1. Distributed Garbage Collection
An Overview
Presented by Dotan Adler
Copyright, 1996 © Dale Carnegie & Associates, Inc.

2. Representation Outline
Present the need for Distributed GC
Present the Distributed Model
Present the problem of Distributed GC
Present some outlines of solutions to the problem (one Direct, and two Indirect)
Conclusion
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Distributed Garbage Collection

3. The need for a Distributed GC
Where do we need Distributed GC ?
Distributed applications (DCOM, CORBA, etc ..)
Internet applications (Java extensions)
Distributed file servers
HTML pages & links
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Distributed Garbage Collection

4. The Distributed Computing Model
A Distributed System is a set of Autonomous systems connected by a network.
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5. The Classical GC Problem
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6. Distributed Garbage Collection
Each computer in a distributed system has :
A local store (memory or disk)
A stack
Local running programs (or RPCs)
A GC algorithm (a part of a global GC algorithm)
Each computer has access only to local store
Access to remote store is achieved by message passing
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7. Distributed Garbage Collection
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Distributed Garbage Collection

8. Distributed Garbage Collection
Because there is no global address space, references to remote cells are necessarily indirect
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9. Problems With Distributed GC
Unreliability (Relaxed)
Duplications of messages
Out of order delivery
Communication fail
Latency - the elapsed time between the issue of a task and when it is executed is undeterministic
Synchronization - it takes a lot of resources to synchronize all computers in a network
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10. Distributed Garbage Collection
Direct Approach
Direct approach means to identify the garbage and mark it, so that it could be removed
Reference counting is a direct GC approach since it marks garbage with count equals to 0
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Distributed Garbage Collection

11. Distributed Garbage Collection
Reference Counting
Every object has a reference count
message increases a’s count
decreases the count
“Inc”, and “Dec” messages are sent to the CPU which hosts the object
Deletion occurs when reference equals 0
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12. Reference Counting (Cont’d)
Problem : If A or C were responsible for incrementing b’s count, then latency is an important factor
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13. Reference Counting (Cont’d)
Solution : When A it first sends an ack-request to B and a copy message to C
B will not accept any request after it gets an ack_req until it sends an ack to C
C will not until it get
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14. Reference Counting (Cont’d)
Problems
Does not destroy inter-site cyclic references
Relies on blocking of operations
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15. Distributed Garbage Collection
Indirect Approach
Indirect approach means to identify all the live objects first, and then reclaim all space not used by “live” objects
Examples of indirect GC algorithms :
Mark-scan
Generation scavenging
Dijkstra’s 3-color algorithm
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16. Centralized Indirect Solution
Let every site run a GC locally on it’s private memory.
RCT = remote cell table ERT = exit reference table
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17. Centralized Indirect Solution (Cont)
Still we have problems with inter-site cycles.
For this we add a centralized service :
Whenever a local site finishes it’s GC it sends a list of RCT to ERT paths, which are inaccessible from the roots to the centralized service.
The centralized service can then find dead inter-site cycles, and report them to the sites.
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18. Distributed Garbage Collection
Cell Migration
We can solve the inter-site cycles problem by migrating cells, so that inter-site cycles become local cycles.
We need to define an order, among the sites, so that we won’t get a cyclic migration. This way a cell migrates only to an inferior site.
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19. Distributed Garbage Collection
Cell Migration
Problems:
Copy operations could be very slow - especially when working with big networks, and big data structures
An order between sites must be set prior to the running of the algorithm
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20. Distributed Garbage Collection
Empirical Results
Little work has been done in the field of DGC because it is hard to compare the execution of two distributed systems. (delay times, synchronization, etc …)
Tests, however, show that Direct methods give better pause times than Indirect methods.
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21. Future Directions of research
Use some hybrid methods (Direct & Indirect)
Try to create a framework for testing Distributed GC algorithms
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22. Distributed Garbage Collection
Conclusions
Distributed GC adds a new dimesion to old-style GC. It adds the dimension of asynchronity & latency.
Alltought standard GCs suffer from the same problems (memory leaks, pause time), creating a robust DGC is much more complicated.
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23. Distributed Garbage Collection
The End
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