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Let's talk about Garbage Collection

Garbage collection is the most famous (infamous) JVM mechanism and it dates back to Java 1.0. Every Java developer knows about its existence yet most of the time we wish we can ignore its behavior and assume it works perfectly. Unfortunately this is not the case and if you are ignoring it, GC may hit you really hard.... in production. Furthermore the information that you may find on the web can be a lot of times misleading. In this event we will try to demystify some of the misconceptions around GC by understanding how different GC mechanisms work and how to make the right decisions in order to make them work for you.

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Let's talk about Garbage Collection

  1. 1. Garbage Collectors Haim Yadid, Head of Performance and Application Infra Group
  2. 2. Motivation & Goals
  3. 3. Why Java has GC? Memory management is hard malloc/free Memory leaks (dangling pointers) Heap corruption (Free an object twice) Reference counting Cyclic graphs Garbage Collectors
  4. 4. GC Me allocate myObject = new MyClass(2015) GC cleans So everything is cool !
  5. 5. Goal: Minimize Memory Overhead Garbage collector memory overhead Internal structures Additional memory required for GC Policy and amount of memory generated before GC Garbage Collectors
  6. 6. Goal: Application Throughput No garbage collection at all -->
 Application throughput 1 (100% of the time) If in average garbage collection consumes x milliseconds Every y milliseconds Throughput is (y-x)/y E.g. if GC consumes 50 ms every second 
 Throughput is 95% (GC overhead is 5%) Garbage Collectors GCtime   —————   Total  Time  
  7. 7. Goal: Responsiveness Pause Time Garbage collector stops the application During that time the application is not responsive What is the maximal delay your application can sustain: Batch applications seconds… Web applications ½ second? Swing UI 100 ms max Trading application milliseconds Robotics microseconds…. Garbage Collectors
  8. 8. Goals Memory footprint Throughput Max pause time Garbage Collectors: G1 Contradicting! Choose 2 of 3
  9. 9. Goal: Fast Allocation Maintaining free list of objects tend to lead to fragmentation Fragmentation increases allocation time A well known problem of C/C++ programs Garbage Collectors
  10. 10. Goal: Locality TLAB - thread local allocation buffer Maintaining locality utilizes CPU cache Linear allocation mechanism Per thread buffer allocated on first object alloc after new Gen GC. Small objects allocated linearly on that buffer FAST 10 machine instructions Resized automatically ( ResizeTLAB=true) Based on TLABWasteTargetPercent=1 Garbage Collectors
  11. 11. Terminology
  12. 12. Sizing Heap Heap the region of objects reside |H| - The size of the heap #H - number of object in the heap Garbage Collectors
  13. 13. GC Root An object which is references from outside the heap or heap region. Java Local JNI Native stack System class Busy monitor Etc… Garbage Collectors
  14. 14. Sizing : Live Set Live Set : Object that are reachable from garbage collection roots #LS - number of object in the live set |LS| - The size of live set Garbage Collectors
  15. 15. Mutator Application threads The stuff that create new objects and changes state The stuff that makes the garbage collector suffer Allocation rate - how fast new objects are allocated Mutation rate - how fast app changes references of live objects Garbage Collectors
  16. 16. Collector Pauseless Collector (Concurrent Collector )
 vs Stop the world (STW) Collector Serial Collector -Single threaded
 vs Parallel collector -Multi threaded Incremental vs Monolithic Conservative vs Precise Garbage Collectors
  17. 17. GC Safepoint A point in thread execution where it is safe for a GC to operate References inside thread stacks are identifiable If a thread is in a safe point GC may commence Thread will not be able to exit a safe-point until GC ends Global Safe-point : All threads enter a safe point Safe points must be frequent Garbage Collectors
  18. 18. Building Blocks: Mark and Sweep Mark: O(#LS) Every object holds a bit initially set to 0 Start with GC roots DFS traversal every visited object change to 1 Sweep O(|H|) Objects with 0 are added to the empty list Downside: Heap fragmentation Garbage Collectors A B C E F D A B C D F
  19. 19. Building Blocks: Copy Collector Mark: O(#LS) Same as mark and sweep Copy: O(|LS|) = O(#LS) All live objects are copied to an empty region(to space) No fragmentation Downside: Need twice as much memory Copy is very expensive STW - mutators cannot work at the same time as copy Garbage Collectors A B C D F A B C D F from space to space
  20. 20. Building Blocks: Mark (Sweep)Compact Mark: O(#LS) Same as mark&sweep Sliding compaction O(|H|+|LS|) move objects (relocate) fix pointers(remap) Compact to the beginning of the heap Do not need twice the memory Copy is more delicate and may be slower Garbage Collectors
  21. 21. The Weak Generational Hypothesis Most objects survive for only a short period of time. Low number of references from old to new objects Garbage Collectors
  22. 22. Generational Garbage Collectors Since JDK 1.2 all collectors are generational and advantage of the WGH Different Collectors can be chosen for each generation New generation collector Tenured generation collector GC roots to young gen are maintained by a “remembered set” Garbage Collectors
  23. 23. Oracle Hotspot Garbage Collectors Serial (Serial, MSC) Train Collector (history) Parallel Collectors (a.k.a throughput collector) Concurrent collector (CMS) iCMS incremental CMS G1GC (Experimental) Garbage Collectors
  24. 24. Major Memory Regions Monitoring the JVM Young PermTenured Code Cache Young generation Further divided into: Eden A “from” survivor space A “to” survivor space Tenured (old) generation Permanent generation/Meta Space Code Cache Heap Non Heap NativeNative
  25. 25. Object Life Cycle Most objects are allocated in Eden space. When Eden fills up a minor GC occurs Reachable object are copied “to” survivor space. There are two survivor spaces surviving objects are copied alternately 
 from one to the other. Eden S0 S1 Eden S0 S1 Eden S0 S1 1 2 3
  26. 26. Object Promotion Objects are promoted to the old generation (tenured) when: surviving several minor GCs Survival spaces fill up
  27. 27. Serial Collectors Single threaded Stop the world Monolithic Efficient (no communication between threads) The default on certain platforms
 Garbage Collectors:Serial
  28. 28. Serial New Collector -XX:+ UseSerialGC Serial new Single threaded young generation collector Triggered when: Eden space is full. An explicit invocation or call to System.gc(). Garbage Collectors: Serial
  29. 29. MSC (Serial Old) Single threaded tenured generation collector Mark & Sweep Compact Events which initiate a serial collector garbage collection Tenured generation space is unable to satisfy an 
 object promotion coming from young generation. An explicit invocation or call to System.gc(). Garbage Collectors: Serial
  30. 30. Serial Collector: Suitability Well suited for single processor core machines CPU affinity (one-to-one JVM to processor core configuration) Tends to work well for applications with small Java heaps, i.e. less than 128mb- 256mb Garbage Collectors:Serial
  31. 31. Train Collector Introduced in Java 1.3 Divides the heap into small chunks Incremental Experimental Discontinued on Java 1.4 Garbage Collectors
  32. 32. Parallel Collector Multi-threaded Monolithic Stop the world Three variants -XX:+UseParallelGC -XX:+UseParallelOldGC The default on most platforms Garbage Collectors
  33. 33. Managing collector threads Number parallel collector threads controlled 
 by -XX:ParallelGCThreads=<N> Defaults to Runtime.availableProcessors(). 
 In a JDK 6 update release, 5/8ths available processors if > 8 Multiple JVM per machine configurations, 
 set -XX: ParallelGCThreads=<N> such that sum of all threads < NCPU Garbage Collectors
  34. 34. Parallel GC Triggering Same as serial collector Events which initiate a minor garbage collection Eden space is unable to satisfy an object allocation request. Results in a minor garbage collection event. Events which might initiate a full garbage collection Tenured generation space unable to satisfy an 
 object promotion coming from young generation. An explicit invocation or call to System.gc(). Garbage Collectors
  35. 35. Parallel Collector:Suitability Reduce garbage collection overhead 
 on multi-core processor systems Reduce pause time on multi-core systems Best throughput Pause time may be reasonable when heap size < 1GB Garbage Collectors
  36. 36. CMS Collector (Mostly) Concurrent mark and sweep tenured space collector Runs mostly concurrently with application threads Do not compact heap! Enabled with -XX:+UseConcMarkSweepGC ParNew - Parallel, multi-threaded young generation collector enabled by default. working with CMS Garbage Collectors
  37. 37. Alas! Lower throughput Requires more memory 20-30% more Concurrent mode failure will fallback to a stop the world full GC can occur when : objects are copied to the tenured space faster than the concurrent collector can collect them. (“loses the race”) space fragmentation -XX:PrintFLSStatistics=1 Garbage Collectors
  38. 38. Concurrent Collector Phases Concurrent collector cycle contains the following phases: Initial mark (*) Concurrent mark Concurrent Pre-clean Remark (*) - second pass Concurrent sweep Concurrent reset Garbage Collectors
  39. 39. The Concurrent Collector Initial mark phase(*) Objects in the tenured generation are “marked” 
 as reachable including those objects which may 
 be reachable from young generation. Pause time is typically short in duration relative 
 to minor collection pause times. Concurrent mark phase Traverses the tenured generation object graph 
 for reachable objects concurrently while Java 
 application threads are executing. Garbage Collectors
  40. 40. The Concurrent Collector Phases Remark(*) Finds objects that were missed by the concurrent 
 mark phase due to updates by Java application 
 threads to objects after the concurrent collector 
 had finished tracing that object. Concurrent sweep Collects the objects identified as 
 unreachable during marking phases. Concurrent reset Prepares for next concurrent collection. Garbage Collectors
  41. 41. PermGen Collection Classes will not be collected during CMS 
 concurrent phases Only during Full (STW) collection Explicitly instructed to do so using 
 -XX:+CMSClassUnloadingEnabled and 
 (the 2nd switch is not needed in post HotSpot 6.0u4 JVMs). Garbage Collectors
  42. 42. Suitability Application responsiveness is more important than application throughput More than one core Large Heaps > 1GB Garbage Collectors
  43. 43. ExplicitGC and CMS Explicit GC used to invoke the stop the world GC This can cause a problem with large heaps
 -XX:+ExplicitGCInvokesConcurrent (Java 6) -XX:+ExplicitGCInvokesConcurrentAndUnloadsClasses 
 (requires Java6u4 or later). Garbage Collectors
  44. 44. CMS Minor Collection Triggering Minor collections are triggered as with the parallel collector The ParNew collector is built in such a way it can work in parallel with the CMS Garbage Collectors
  45. 45. CMS Collection Triggering Start if the occupancy exceeds a percentage threshold Default value is 92% -XX:CMSInitiatingOccupancyFraction=n 
 where n is the % of the tenured space size. Garbage Collectors
  46. 46. iCMS Deprecated on Java 8 CMSIncrementalMode enables the concurrent 
 modes to be done incrementally. Periodically gives additional processor back to 
 the application resulting in better application responsiveness by doing the concurrent work 
 in small chunks. Garbage Collectors
  47. 47. Tune iCMS CMSIncrementalMode has a duty cycle that controls the amount of work the concurrent collector is allowed to do before giving up the processor. Duty cycle is the % of time between minor collections 
 the concurrent collector is allowed to run. Duty cycle by default is automatically computed using what's called automatic pacing. Both duty cycle and pacing can be fine tuned. Garbage Collectors
  48. 48. Enabling iCMS Java 6 On JDK 6, recommend using the following two switches together: -XX:+UseConcMarkSweepGC and -XX:+CMSIncrementalMode Or use: -Xincgc Garbage Collectors
  49. 49. Enabling iCMS Java5 On JDK 5 use all of the following switches together: -XX:+UseConcmarkSweepGC -XX:+CMSIncrementalMode -XX:+CMSIncrementalPacing -XX:CMSIncrementalDutyCycleMin=0 -XX:CMSIncrementalDutyCycle=10 JDK 5 settings mirror the default settings decided upon for JDK 6. JDK 5's -Xincgc!= CMSIncrementalMode, it enables CMS Garbage Collectors
  50. 50. iCMS Fine Tuning If full collections are still occurring, then: Increase the safety factor using 
 The default value is 10. Increasing safety factor 
 adds conservatism when computing the duty cycle. Increase the minimum duty cycle using
 The default is 0 in JDK 6, 10 in JDK 5. Disable automatic pacing and use a fixed duty cycle using 
 -XX:-CMSIncrementalPacing and 
 The default duty cycle is 10 in JDK 6, 50 in JDK 5. Garbage Collectors
  51. 51. G1 Garbage collector Garbage Collectors: G1
  52. 52. Garbage First GC Stop the world Incremental Beta stage in Java6 and Java 7 Supported since Java7u4 Garbage Collectors: G1
  53. 53. Garbage First GC Apply New generation harvesting to the tenured Gen Achieve soft real time goal 
 consume no more than x ms of any y ms time slice while maintaining high throughput for programs with large heaps and high allocation rates, running on large multi-processor machines. Garbage Collectors: G1
  54. 54. Heap Layout divided into equal-sized heap regions, each a contiguous range of virtual memory Region size is 1-32MB based on heap size target 2000 regions A linked list of empty regions Heap is divided to New generation regions and old generation regions Garbage Collectors: G1
  55. 55. Each region is either Marked Eden Survivor Space Old Generation Empty Humongous Heap Layout Garbage Collectors: G1 E E S S E E E E O O O O O H H H E E E
  56. 56. G1 New Gen GC Live objects from young generation are moved to Survivor space regions Old gen regions STW pause Calculate new size of eden and new Survivor space Garbage Collectors: G1
  57. 57. G1 Concurrent Mark Triggered when entire heap reaches certain threshold Mark regions Calculate liveliness information for each region Concurrent Empty regions are reclaimed immediately Garbage Collectors: G1
  58. 58. OldGen collection Choose regions with low liveliness Piggyback some during next young GC Denoted GC Pause (mixed) Garbage Collectors: G1
  59. 59. Humongous Objects 1/2 of the heap region size allocated in dedicated (contiguous sequences of) heap regions; these regions contain only the humongous object GC is not optimized for these objects Garbage Collectors: G1
  60. 60. Command line Options -XX:+UseG1GC -XX:MaxGCPauseMillis=200 -XX:InitiatingHeapOccupancyPercent=45 Garbage Collectors: G1
  61. 61. Valid Combinations Tenured Garbage Collectors: G1 Young G1GC Parallel Scavenge ParNewSerial Serial Old CMS Parallel Old
  62. 62. Summary: -XX flags for JDK6 Garbage Collectors: G1 Collector -XX: Param "Serial" + "Serial Old“ UseSerialGC "ParNew" + "Serial Old“ UseParNewGC "ParNew"+"CMS" + "Serial Old“ * UseConcMarkSweepGC "Parallel Scavenge" + "Serial Old" UseParallelGC "Parallel Scavenge" + "Parallel Old" UseParallelOldGC G1 Garbage collector ** UseG1GC
  63. 63. Alternatives Garbage Collectors: G1
  64. 64. Azul C4 A proprietary JVM Commercial Pause-less Requires changes in the OS (Linux only) Achieves Throughput and Pause time May require more memory…. 32GB and above Useful for large heaps (1TB is casual) Garbage Collectors: C4
  65. 65. Shenandoah JEP 189 for Open JDK Developed in RedHat An Ultra-Low-Pause-Time Garbage Regions same as G1 Concurrent collection w/o memory barrier Not a generational collector Brooks forwarding pointer Garbage Collectors: Shenandoah Ref Object
  66. 66. Weird References Garbage Collectors: G1
  67. 67. Object Life Cycle Garbage Collectors: References Created Initialized Strongly Reachable Softly Reachable Weakly Reachable Finalized Phantom Reachable
  68. 68. Finalizers Create performance issues For example: 
 do not rely on a finalizer to close file descriptors. Try to limit use of finalizer as safety net Use other mechanisms for releasing resources. Keep the work being done as short as possible. Garbage Collectors: References
  69. 69. Finalizers and GC Inside a finalizer you have a reference to your object and technically you may resurrect it. Objects which have a finalize method will need two cycles of GC in order to be collected Can lead to OOM errors A resurrected object may be reachable again but it finalize method will not run again. In order to prevent this problem use phantom references…. Garbage Collectors: References
  70. 70. Finalizers and Thread Safety Finalizers are executed from a special thread According to the Java memory model
 updates to local variable may not be visible to the Finalization thread Occurs when GC happen too soon In order to ensure correct memory visibility one need to use a sync block to force coherency Garbage Collectors: References
  71. 71. Reference Objects drawbacks lots of reference objects also give the garbage collector more work to do since unreachable reference objects need to be discovered and 
 queued during garbage collection. Reference object processing can extend the time 
 it takes to perform garbage collections, especially 
 if there are consistently many unreachable reference objects to process. Garbage Collectors: References
  72. 72. GC Tuning Tuning Memory: Tuning GC
  73. 73. Sizing Generations VisualVM's Monitor tab Jconsole's Memory tab VisualGC's heap space sizes GCHisto jstat's GC options -verbose:gc heap space sizes -XX:+PrintGCDetails heap space sizes Tuning Memory: Tuning GC
  74. 74. Logging GC activity The three most important parameters are: -XX:+PrintGCDetails -XX:+PrintGCTimeStamps -XX:+PrintGCDateStamps -Xloggc:<logfile> If you want less data -verbose:gc Tuning Memory: Tuning GC
  75. 75. Log File Rotation -XX:+UseGCLogFileRotation Control log file size -XX:GCLogFileSize=1000 Number of log files -XX:NumberOfGCLogFiles=10 Starting from Java 7 Tuning Memory: Tuning GC
  76. 76. Verbose GC on runtime Java.lang.Memory Set verbose attribute to true Tuning Memory: Tuning GC
  77. 77. The GC Log Log every GC occurs PrintGCDetails (more verbose) [GC [DefNew: 960K->64K(960K), 0.0047410 secs] 
 3950K->3478K(5056K), 0.0047900 secs] Verbose:gc (less verbose) [GC 327680K->53714K(778240K), 0.2161340 secs] Before->After(Total), time Tuning Memory: Tuning GC
  78. 78. CMS Collector Minor collections follow serial collector format. [Full GC [CMS: 5994K->5992K(49152K),
 0.2584730 secs] 6834K->5992K(63936K), 
 [CMS Perm: 10971K->10971K(18404K)], 0.2586030 secs] [GC [1 CMS-initial-mark: 13991K(20288K)] 
 14103K(22400K), 0.0023781 secs] [CMS-concurrent-preclean: 0.044/0.064 secs] [GC [1 CMS-remark: 16090K(20288K)] 
 17242K(22400K), 0.0210460 secs] Tuning Memory: Tuning GC
  79. 79. Additional information More information with the flags -XX:+PrintGCApplicationStoppedTime -XX:+PrintGCApplicationConcurrentTime -XX:+PrintTenuringDistribution Tuning Memory: Tuning GC
  80. 80. GC Log Quirks CMS GC logs may get garbled Due to concurrency of the different GC mechanisms Analysis tools should be able to overcome this problem Not easy at all and may need to fix manually. Tuning Memory: Tuning GC
  81. 81. J9 -Xverbosegclog:<file path> Structured xml Tuning Memory: Tuning GC <gc-end id="92" type="scavenge" contextid="88" durationms="4.464" usertimems="4.000" systemtimems="0.000" timestamp="2014-01-21T16:37:18.953"> <mem-info id="93" free="5472664" total="8388608" percent="65"> <mem type="nursery" free="655360" total="2097152" percent="31"> <mem type="allocate" free="655360" total="1179648" percent="55" /> <mem type="survivor" free="0" total="917504" percent="0" /> </mem> <mem type="tenure" free="4817304" total="6291456" percent="76"> <mem type="soa" free="4502936" total="5977088" percent="75" /> <mem type="loa" free="314368" total="314368" percent="100" /> </mem> <pending-finalizers system="2" default="0" reference="24" classloader="0" /> <remembered-set count="1770" /> </mem-info> </gc-end>
  82. 82. Visualization Tools Tuning Memory: Tuning GC
  83. 83. JClarity: Cesnum Commercial product Give recommendations Tuning Memory: Tuning GC
  84. 84. GCViewer Open source originally by Tagtraum industry A new fork exists in github latest version 1.33 Supports G1GC Java6/7 Not bullet proof Tuning Memory: Tuning GC
  85. 85. GCViewer Tuning Memory: Tuning GC
  86. 86. GCViewer View Choose which info to 
 view Tuning Memory: Tuning GC
  87. 87. GCViewer -Summary Tuning Memory: Tuning GC
  88. 88. GCViewer - Memory Tuning Memory: Tuning GC
  89. 89. GCViewer -Pause Tuning Memory: Tuning GC
  90. 90. VisualGC Tuning Memory: Tuning GC
  91. 91. Explicit GC Do not use System.gc() unless there is a specific 
 use case or need to. Disable Explicit GC: -XX:+DisableExplicitGC. Default RMI distributed GC interval is once per minute, (60000 ms). Use: 
 -Dsun.rmi.dgc.client.gcInterval =3600000 
 -Dsun.rmi.dgc.server.gcInterval =3600000 When using JDK 6 and the Concurrent collector also use 
 -XX:+ExplicitGCInvokesConcurrent Tuning Memory: Tuning GC
  92. 92. Interpretation Tuning Memory: Tuning GC
  93. 93. The Jigsaw effect Garbage collection makes heap diagrams over time look like a jigsaw. Minor collections visualize as small teeth Full collections visualize as large teeth Tuning Memory: Tuning GC
  94. 94. Measuring Memory Usage Best way is heap dump From GC logs look on the lower points of the Full GC lines. Tuning Memory: Tuning GC 60.0 90.0 120.0 150.0 180.0
  95. 95. Memory Leak Draw the line between the full gc points Increasing over time --> memory leak Tuning Memory: Tuning GC 60.0 90.0 120.0 150.0 180.0
  96. 96. Low Throughput Under 95% Increase Heap: Low throughput is usually a result of insufficient memory GC kicks in too frequently and frees small amounts New Gen too small New gen GC kicks in too fast Not able to release enough as a result mid term objects spill to old gen. Old Gen too small Application state spills to new Gen. Breaks the Generational Hypothesis Tuning Memory: Tuning GC
  97. 97. Contradicting Goals Goal1: Retain as many objects as possible in the survivor spaces Less promotion into the old generation Less frequent old GCs Goal2: Do not copy very long- lived objects between the survivors Unnecessary overhead on minor GCs Tuning Memory: Tuning GC
  98. 98. High Pause Time Choose The correct GC scheme: CMS/G1 when low pause is required Reduce footprint of user interactive processes Reduce memory allocations do not allocated too many related temporary objects chunking of work. Tuning Memory: Tuning GC

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Garbage collection is the most famous (infamous) JVM mechanism and it dates back to Java 1.0. Every Java developer knows about its existence yet most of the time we wish we can ignore its behavior and assume it works perfectly. Unfortunately this is not the case and if you are ignoring it, GC may hit you really hard.... in production. Furthermore the information that you may find on the web can be a lot of times misleading. In this event we will try to demystify some of the misconceptions around GC by understanding how different GC mechanisms work and how to make the right decisions in order to make them work for you.


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