seminar 15-06-2004 sanchez distributedgarbagecollection

7/29/2019 Seminar 15-06-2004 Sanchez DistributedGarbageCollection

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1Concurrency Seminar: Distributed Garbage Collection for Wide Area Replicated Memory

Distributed Garbage Collection for

Wide Area Replicated Memory

from Alfonso Snchez, Lus Veiga, Paulo Ferreira

presented by Andrea Rezzonico


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Overview

1.Introduction

2.Memory Model

3.Algorithm4.Example


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Introduction

local root

x

z

y


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Introduction (2)

local root

x

z

y

y is not reachable from the process's local root


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Introduction (3)

local root

x

z

y

other processes may have a reference to y

local root

w


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Current Solutions

Distributed reference counting

Problem with cycles

Reference listing

Mark & sweep

count = 1

count = 1

What happen if we allow object replication?


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Replication

local root

z

local root

x

local root

x

Mark & sweep consider z garbage

process i process j


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Replication (2)

local root

z

local root

x

local root

x

but after a propagation z is reachable

propagate(x)i->j

process i process j


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Motivation

Shortcomings of Current Solutions

Replication is not considered

Constraints on scalability

Solution

Deals with replication

Does not impose constraints on scalability

It is indipendent of the used coherence protocol Does not deal with failures (i.e. communication failures)


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Overview

1.Introduction

2.Memory Model

3.Algorithm4.Example


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WARM Model

Several communicating processes

Memory

Mutator: modify the reference graph throught

assignment (i)

Coherence engine: propagte the object replicas to

keep them coherent (propagate(y)i->j

)


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What is Garbage?

local root

z

local root

x

local root

x

process i process j


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What is Garbage? (2)

Unreachable objects are considered garbage

Union RuleA target object z is considered

unreachable only if the union of all

the replicas of the source objects

do not refer to it.


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Overview

1.Introduction

2.Memory Model

3.Algorithm

4.Example


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Data Structures

stub (OutRef, SourceObj, Scion, Chain)

scion (InRef, Stub, Chain)

local root

z

local root

x

process i

process j

Z

Z

scionstub


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Data Structures (2)

inPropList / outPropList

(propObj, propProc, sentUmess/recUmess)

process i

local root

process j

local root

process k

local root

x x

z

inPropList outPropList


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Data Structures (3)

inPropList / outPropList

(propObj, propProc, sentUmess/recUmess)

process i

local root

process j

local root

process k

local root

x x

z


x j 0 x i 0


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Garbage Collector Root

Local root (stacks and static variables)

Set of scions

inPropList

outPropList


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Messages

unreachable

object replica is reachable only from the inPropList

process i

local root

process j

local root

x x


x i 0x j 0


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Messages (2)

unreachable

object replica is reachable only from the inPropList

process i

local root

process j

local root

x x


x i 1x j 1

unreachable(x)


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Messages (3)

reclaim

all object replicas are reachable only from the inPropList

process ilocal root

process j

local rootx

x

inPropList

outPropList

x i 1

x j 1

process k

local root

x

inPropList

x j 1

x k1


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Messages (4)

reclaim

all object replicas are reachable only from the inPropList

process ilocal root

process j

local rootx

x

inPropList

outPropList

x j 1

process k

local root

x

inPropList

x j 1

reclaim(x)

reclaim(x)


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Messages (5)

newSetStubs

when a new set of stubs is available

process i

local root

process j

local root

y

x

y

y


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Messages (6)

newSetStubs


process i

local root

process j

local root

y

x

y

y

newSetStubs


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Messages (7)

newSetStubs


process i

local root

process j

local root

y

x


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DGC Algorithm

Local Collector

Starts the trace from the process's local root, the set of scions,

the inPropList and the outPropList

For every outgoing inter-process reference a stub is createdand added to the new set of stub

For objects reachable only from the inPropList an

unreachable message is sent and the sentUmess bit is set

For objects reachable only from the outPropList for which an

unreachable message from all process was received a

reclaim message is sent


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DGC Algorithm (2)

Distributed Collector

Does not require waiting for replicas to be coherent

From time to time a newSetStubs message is sent to the

processes holding the scions corresponding to the stubs in theprevious stub set

unreachable message received

set recUmess bit

reclaimmessage received

delete the corresponding entry in the inPropList


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Overview

1.Introduction

2.Memory Model

3.Algorithm

4.Example


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Example

propagate(y)j->i

i

propagate(x)i->j

j

propagate(y)j->i

i

xj

:= 0

xi

:= 0

process i

local root

process j

local root

process k

local root

x

y

x

y

z

S1

S2

Z

Z

x j 0

y j 0

x i 0

y i 0



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Example (2)

propagate(y)j->i

i

propagate(x)i->j

j

propagate(y)j->i

i

xj

:= 0

xi

:= 0

process i

local root

process j

local root

process k

local root

x

y

x

y

z

S1

S2

Z

Z

x j 0

y j 0

x i 0

y i 0


S4

Z

S3

Z


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Example (3)

propagate(y)j->i

i

propagate(x)i->j

j

propagate(y)j->i

i

xj

:= 0

xi

:= 0

process i

local root

process j

local root

process k

local root

x

y

x

y

z

S1

S2

Z

Z

x j 0

y j 0

x i 0

y i 0


S4

Z

S3

Z


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Example (4)

propagate(y)j->i

i

propagate(x)i->j

j

propagate(y)j->i

i

xj

:= 0

xi

:= 0

process i

local root

process j

local root

process k

local root

x

y

x

y

z

S1

S2

Z

Z

x j 0

y j 0

x i 0

y i 0


S4

Z

S3

Z

S6

Z

ZS5

S7Z


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Example (5)

propagate(y)j->i

i

propagate(x)i->j

j

propagate(y)j->i

i

xj

:= 0

xi

:= 0

process i

local root

process j

local root

process k

local root

x

y

x

y

z

S1

S2

Z

Z

x j 0

y j 0

x i 0

y i 0


S4

Z

S3

Z

S6

Z

ZS5

S7Z


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Example (6)

propagate(y)j->i

i

propagate(x)i->j

j

propagate(y)j->i

i

xj

:= 0

xi

:= 0

process i

local root

process j

local root

process k

local root

x

y

x

y

z

S1

S2

Z

Z

x j 0

y j 0

x i 0

y i 0


S4

Z

S3

Z

S6

Z

ZS5

S7Z


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Example (7)

propagate(y)j->i

i

propagate(x)i->j

j

propagate(y)j->i

i

xj

:= 0

xi

:= 0

process i

local root

process j

local root

process k

local root

x

y

x

y

z

S1

S2

Z

Z

x j 0

y j 0

x i 0

y i 0


S4

Z

S3

Z

S6

Z

ZS5

S7Z


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Example (8)

propagate(y)j->i

i

propagate(x)i->j

j

propagate(y)j->i

i

xj

:= 0

xi

:= 0

process i

local root

process j

local root

process k

local root

x

y

x

y

z

S1

S2

Z

Z

x j 0

y j 0

x i 0

y i 0


S4

Z

S3

Z

S6

Z

ZS5

S7Z


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Example (9)

propagate(y)j->i

i

propagate(x)i->j

j

propagate(y)j->i

i

xj

:= 0

xi

:= 0

process i

local root

process j

local root

process k

local root

x

y

x

y

z

S1

S2

Z

Z

x j 0

y j 0

x i 0

y i 0


S4Z

S3

Z

S6

Z

ZS5

S7Z


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Remarks

xiis reachable only

from the inPropList

xj

is reachable only

from the outPropList

zk

is reachable only

from the scion S1

no object can becollected yet

process i

local root

process j

local root

process k

local root

x

y

x

y

z

S1

S2

Z

Z

x j 0

y j 0

x i 0

y i 0


S4Z

S3

Z

S6

Z

ZS5

S7Z


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Collecting Garbage

LGC in process i

xiis reachable only

from the inPropList

an unreachable

message is sent

from i to j

the sentUmess bit

is set

process i

local root

process j

local root

process k

local root

x

y

x

y

z

S1

S2

Z

Z

x j 1

y j 0

x i 1

y i 0


S4Z

S3

Z

S6

Z

ZS5

S7Z

unreachable(x)


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Collecting Garbage (2)

LGC in process j

xj

is reachable only

from the outPropList

delete the entry from

the outPropList in j

send a reclaim

message

delete the entry from

the inPropList in i

process i

local root

process j

local root

process k

local root

x

y

x

y

z

S1

S2

Z

Z

y j 0 y i 0


S4Z

S3

Z

S6

Z

ZS5

S7Z

reclaim(x)


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LGC in process j

xj

is collected

S7 is deleted

a newSetStubs

message is sent

to process i

S6 is deleted

process i

local root

process j

local root

process k

local root

x

y y

z

S1

S2

Z

Z

y j 0 y i 0


S4Z

S3

Z

S6

Z

ZS5

S7Z

newSetStubs


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LGC in process i

x is reclaimed in

process i

S5 and S4 are

deleted

a newSetStubs

message is sent to

process j

the distributed collector

deletes S3 in process j

process i

local root

process j

local root

process k

local root

y y

z

S1

S2

Z

Z

y j 0 y i 0


S4Z

S3

Z

ZS5

newSetStubs


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LGC in process j

S2 is deleted

a newSetStubs

message is sent

from j to k

S1 is deleted

process i

local root

process j

local root

process k

local root

y y

z

S1

S2

Z

Z

y j 0 y i 0


newSetStubs


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LGC in process k

z can now be

collected

process i

local root

process j

local root

process k

local root

y y

z

y j 0 y i 0



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Conclusion

Advantages

Deals with replication

Scalability

Orthogonal to the coherence engine

Disadvantages

Not fault-tolerant


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Questions?

A di S f R l


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Appendix 1: Safety Rules

Safety rule 1 (Clean Before Send Propagate)Before sending a propagate message for an object y from a

process j, y must be cleaned (i.e. it must be scanned for

references) and the corresponding scions created in j

Safety rule 2 (Clean Before Deliver Propagate)Before delivering a propagate message for an object y in a

process i, y must be cleaned (i.e. it must be scanned for

outgoing inter-process references) and the corresponding stubs

created in i, if they do not exist yet.

seminar 15-06-2004 sanchez distributedgarbagecollection

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