in memory that is committed to the VM at init. The JVM can grow to the size of -Xmx.
• The difference between -Xmx and -Xms is virtual memory (virtually committed)

3.1 Total Heap

• Total available memory is the most important factor affecting GC performance
• By default the JVM grows or shrinks the heap at each GC to keep the ratio of free space to live objects at each collection within a specified range.
• -XX:MinHeapFreeRatio - when the percentage of free space in a generation falls below this value the generation will be expanded to meet this percentage. Default is 40
• -XX:MaxHeapFreeRatio - when the percentage of free space in a generation exceeded this value the generation will shrink to meet this value. Default is 70
• For server applications
• Unless you have problems with pauses grant as much memory as possible to the JVM
• Set -Xms and -Xmx close to each other or equal for a faster startup (removes constant resizing of JVM). But if you make a poor choice the JVM can't compensate for it.
• Increase memory sa you increase # of processors because memory allocation can be parallelized.

3.2 The Young Generation

• The bigger the young generation the less minor GC's, but this implies a smaller tenured generation which increases the frequency of major collections.
• You need to look at your application and determine how long your objects live for to tune this.
• -XX:NewRatio=3 - the young generation will occupy 1/4 the overall heap
• -XX:NewSize - Size of the young generation at JVM init. Calculated automatically if you specify -XX:NewRatio
• -XX:MaxNewSize - The largest size the young generation can grow to (unlimited if this value is not specified at command line)

3.2.1 Young Generation Guarantee

• The -XX:SurvivorRatio option can be used to tune the number of survivor spaces.
• Not often important for performance
• -XX:SurvivorRatio=6 - each survivor space will be 1/8 the young generation
• If survivor spaces are too small copying collection overflows directly into the tenured generation.
• Survivor spaces too large uselessly empty
• -XX:+PrintTenuringDistribution - shows the threshold chosen by JVM to keep survivors half full, and the ages of objects in the new generation.
• Server Applications
• First decide the total amount of memory you can afford to give the virtual machine. Then graph your own performance metric against young generation sizes to find the best setting.
• Unless you find problems with excessive major collection or pause times, grant plenty of memory to the young generation.
• Increasing the young generation becomes counterproductive at half the total heap or less (whenever the young generation guarantee cannot be met).
• Be sure to increase the young generation as you increase the number of processors, since allocation can be parallelized.

4 Types of Collectors

• Everything to this point talks about the default garbage collector, there are other GC's you can use
• Throughput Collector - Uses a parallel version of the young generation collector
• -XX:+UseParallelGC
• Tenured collector is the same as in default
• Concurrent Low Pause Collector
• Collects tenured collection concurrently with the execution of the app.
• The app is paused for short periods during collection
• -XX:+UseConcMarkSweepGC
• To enable a parallel young generation GC with the concurrent GC add -XX:+UseParNewGC to the startup. Don't add -XX:+UseParallelGC with this option.
• Incremental Low Pause Collector
• Sometimes called Train Collector
• Collects a portion of the tenured generation at each minor collection.
• Tries to minimize large pause of major collections
• Slower than the default collector when considering overall throughput
• Good for client apps (my observation)
• -Xincgc
• Don't mix these options, JVM may not behave as expected.

4.1 When to use Throughput Collector

• Large number of processors
• Reduces serial execution time of app, by using multiple threads for GC
• App with lots of threads allocating objects should use this with a large young generation
• Server Applications (my observation)

4.2 The Throughput collector

• By default the throughput collector uses the number of CPU's as its value for number of GC threads.
• On a computer with one CPU it will not perform as well as the default collector
• Overhead from parallel execution (synchronization costs)
• With 2 CPU's the throughput collector performs as well as the default garbage collector.
• With more then 2 CPU's you can expect to see a reduction in minor GC pause times
• You can control the number of threads with -XX:ParallelGCThreads=n
• Fragmentation can occur
• Reduce GC threads
• Increase Tenured Generation size

4.2.1 Adaptive Sizing

• Keeps stats about GC times, allocation rates, and free space then sizes young and tenured generation to best fit the app.
• J2SE 1.4.1 and later
• -XX:+UseAdaptiveSizePolicy (on by default)

4.2.2 Aggressive Heap

• Attempts to make maximum use of physical memory for the heap
• Inspects computer resources (memory, num processors) and sets params optimal for long running memory allocation intensive jobs.
• Must have at least 256MB of RAM
• For lots of CPU's and RAM, but 1.4.1+ has shown improvements on 4-Way machines.
• -XX:+AggressiveHeap

4.3 When to use the Concurrent Low Pause Collector

• Apps that benefit from shorter GC pauses, and can share resources with GC during execution.
• Apps with large sets of long living data (tenured generation)
• Two or more processors
• Interactive apps with modest tenured generation size, and one CPU

4.4 The Concurrent Low Pause Collector

• Uses a separate GC thread to do parts of the major collection concurrently with the app threads.
• Pauses App threads in the beginning of a collection and toward the middle (longer pause in middle)
• The rest of the GC is in a single thread that runs at the same time as the app

4.4.1 Overhead of Concurrency

• Doesn't provide much of an advantage on single processor machines.
• Fragmentation can occur.
• Two processor machine eliminates pauses due to the GC thread.
• The more CPU's the advantages of concurrent collector increase.

4.4.2 Young Generation Guarantee

• There has to be enough contiguous space available in the tenured generation for all objects in the eden and one survivor space.
• A larger heap is needed compared to the default collector.
• Add the size of the young generation to the tenured generation.

4.4.3 Full Collections

• If the concurrent collector is unable to finish collecting the tenured generation before the tenured generation fills up, the application is paused and the collection is completed.
• When this happens you should make some adjustments to your GC params

4.4.4 Floating Garbage

• Floating Garbage - Objects that die while the GC is running (after they have been checked).
• Increase the tenured generation by 20% to reduce floating garbage.

4.4.5 Pauses

• First Pause - marks live objects - initial marking
• Second Pause - remarking phase - checks objects that were missed during the concurrent marking phase due to the concurrent execution of the app threads.

4.4.6 Concurrent Phases

• Concurrent Marking phase occurs between initial mark and remarking phase.
• Concurrent sweeping phase collects dead objects after the remarking phase.

4.4.7 Measurements with the Concurrent Collector

• Use -verbose:gc with -XX:+PrintGCDetails
• vCMS-initial-mark shows GC stats for the initial marking phase
• CMS-concurrent-mark - shows GC stats for concurrent marking phase.
• CMS-concurrent-sweep - shows stats for concurrent sweeping phase
• CMS-concurrent-preclean - stats for determining work that can be done concurrently
• CMS-remark - stats for the remarking phase.
• CMS-concurrent-reset - concurrent stuff is done, ready for next collection.

4.4.8 Parallel Minor Collection Options with Concurrent Collector

• -XX:+UseParNewGC - for multiprocessor machines, enables multi threaded young generation collection.
• -XX:+CMSParallelRemarkEnabled - reduce remark pauses

4.5 When to use the Incremental Low Pause Collector

• Use when you can afford to tradeoff longer and more frequent young generation GC pauses for shorter tenured generation pauses
• You have a large tenured generation
• Single Processor

4.6 The Incremental Low Pause Collector

• Minor collections same as default collector.
• Don't use try to use parallel GC with this collector
• Incrementally Collects parts of the tenured generation at each young collection.
• Tries to avoid long major collections by doing small chunks each minor collection.
• Can cause fragmentation of the heap. Sometimes need to increase tenured generation size compared to the default.
• There is some overhead required to maintain the position of the incremental collector. Less overhead than is required by the default collector.
• First try the default collector, and adjust heap sizing. If major pauses are too long try incremental.
• If the incremental collector can't collect the tenured generation fast enough you will run out of memory, try reducing the young generation.
• If young generation collections do not free any space, could be because of fragmentation. Increase tenured generation size.

4.6.1 Measurements with the Incremental Collector

• -verbose:gc and -XX:+PrintGCDetails
• Look for the Train: to see the stats for the incremental collection.

5 Other Considerations

• The permanent generation may be a factor on apps that dynamically generate and load many classes (JSP, CFM application servers)
• You may need to increase the MaxPermSize, eg: -XX:MaxPermSize=128m
• Apps that rely on finalization (finalize method, or finally clauses) will cause lag in garbage collection. This is a bad idea, use only for errorious situations.
• Explicit garbage collection calls (System.gc()) force a major collection. You can measure the effectiveness of these calls by disabling them with -XX:+DisableExplicitGC
• RMI garbage collection intervals can be controlled with
• -Dsun.rmi.dgc.client.gcInteraval=3600000
• -Dsun.rmi.dgc.server.gcInterval=3600000
• On Solaris 8+ you can enable libthreads, lightweight thread processes, these may increase thread performance.
• To enable add /usr/lib/lwp to LD_LIBRARY_PATH
• Soft References cleared less aggressively in server.
• -XX:SoftRefLRUPolicyMSPerMB=10000
• Default value is 1000, or one second per MB

6 Conclusion

• GC can be bottleneck in your app.

More Information

• FAQ: http://java.sun.com/docs/hotspot/gc1.4.2/faq.html
• Concurrent collector research paper: http://research.sun.com/techrep/2000/abstract-88.html
• HotSpot VM command line options: http://java.sun.com/docs/hotspot/VMOptions.html

Java Performance Books

• Java Performance Tuning Book ISBN: 0596003773
• Better Faster Lighter Java ISBN: 0596006764