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Size of in java
- 1. SizeOf in Java because size does matter! Ludovic Orban — Software engineer, Terracotta
- 2. JavaZone 2012 Take aways Better understanding of heap utilization & VM memory management Make better use of that memory Look at some hotspot’s source code! ! ! ! 2
- 3. JavaZone 2012 Me, myself and I … Software Engineer @ Terracotta Ehcache, Quartz, Terracotta core & toolkit Author of the Bitronix Transaction Manager ! – ! 3
- 4. JavaZone 2012 Basic heap intro Object (and other) overheads (on different platforms) How to measure objects Reflection sun.misc.Unsafe JVMTI Attaching to a VM at runtime and loading an agent Some platform gotchas What this means for caching Q&A – – – 4 Agenda
- 5. Heap?
- 6. JavaZone 2012 Java needs memory 6 Class definitions (and classloaders) Threads (and their stack) NIO calls JNI calls JIT (compilation & executable code) ! ! ! ! !
- 7. JavaZone 2012 Java needs memory for you Needs to store your object state myCousin = new Relative(name, Gender.MALE, 23); All that state (or almost) goes on heap Where in heap depends on your VM implementation ... but we don’t really care about this here and now ! – ! – – 7
- 8. JavaZone 2012 Java Heap Controlled with -Xms -Xmx Lets you allocate a lot of memory ~4GB on 32bit system (theoretically) 16PB on 64bit system (I guess that’s still theoretical too) ! ! – – 8
- 9. JavaZone 2012 All managed memory No free... only new GC will free all unused objects for you … unreachable object would me more accurate! So let us use “new” and create objects … ! ! – ! 9
- 10. Object? (this is all Java 1.5 onwards!)
- 11. JavaZone 2012 Object memory consumption 11 Every field consumes memory … … depending on its type ! – Type Bit representation Size in byte boolean, byte 1 or 8 1 char, short 16 2 int, float 32 4 long, double 64 8
- 12. JavaZone 2012 Thanks for attending ! So that’s easy... a class with 32 primitive byte fields would consume 32 bytes! Let’s verify this first though (before leaving) … ! – ! 12
- 13. JavaZone 2012 We should be almost done … Start a VM Load the class (simple class, declaring 32 fields of type byte) Measure the Memory used Create an instance of that Class (and keep a reference to it) Measure the memory used (again) Diff the two ! ! ! ! ! ! 13
- 15. … alright not then!
- 16. JavaZone 2012 So… what’s wrong ? 16 Firstly, don’t call System.gc() ;-) Secondly … we’re missing 8 bytes Maybe heap consumption isn’t that straight forward! ! ! – !
- 17. JavaZone 2012 Object header Every object state on heap, also includes an object header Two words big mark word : bitfields for stuff (including synchronization state) klass pointer : pointer to another object (a meta-object) ! – – – 17
- 18. JavaZone 2012 2 words in size? These are our missing 8 bytes! But wait... are these always twice 4 bytes? What about 64 bit systems? Are these arch words? These should, let’s try … ! ! ! – – 18
- 19. Second conclusion demo … this better works this time!
- 20. JavaZone 2012 Almost there Looks like removing a byte doesn’t buy us one byte … even more, adding one byte consumes more than expected … more so, on 64 bit arch, we’re really not getting what we expected! … there seems to be yet more to it. ! – – – 20
- 21. JavaZone 2012 Memory alignment Aligning addresses in memory So a new address only starts at every X bytes CPU limitations Cache line faulting / invalidation The VM will (generally) align on 8 bytes JRockit does sometimes 16 bytes ! – – – ! – 21
- 22. JavaZone 2012 … what about 64 bit arch? compressed oops 64 bit address space? Are you really using this? If feasible, the VM will represent managed pointers as 32-bit values Avoiding wasting too much memory when not required Still allowing applications to address up to four billion objects (not bytes!), or a heap size of up to about 32Gb ! ! – – – – 22
- 23. JavaZone 2012 Right … pointers! References to other objects are effectively pointers A pointer will differ in size Depending on the arch Whether Compressed OOPs are used or not ! ! – – 23
- 26. JavaZone 2012 The truth about class layouting 26 VM will layout the instance’s state in memory Not necessarily the way you’ve declared fields in the class Avoiding wasting memory ! ! –
- 27. JavaZone 2012 Here’s what going on… That information is kept by the VM Referenced by the klass pointer The VM first lays out 64 bit, 32 bit, 16 bit, 8 bit primitives and finally OOP … but also backpads ! – ! – – 27
- 28. Class layouting
- 30. JavaZone 2012 sun.misc.Unsafe.theUnsafe 30 Sun Oracle VM specific Lets you do bunch of … unsafe stuff ! !
- 31. JavaZone 2012 java.util.concurrent.Atomic* how AtomicInteger uses Unsafe internally :! 31 10: public final boolean compareAndSet(int expect, int update) { 11: return unsafe.compareAndSwapInt(this, valueOffset, expect, update); 12: } static { try { valueOffset = unsafe.objectFieldOffset (AtomicInteger.class.getDeclaredField("value")); } catch (Exception ex) { throw new Error(ex); } }
- 32. JavaZone 2012 UnsafeSizeOf Looking at the “last” field’s offset Math is easily done Still have to take care about the previous concerns Object Alignment ! – – • 32
- 34. JavaZone 2012 JVMTI 34 java.lang.instrument.Instrumentation has getObjectSize(Object) calls into the VM to figure the memory footprint of an instance and … … returns it in bytes … … that’s it! ! – ! – – –
- 35. JavaZone 2012 Now that sounds like … cool ! All we need is an instance of java.lang.instrument.Instrumentation … which is an interface … implemented by sun.instrument.InstrumentationImpl … which has a private constructor The way to get such an instance is … A Java Agent ! – – – ! – 35
- 36. JavaZone 2012 Java Agent All we need is package it properly … with an MANIFEST.MF file in the jar declaring an Premain-Class The class needs a premain method public static void premain(String agentArgs, Instrumentation inst); ! – – ! – • 36
- 37. Java Agent? … as in -javaagent: ?! You’re kidding me, right?
- 38. JavaZone 2012 agentmain invoked by the VM when the agent is loaded … … the agent is loaded by providing a -javaagent:<locationToAgentJar> argument at VM startup Modifying VM startup scripts / arguments is painful in … real life We couldn’t settle for that! ! – ! ! 38
- 39. JavaZone 2012 Introducing the AttachAPI Introduced in JDK6 Let’s you attach to a VM process, while it is running Including yourself … and load an agent (jar) Additional agentmain method required Additional MANIFEST.MF’s Agent-Class ! ! – ! – – 39
- 40. JavaZone 2012 … now, where is it ? The AttachAPI is part of the tools.jar Which is on the classpath … … on OS X … but not on Windows, nor Linux But it’s there … … as part of the install ! ! – – ! – 40
- 41. JavaZone 2012 Loading the AttachAPI Since it is not on the classpath … but present We load it ourselves … search for it “around” JAVA_HOME … if we find it load it ! And finally attach, load the agent… and get the Instrumentation instance! ! – ! – – ! 41
- 42. Victory ! … and should that fail, we fall back to Unsafe, … … and if not present, fallback to Reflection based one
- 43. Gotcha! You didn’t expect it to be that simple, did you?
- 44. JavaZone 2012 One last thing ! There is one gotcha … Remember the GC we talked about? It needs to store some information too … … sometimes that information has a minimal footprint … like with CMS! ! ! – – – 44
- 45. JavaZone 2012 HotSpot CMS’ FreeChunk 1: class FreeChunk VALUE_OBJ_CLASS_SPEC { 2: friend class VMStructs; 3: // For 64 bit compressed oops, the markOop encodes both the size and the 4: // indication that this is a FreeChunk and not an object. 5: volatile size_t _size; 6: FreeChunk* _prev; 7: FreeChunk* _next; (...) } 45
- 46. Now we’re there … … but why is that cool again?
- 47. JavaZone 2012 Why should I still care? The VM manages a lot of burden for us … But we still need to manage our heap size … heap size that can grow pretty large! But what should we do with all this memory? ! ! – ! 47
- 48. JavaZone 2012 Caching Make your app faster! Keep data close to the processing unit … … no, that’s not the DB, but in memory! Make use of the large heap sizes VM offer today Letting the cache manage the resources ! – – ! ! 48
- 49. JavaZone 2012 Ehcache ARC Automatic Resource Control Lets you size cache sizes in bytes, … … rather than in elements count (when I say byte, I mean byte, kb, mb, gb, tb) Not only caches but even all your caches ! ! – – – ! 49
- 50. JavaZone 2012 Adding to the cache cache.put( new Element(key, value) ) Will size the the value (Object graph) the key (Object graph) the container(s) ! ! – – – 50
- 51. JavaZone 2012 How much would you like? Ehcache has CacheManagers who manages … Caches Define your Cache or whole CacheManager sizes in maxBytesLocalHeap ! – – ! – 51
- 52. JavaZone 2012 ehcache.xml <ehcache maxBytesLocalHeap="512m"> <cache name="CacheOne" /> <cache name="CacheTwo" /> <cache name="CacheThree" maxBytesLocalHeap="256m" /> </ehcache> 52
- 53. It’s that easy … … thanks to all the rest we’ve covered!
- 54. JavaZone 2012 Conclusion … that’s not all, there is more to all that. But it’s open source so go and see There might still be bugs! Bunch of VM options & optimizations to account for We might have missed some So, please, go download and try it out! ! – ! – – – 54
- 55. Questions? Using SizeOf with ehcache, go visit www.ehcache.org