Design'Ra*onales'in'the'JRockit'JVM'Marcus'Lagergren'

Java'Language'team,'Oracle'

Design'Ra*onales'in'the'JRockit'JVM'Marcus'Lagergren'

Java'Language'team,'Oracle'

Design'Ra*onales'in'the'JRockit'JVM'Marcus'Lagergren'

Java'Language'team,'Oracle'

The following is intended to outline our general product direction. It is intended for information purposes only, and may not be incorporated into any contract. It is not a commitment to deliver any material, code, or functionality, and should not be relied upon in making purchasing decisions. The development, release, and timing of any features or functionality described for Oracle�s products remains at the sole discretion of Oracle.

Safe'Harbor'Statement'

Agenda'•  In'the'beginning…'•  What'did'we'accomplish'/'Internals'–  Code'Genera*on'– Memory'Management'–  Threads'&'Synchroniza*on'

•  Externals'–  The'Java'Mission'Control'suite'– A'parenthesis'on'JRockit'VE'

•  Q&A'

About'the'speaker'

@lagergren'

About'the'speaker'

@lagergren'

Buy'this'book!'

About'the'speaker'

•  M.'Sc.'from'KTH,'Stockholm'– Narrowly'escaped'doing'a'PhD'on'bit'security'in'cryptographic'systems'

•  Run*me,'OS'and'compiler'engineer'since'1999,'with'some'startup'breaks'

•  One'of'the'original'creators'of'the'JRockit'JVM'

In'the'beginning'

Appeal'Virtual'Machines'

•  Appeal'SoYware'Solu*ons'– Consul*ng,'almost'exclusively'Java'by'1997'

•  S*ll'the'pre[app'server'era'

Appeal'Virtual'Machines'•  We'saw'that'Java'would'be'great'on'the'server'side'

Appeal'Virtual'Machines'•  We'saw'that'Java'would'be'great'on'the'server'side'– Shorter'development'cycles'–'money'in'the'bank'•  Buffer'overrun'protec*on'•  Automa*c'memory'management ''• Write'once'run'everywhere'

Appeal'Virtual'Machines'•  We'saw'that'Java'would'be'great'on'the'server'side'– Shorter'development'cycles'–'money'in'the'bank'•  Buffer'overrun'protec*on'•  Automa*c'memory'management ''• Write'once'run'everywhere'

•  Tremendous'scalability'problems'•  Sun'ClassicVM'was'all[encompassing'

JavaOne'1997'

•  Sun'Microsystems'presents'the'HotSpot'virtual'machine'

JavaOne'1997'

•  Sun'Microsystems'presents'the'HotSpot'virtual'machine'– “WOW!'This'is'the'way'to'do'it!'Adap*ve'run*mes!”'

JavaOne'1998'

•  Sun'Microsystems'presents'the'HotSpot'virtual'machine'again%

JavaOne'1998'

•  Sun'Microsystems'presents'the'HotSpot'virtual'machine'again%– “WTF!'This'is'slide[by[slide'the'exact'same'presenta*on'as'last'year!?!”'

– We'can’t'wait'any'longer.'Let’s'build'our'own'VM.'How'hard'can'it'be?''

Crea*ng'our'own'JVM'['JRockit'

Produc*ze'a'narrower'domain?'

•  Server[side'usage'only.'Headless.''– We'need'to'help'the'early'app'server'vendors'get'performance'and'scalability'

Produc*ze'a'narrower'domain?'

•  Server[side'usage'only.'Headless.''– We'need'to'help'the'early'app'server'vendors'get'performance'and'scalability'

•  No'interpreter''– “startup'*me'doesn’t'mader'on'the'server'anyway”'

Produc*ze'a'narrower'domain?'

•  Server[side'usage'only.'Headless.''– We'need'to'help'the'early'app'server'vendors'get'performance'and'scalability'

•  No'interpreter''– “startup'*me'doesn’t'mader'on'the'server'anyway”'

•  Green'threads'or'n'x'm'threads.''– Explicit'parallelism'was'all[pervasive.'

Produc*ze'a'narrower'domain?'

•  Incremental'GC''– We'thought'something'like'[Seligman,'Grarup]'would'suffice.''

Produc*ze'a'narrower'domain?'

•  Incremental'GC'– We'thought'something'like'[Seligman,'Grarup]'would'suffice.''

•  Support'ourselves'on'consul*ng'only.'– Nope'–'needed'venture'capital'

The'Java'License'

•  You'can’t'call'yourself'“Java”'without'a'Java'license'

•  You'need'to'pass'the'TCK'test'suite'– Not'available'without'license'

•  To'get'a'Java'License'you'need'a'“value'add”'

The'Java'License'

•  What’s'a'“value'add”?'

The'Java'License'

•  What’s'a'“value'add”?'

The'Java'License'

•  What’s'a'“value'add”?'

The'Java'License'

•  What’s'a'“value'add”?'– Superior'performance!'

The'Java'License'

•  What’s'a'“value'add”?'– Superior'performance!'– What?'You'didn’t'like'that?'

The'Java'License'

•  What’s'a'“value'add”?'– Superior'performance!'– What?'You'didn’t'like'that?'– OK…'Let’s'see…'Err..'“managability”'

The'Java'License'

•  Java'License'was'granted'2001'– Helped'us'partner'up'with'BEA'Systems'and'Intel'

– BEA'acquired'us'in'2002'– Oracle'acquired'BEA'in'2008'– Oracle'acquired'Sun'in'2010'

What'did'we'accomplish?''!

The'real'value'adds'turned'out'to'be:''

•  Mul*'*ered'support'for'paying'customers'– Part'of'the'WLS'stack'

'

•  Monitoring'and'Serviceability'–  JRockit'Mission'Control'(now'Java'Mission'Control)'

– Record'and'introspect'produc*on'systems'with'zero'overhead.'

'

The'real'value'adds'turned'out'to'be:''

•  Pioneered'“SoY'real*me”'GC'– Determinis*c'GC'– Low'latency'GC'

The'real'value'adds'turned'out'to'be:''

•  Virtualiza*on'–  JRockit'Virtual'Edi*on'–'an'opera*ng'system'for'Java'

– Shorter'paths'between'Java'and'hardware'– Hypervisor'required'–  JRockit'VE'on'virtual'hardware'outperformed'physical'Linux!'

'

The'real'value'adds'turned'out'to'be:''

•  The'benchmark'wars'– Constantly'keeping'it'going'with'Sun'and'IBM,'driving'Java'server[side'performance'

'

The'real'value'adds'turned'out'to'be:''

•  JRockit'became'the'default'JVM'in'the'Oracle'stack'in'2008'

•  ExaLogic''

…'and'then'

INTERNALS!

@SimmsUpNorth

Code'Genera*on'

Code'genera*on'–'No'Interpreter'

•  Keep'test'matrix'small'•  Keep'opera*onal'complexity'down'•  Targe*ng'server'side'apps'–'warmup'a'small'issue'

•  “Code'caching'/'AOT'can'be'done'later”'

Code'genera*on'–'One'JIT'

•  Keep'test'matrix'small'•  Keep'opera*on'complexity'down'•  Run'it'in'different'modes,'with'maximum'code'reuse'

•  Same'IR'throughout'– With'gradual'augmenta*ons'

But…'

•  Startup'became'a'problem'– We'removed'op*mizers'and'added'as'a'“spine”'to'the'normal'JIT'pipeline.''

•  Lazy'code'genera*on'through'trampolines'•  Same'mechanism'for'code'invalida*on'•  Bookkeeping'to'iden*fy'a'program'point'down'to'any'individual'machine'instruc*on'

Code'Genera*on'

•  Same'“spine”'used'in'all'*ers'of'code'genera*on'

Code'Genera*on'

•  Same'“spine”'used'in'all'*ers'of'code'genera*on'

Op*miza*ons'

•  In'and'out'of'SSA'•  Applied'to'all'levels'of'IR'

–  Loop%peeling,%value%numbering,%String%append%explosion,%Type%check%removal,%sign%extension%elimina;on,%copy%propaga;on,%bounds%check%removal,%virtual%to%fixed%calls,%inlining,%if%short%circui;ng,%straightening,%strength%reduc;on,%constant%propaga;on,%dead%code%removal,%out%of%loop%hois;ng,%explode%objects%and%array%copies,%boxing%&%unboxing%removal,%local%escape%analysis,%ASM%peephole%op;miza;on,%redundant%memory%access%removal,%etc%etc%etc…%

•  Support'for'regionalized'IRs'•  Graph'Fusion'Register'Allocator'

Op*miza*on'Targets'

•  Thread'sampling'•  Partly'taken'over'by'safe'point'based'approach'in'R28'

•  Some'code'instrumenta*on,'for'example'for'inlining'path'– Not'in'the'general'case,'e.g'invoca*on'counters'

Op*miza*on'Targets'

•  Hardware'sampling'where'available'– Only'good'thing'about'IA64?'– Could'also'match'e.g.'L2'misses'to'program'points'

•  Bugging'the'processor'manufacturers'since'2002'about'userland'PC'sample'buffer.'

•  JRockit'VE'x'1000'more'samples'–'significantly'proven'shorter'warmup'

HotSpot'style?'

•  On[stack'replacement?'•  Deop*miza*on?'

HotSpot'style?'

•  On[stack'replacement?'•  Deop*miza*on?'•  Never'much'cared'for'any'it';[)'

HotSpot'Style'OSR'and'Deop*miza*on'

•  We’ve'never'found'a'prac*cal'use'case.''–  So'we'can’t'ever'swap'out'the'main'func*on'with'the'microbenchmark'loop.'Who'cares?'

•  An'assump*on'is'invalidated'–  Either'patch'code'directly'or'use'a'guard'when'genera*ng'it'in'the'first'place'

•  A'large'assump*on'– Write'a'trap'in'the'code'and'schedule'lazy'regenera*on'of'en*re'method'

•  Not'strictly'true'for'dynamic'languages'

HotSpot'Style'OSR'and'Deop*miza*on'

•  We’ve'never'found'a'prac*cal'use'case.''–  So'we'can’t'ever'swap'out'the'main'func*on'with'the'microbenchmark'loop.'Who'cares?'

•  An'assump*on'is'invalidated'–  Either'patch'code'directly'or'use'a'guard'when'genera*ng'it'in'the'first'place'

•  A'large'assump*on'– Write'a'trap'in'the'code'and'schedule'lazy'regenera*on'of'en*re'method'

•  Not'strictly'true'for'dynamic'languages'

HotSpot'Style'OSR'and'Deop*miza*on'

•  We’ve'never'found'a'prac*cal'use'case.''–  So'we'can’t'ever'swap'out'the'main'func*on'with'the'microbenchmark'loop.'Who'cares?'

•  An'assump*on'is'invalidated'–  Either'patch'code'directly'or'use'a'guard'when'genera*ng'it'in'the'first'place'

•  A'large'assump*on'– Write'a'trap'in'the'code'and'do'regenera*on'of'en*re'method'

•  Not'strictly'true'for'dynamic'languages'

HotSpot'Style'OSR'and'Deop*miza*on'

•  We’ve'never'found'a'prac*cal'use'case.''–  So'we'can’t'ever'swap'out'the'main'func*on'with'the'microbenchmark'loop.'Who'cares?'

•  An'assump*on'is'invalidated'–  Either'patch'code'directly'or'use'a'guard'when'genera*ng'it'in'the'first'place'

•  A'large'assump*on'– Write'a'trap'in'the'code'and'do'regenera*on'of'en*re'method'

•  Not'strictly'true'for'dynamic'languages'

“Garbage'collec*ng'code”'

•  Code'kept'in'binary'tree'of'code'blocks'~'64M'– More'if'large'pages'enabled'

•  Class'loader'unloading'!'garbage'collec*on'•  Reference'count'to'ac*ve'code'modified'when'backpatching'

•  Specialized'usage'of'code'blocks.'– Trampolines'only'– Op*mized'code'only'

Bytecode'is'bad'–'kill'it'quickly'•  What’s'with'the'goto:s?'•  Why'can'it'express'more'than'Java'source'code?''– OK'we'understand'the'mul*'language'concept,'we'sorta'forgive'you.'

– But'man,'dominators'and'loop'analysis'–'that’s'a'lot'of'compile'*me'

Bytecode'is'bad'–'kill'it'quickly'•  …and'why'is'it'a'stack'machine'AND'a'register'machine'with'65535'registers'at'the'same'*me!? ' ''

•  Ini*ally'tried'to'reconstruct'ASTs'– Obfuscators'etc'made'this'predy'hopeless.'

•  ~15%'of'the'klocs'in'JRockit/codegen'do'flow'control'analysis'on'the'goto:s'

The'IR'•  Use'IR'everywhere'(or'Java)'•  The'IR'should'ideally'reflect'any'of'several'pluggable'frontends.' ''– We'played'around'with'CLR'a'bit.'– These'days'–'dynamic'languages':[)'

•  No'Sea'of'Nodes'•  No'HotSpot'style'“high'level'IR'is'low'level”'

The'IR'

•  Simple'IR'in'MIR'form'(playorm'independent)'

The'IR'–'Design'Ra*onale'•  We'had'some'compiler'experience'–'wanted'to'be'on'track'quickly.'Do'it'the'tradi*onal'way.'

•  We'are'not'“wrong”.'LLVM'is'very'similar.'

The'IR'–'Design'Ra*onale'•  Tiered:%highest%;er%=='always'high'level'•  Hardware'agnos*c.''•  No'architecture'specific'memory'ops'

•  Tiered:%lowest%;er%=='always'the'na*ve'architecture'instruc*on'for'instruc*on.''•  A'gradual'transi*on.''•  A'CPU'has'no'sea'of'nodes.'

The'IR'•  Highest'IR'level'may'have'opera*ons'as'operands'

•  Intrinsics'everywhere'–  arraycopy, membar, cmpuXX, sse4IndexOf, doubleToLongBits, crypto, Math.sin%and%so%on%…'

•  Regret'not'doing'more'in'SSA'form'

The'IRInfo'“database”'•  Lazily'computable'informa*on'–  Liveness'– Dominators'–  Loop'informa*on'– Aliases'–  Type'inference'–  Ranges'– Nullness'analysis'– …'–  Invalidate'on'modifica*on.'

•  Not'a'very'stable'model.'

Memory'management'!

Transi*on:'object'layout,'types'and'livemaps…'

Object'layout'and'types'

•  Object'headers'should'be'fixed'sized.'•  JRockit'Object'header'is'32'+'32'bits'on'all'playorms'with'some'content'varia*ons.'

•  [Grove]'ramblings'on'object'models'•  Type'tree'maintained'similar'to'[Krall,'Vitek,'Horspool]''

'

Livemaps'(oopmaps)'

•  Registers'and'stack'slots'on'the'local'frame'that'contain'objects.''

•  Nothing'strange'here.'Required'for'non[conserva*ve'garbage'collec*on'of'any'sort.'

•  Internal'pointer'bit'•  Forms'the'root'set.''•  Rollforwarding'vs'the'safepoint'approach'

Transi*on'['Livemaps'

Memory'management'•  Garbage'collectors'– Concurrent'– Parallel'– Determinis*c'

•  With'or'without'genera*ons'

Memory'management'•  Concurrent'collec*on'–  Your'basic'genera*onal'concurrent'mark'and'sweep'collector'[Printezis,'Detlefs]'

–  Supports'mul*'genera*on'(>1)'young'spaces.'•  Combats'heavy'object'alloca*on'situa*ons.'•  Adap*vely'balanced'against'copy'overhead'

– Write'barriers'before'object'writes'– Minimize'stopping'the'world'–  Young'collec*ons'use'a'variant'of'stop'&'copy'

Memory'management'•  Can'also'run'with'a'parallel'policy'– Stop'the'world'and'clean'up'quickly'– Only'throughput'oriented'– No'write'barriers,'as'there'is'no'need'for'a'card'table'

Mark'&'Sweep'•  Backbone'of'GC'based'on'tradi*onal'tri[color'mark'and'sweep'

•  Adap*ve'thread'usage'and'addi*onal'concurrency'

Mark'&'Sweep'•  Two'colors'–'not'three.'– Object'is'in'one'of'two'sets'–  Live'objects:'grey'bits'(mix'of'grey'&'black'objects'in'tradi*onal'tri[coloring)'

– Dis*nc*on'handled'by'pu~ng'grey'objects'in'thread'local'queues'for'each'GC'thread.''

–  Parallel'threads'can'work'on'thread'local'data'–  Efficient'prefetching'is'possible'due'to'FIFO'order.'

No'permgen!'Ever!'

Other'nice'features'•  No'permgen!!!'Ever!'

Other'nice'features'•  No'permgen!!!'Ever!'•  Pinned'objects.'– Fast'memory'buffers'– Also'enable'non[con*guous'heaps'

Other'nice'features'•  No'permgen!!!'Ever!'•  Pinned'objects.'– Fast'memory'buffers.'– Also'enable'non[con*guous'heaps.'

•  Compac*on'– “Internal'and'external”.''– G1'evacuates'regions'instead'with'a'stop'the'world[and[copy'policy'similar'to'JRockit'YC'

Memory'management'•  Concurrent'GC'has'an'addi*onal'set:'live'bits'–  Contains'all'live'objects'in'the'system,'including'the'newly'created'ones.''

–  JRockit'can'quickly'find'objects'that'have'been'created'during'a'concurrent'mark'phase.''

–  Card'tables''•  Not'just'for'genera*onal'GC'•  Also'to'avoid'searching'the'en*re'live'object'graph'when'a'concurrent'mark'phase'cleans'up.'

•  Just'look'at'dirty'cards'at'the'end'of'the'mark'phase.'

Young'Collec*ons'•  A'variant'of'stop'and'copy'is'used.''– All'threads'are'halted'and'objects'are'deleted'or'promoted'– Hierarchical'breadth'first'copy'for'cache'locality'• Parallelizes'nicely'• Many'threads'always'harvest'a'young'space'

Young'Collec*ons'•  Young'and'old'collec*ons'may'occur'at'same'*me.'– All'bit'sets'and'data'structures'can'be'shared'as'long'as'the'old'collec*on'is'guaranteed'to'see'all'cards'that'have'become'dirty'during'a'concurrent'phase.'(Extra'card'table'to'record'this'“difference”'–'“modified'union'set”)''

– Keep'this'intact'for'old'collec*on'

Thread'Local'Alloca*on'•  Thread'local'alloca*on''•  Thread'local'areas'are'roughly'L2'cache'sized'and'objects'are'allocated'here'before'they'are'forced'upon'the'heap'

Compressed'References'•  For'less'than'4'(or'4'*'x)'GB'of'maximum'heap'size'

•  Use'32'bit'pointers'(or'32'+'log2(x)'bits)'CompRef compress(Ref ref) {

return (uint32_t)ref; //truncate reference to 32-bits

}

Ref decompress(CompRef ref) {

return globalHeapBase | ref;

}

Compressed'References'•  For'less'than'4'(or'4'*'x)'GB'of'maximum'heap'size'

•  Use'32'bit'pointers'(or'32'+'log2(x)'bits)'CompRef compress(Ref ref) {

return (uint32_t)ref; //truncate reference to 32-bits

}

Ref decompress(CompRef ref) {

return globalHeapBase | ref;

}

CompRef compress(Ref ref) {

return (uint32_t)(ref >> log2(objectAlignment));

}

Ref decompress(CompRef ref) {

return globalHeapBase | (ref << log2(objectAlignment));

}

Determinis*c'GC'•  QoS'level'for'latencies.'“No'more'than'X'ms”'•  Down'to'single'digits'on'modern'x86'hardware'

•  Caveat:'live'data'on'heap'is'the'main'constraint.''– Up'to'50%'of'heap'live'data's*ll'feasible'

Determinis*c'GC'

Determinis*c'GC'–'How?'

•  Greedy'strategy'– Postpone'stopping'the'world'for'as'long'as'possible.'

– Maybe'the'problem'goes'away'and'we'don’t'have'to'stop'the'world'

•  Split'up'everything'into'work'packets'– Drop'them'at'any'*me.'

Determinis*c'GC'–'How?'

•  Efficient'paralleliza*on.'– Mark'phase'is'90%'of'GC'*me''

•  Efficient'heuris*cs'– Some'more'work'in'e.g.'write'barriers'

Threads'and'Synchroniza*on'!

Threads'and'Synchroniza*on'•  A'java.lang.Thread'is'a'na*ve'thread.'–  Interes*ng,'though:'thread'pooling'and'pseudo'thin[threads'are'back,'for'example'in'Akka.'

–  Java'8'–'Collection.parallelStream– The'world'is'moving'towards'implicit'parallelism'in'general'

•  Most'of'the'JRockit'thread'code'and'adap*vity'logic'is'wriden'in'Java'

Threads'and'Synchroniza*on'•  Locks'are'thin'or'fat''– Adap*ve'infla*on'and'defla*on'

•  Lazy'locking'(biased'locking'supported)'– Adap*ve'heuris*cs'for'banning'and'retrying'the'lazy'approach.'

Threads'and'Synchroniza*on'public class PseudoSpinlock {

private static final int LOCK_FREE = 0; private static final int LOCK_TAKEN = 1;

public void lock() { //burn cycleswhile (cmpxchg(LOCK_TAKEN, &lock) == LOCK_TAKEN) {

micropause(); //optional}

}

public void unlock() { int old = cmpxchg(LOCK_FREE, &lock); //guard against recursive locksassert(old == LOCK_TAKEN);

} }

Threads'and'Synchroniza*on'•  Locks'are'thin'when'first'taken'•  Time'spent'in'lock'and'*mes'taken'triggers'infla*on'

•  wait'or'notify'immediately'inflates'a'lock'

•  Fat'locks'are'also'deflated'when'uncontended'for'too'long'

'

Threads'and'Synchroniza*on'

Threads'and'Synchroniza*on'•  Thin'lock'lifecycle'

Threads'and'Synchroniza*on'Thin'&'fat'lock'lifecycle'

Lock'Pairing'•  Bytecode'again'–'no'restric*on'on'matching'monitorenter'with'monitorexit

•  Not'all'of'them'can'be'analyzed'by'the'JIT'

Lock'Pairing'•  We'can'store'what'we'know,'and'make'unlocks'quick.''– Lock'tokens'(the'object'OR'3'bits)'•  Thin,'fat,'recursive,'lazily'taken,'unmatched'

– Livemap'system'contains'nes*ng'order.'

Op*miza*ons'•  A'lot'of'smallish'code'gen'transforms:'e.g.'Lock'fusion'•  “Fat'spin”'•  Lazy'unlocking'(biased'locking)'

–  Start'assuming'all'locks'are'lazy.'Tag'thin'locks'as'lazily'locked.'

–  If'object'already'lazily'locked'•  If'it’s'the'same'thread:'profit'•  Else'–'stop'the'lock'holder,'detect'the'“real”'lock'state'by'stack'walk.'Convert'to'thin'lock'or'forcefully'unlock'it'

–  Transfer'bits'–  Heuris*cs:'object'and'class'banning.'Ageing.'

Threads'and'Synchroniza*on'

•  Thin,'fat'&'lazy'lock'lifecycle'

Export'it'all!'–'JRockit'Mission'Control'

(now'Java'Mission'Control)'

!

@javamissionctrl!$JAVA_HOME/bin/jmc

Mission'Control'•  Use'“free”'run*me'informa*on!'–  JRockit'(Java)'Mission'Control'

•  JRockit'(Java)'flight'recorder'•  Memory'leak'detector'(JRockit'only)'•  Management'console'

•  $JAVA_HOME/bin/JCMD'(used'to'be'JRCMD)'•  Everything'in'the'VM'abstracted'into'an'event'that'may'or'may'not'have'a'dura*on'

•  Soon:'public'API'

Java'Flight'Recorder'•  Always'on'–  Excellent'for'debugging'and'analysis'of'crashes'–  Can'be'set'to'record'more'intrusively'for'periods'in'produc*on'•  E.g.'extensive'lock'profiling'

•  Everything'is'an'event'•  Buffered'recording'–'the'last'n'seconds'available'at'any'crash'or'when'a'command'is'given.'

•  Very'fine'precision.'– Mul*media'*mers'and'system'hardware'support'required'for'e.g.'latencies'

Latency'Analysis'

The'Management'Console'•  Peek'into'the'running'produc*on'JVM'•  Add'triggers'on'events'•  Interact'with'the'VM:'force'GC'etc.'

The'Memory'Leak'Detector'•  Introspect'the'type'graph'in'real*me.'Look'for'types'that'are'growing'despite'GC:s'

•  “Trending'alloca*ons”'

Studying'a'recording'offline'

JRockit'Virtual'Edi*on'

Is'the'JVM'an'OS?'

Is'the'JVM'an'OS?'

•  Add'a'coopera*ve'aspect'to'thread'switching'•  Zero[copy'networking'code'•  Reduce'cost'of'entering'OS'•  Balloon'driver'•  Runs'only'on'hypervisor'•  Facilitates'pauseless'GC'

Is'the'JVM'an'OS?'

Thank'you!''

Would!you!like!to!!

know!more?!

Oracle'JRockit'–''

the'Defini3ve'Guide'