Inside the Garbage Collector - SDD...

InsidetheGarbageCollector

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Agenda

• ApplicationsandResources• TheManagedHeap• MarkandCompact• Generations• Non-MemoryResources• Finalization• HinderingtheGC• HelpingtheGC

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ApplicationsandResources

• Applicationsusemanydifferentkindsofresourceswhenexecuting– Memory– Filehandles– Databaseconnections– Sockets

• .NETprovidesinfrastructureandpatternsformanaginganapplicationsresources

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MemoryandtheManagedHeap

• .NEThasspecificfunctionalitytoautomatememorymanagement– Applicationsrequestmemoryusingnewkeyword– MemoryisallocatedinareacalledtheManagedHeap

• RuntimecleansupmemorywhennecessaryusingGarbageCollection (GC)– Onlymanagedheapisgarbagecollected.Unmanagedandstack

allocatedmemoryunaffected– GarbageCollectorremovesobjectsnolongerinuse

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HowtheGarbageCollectorWorks

• Applicationsrunasifmemoryisinfinite• GCisinvokedwhenresourcethresholdbreached• Assumesallobjectsaregarbage• Collectsthosenotinuse

A B C D E F B D E G

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WhatObjectsareinUse?

• Thegarbagecollectordoesnotcollectobjectsinuse– Howdoesitknowwhatisinuseandwhatisnot?

• GChasconceptofobjectsbeingreachable– Objectsthatcouldbeaccessedfromcodethatisyettorun

• GCwalksobjectsgraphunderliveroots– Non-nullstaticreferences– Localvariablesonthestackthatarestillinuse– Afewmoreexotickinds,e.g.thenativecodeinterop infrastructure

• AnyobjectreachablefromaliverootsurvivestheGC

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SpottheLiveRoots

• WhatreferencesareliverootswhentheGCruns?

static object o1 = new object();

void RunTest(){object o2 = new object();object o3 = new object();

GC.Collect();

o1 = null;

GC.Collect();

ProcessObject(o3);

GC.Collect();}

Whatisgarbagehere?

Whatisgarbagehere?

Whatisgarbagehere?

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GCOptimization- Generations

• ExaminingtheentireheapeveryGCtooexpensive– Optimization:ifobjectsurvivedlastGCthenprobablystillalive– Bydefaultonlychecksthemostrecentlyallocatedobjects

• .NETGCisGenerational– Threegenerations– Allobjectsallocatedintogen0– Anyobjectsurvivingacollectionispromotedtonextgeneration

A B C D E F B D E G

Gen0 Gen1 Gen0

GC

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TheImplicationsofGenerations

• RelativecostofGC– Gen0isverycheaptocollect– Gen1ismoreexpensivetocollectthatgen0– Gen2canbeveryexpensivetocollect

• HealthyGC– Orderofmagnitudebetweennumberofcollectionsofeachsuccessive

generationregardedashealthy– gen0:gen1:gen2=>100:10:1

• Generallydon’ttrytosecondguesstheGC

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GCOptimization– GCStyles

• ThreestylesofGC– Non-concurrent– Concurrent– Asynchronous

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ConcurrentandNon-Concurrent

• Non-concurrent– GCstartsandrunsthroughtocompletionwithoutinterruption– AlwaysusedforGen0andGen1collects

• Concurrent– Applicationthreadsinterleavedwithcollection– Keepsapplicationresponsive– OnlyusedforsomeGen2collects– CancontinueapplicationexecutionuntilanotherGCrequired

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Async

• Asynchronous– Introducedinversion4.0– Similartoconcurrentbutcanperformgen0andgen1collectsduringa

gen2collect

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ConcurrentGC

Gen2GCThread

ApplicationThread

ThreadSuspended

new()

new()

new()

Allocation requires Gen0 collect

Gen 2 collection complete

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BackgroundGC

Gen2GCThread

ApplicationThread

ThreadSuspended

new()

new()

new()

Allocation requires Gen0 collect

Gen 2 collection complete

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GCModes

• TherearethreeGCmodes– WorkstationConcurrent(default)– WorkstationNon-Concurrent– Server

• Controlledbyconfig flags<configuration>

<runtime><gcServer enabled="true"/><gcConcurrent enabled="false"/>

</runtime></configuration>

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WorkstationConcurrent

• DesignedtokeepUIapplicationsinteractive– UIthreadpausedasshortatimeaspossible

• Gen0andGen1usenon-concurrentGC– UIthreadpausetimewouldbeinsignificant

• Gen2canuseconcurrent– Gen2canbeexpensive– KeepsUIthreadpumpingthemessagequeue

• Gen2canuseasynchronous– MeansUIthreadstaysfree-runningevenifanotherGCisrequired– Onlyavailableon.NET4.0andlater

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Workstationnon-concurrent

• WholeGCisnon-concurrent– Optimizedfornon-UIcode– GCkidnapsallocatingthread– FewerCPUcyclesthanworkstationconcurrent

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ServerGC

• Tunedformultipleprocessors– Eachprocessorgetsownblockofmanagedheapforallocation/GC

calledarena– Allowslockfreeallocationacrossmultiplecores

• Alwaysnon-concurrent– GCtakesplaceonallprocessorsatthesametime

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GCOptimization– SelfTuning

• GCbehavior dependentonyourapplication– Usesheuristicstoworkoutefficientstrategy– IfGCofagenerationiseffectivethenperformsmore– IfGCofagenerationreclaimslittlememorythenperformsless

• GCselftuningnormallyproducesoptimalresults– Beststrategyforyourapplication

• AvoidGC.Collect– DisruptsGCstatisticsandsoselftuninglesseffective

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LargeObjects

• Movinglargeobjectstooexpensive– Largeobjectover85000bytes

• AllocatedinLargeObjectHeap

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MemoryisnottheonlyResource

• Onlymemorymanagementisautomated– Runtimedoesnotknowwhenuseofotherresourcesiscomplete

• Typesthatcontrolnon-memoryresourcesfollowstandardpattern– ImplementIDisposable– ConsumercallsDisposewhentheyarefinishedpublic interface IDisposable{

void Dispose();}

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WhenShouldYouImplementIDisposable?

• MostclassesdonotneedtoimplementIDisposable– Donotconsumeresourcesotherthanmemory

• ClassesthatdirectlycontrolresourcesacquiredviainteropmustimplementIDisposable– Noothermanagedtypecanreleasethoseresources

• ClassesthataggregateothertypesthatimplementIDisposable shouldimplementIDisposable themselves– ProvidesahooktoDisposeaggregatedobjects

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WhatifIdon’tCallDispose?

• Onlyanissuefortypesthatdirectlycontrolunmanagedresources– FileStream– SqlConnection– Icon

• Needlastchancemechanismforfreeingupresources– Nooneelsecanfreeupresources

• RuntimehasconceptofFinalization– Veryunlikelythatyourclasseswillneedafinalizer

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HowFinalizationWorks

• TypesthatsupportfinalizationoverrideFinalizevirtualmethodonSystem.Object– C#syntaxlookslikeaC++destructor

• Runtimeseestypehasfinalizer wheninstantiated– Putsobjectinfinalizationqueue

class SharedMemory{

~SharedMemory(){}

}

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FinalizationandGC

• WhenGCdecidestocollectobjectitseesit’sfinalizable– Movestheobjectontofreachable queue– freachable queueactsasliveroot(finalizationreachable)

• Separatefinalizationthreadprocessesfreachable queue– Callsfinalizer andremovesobjectfromqueue– ObjectcanbecollectednexttimeGClooksatit

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ConsequencesofFinalization

• Finalizable objectswillalwayssurviveoneGC– Alwayspromotedtogen1orgen2– Consequentlymoreexpensivetocollect

• Timingoffinalizer executionnon-deterministic– AfterGCnoticesitisgarbage– WhenFinalizationthreadgetsroundtoit

• Limitedfunctionalityinfinalizer– Shouldnotcallotherobjectsasmayalreadyhavebeenfinalized

• Finalizable classesshouldalwaysimplementIDisposable– Cansuppressfinalization inDispose

public void Dispose(){

GC.SuppressFinalize(this);}

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HinderingandHelpingtheGC

• ThreemainissuesforGC– Unnecessarypromotion– Referenceheavydatastructures– Pinning

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UnnecessaryPromotion

• WanttominimiseGen2collections– IdeallyallGen2objectsliveforever– Notrealistic

• Promotethendieisworstscenario– Badlytunedcaches– Finalizers– “ad-hoc”caching– Notreleasingdeadobjects

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UnnecessaryPromotion

• Tunecachetimeouts– DropcacheinGen1– Makeitlonglivedtomakethehitworthwhile

• Removeunnecessaryfinalizers– IfabsolutelyrequiredrefactorcodeintoSafeHandle

public abstract class SafeHandle : CriticalFinalizerObject,IDisposable

{...

}

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“Ad-hoc”Caching

• ConsiderWeakReference– ObjectaccessibleunlessGC’d

“ProbablyneedthisobjectagainsoonsoI’llhangontoit”

WeakReference wr = new WeakReference(new object());

object temp = wr.Target;if(temp == null){

Console.WriteLine("Collected");}else{

Console.WriteLine("Still alive");}

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HangingontoDeadObjects

• Setmemberandstaticreferencestonullwhendone– GCcancleanupassoonaspossible

• Especiallyimportantwhenreplacingobject– Becarefulofthreadingissues

class ReferenceData{

XElement data;

public void ReloadData(string filename){

data = null;data = XElement.Load(filename);

}}

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ReferenceHeavyStructures

• Markphasemadeharderbymanyreferences– GChastochasedownembeddedobjectgraphs

• Thinkofrevisedoroptimisedwaysofrepresentingdata– Arrays– AdjacencyMatrices

• Example:ThreadPoolQueuebefore4.0

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ThreadPoolQueueBefore4.0

• Linkedlistofworkitems– Rootisnevergarbage– Lotsofwork=lotsofreferences

Node WorkItem

Node WorkItem

Node WorkItem

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ThreadPoolQueueAfter4.0

• Linkedlistofarraysofworkitems– Lotsofwork=fewreferences

WorkItem

WorkItem

WorkItem

WorkItem

WorkItem

WorkItem

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Pinning

• PinningmeansGCcan’tmoveobject– Requiredininterop scenarios

• ConsiderinflatinglargebufferstobeallocatedinLOH– PinningirrelevantinLOH– ReuseLOHbuffers

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Summary

• GCautomatesmemoryreclamation• GCisheavilyoptimized• ReleasemodeGCmuchmoreaggressivethandebug• NonmemoryresourcesfreedupusingIDisposable pattern• Finalizationexistsasalastresortforframeworkclasses.You

shouldnotneeditinyourcode• YoucanhelptheGC• YoucanhindertheGC