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Uniprocessor Garbage Collection Techniques

This article explores various garbage collection techniques such as uniprocessor, Cheney copying, mark-sweep, tricolor, and incremental tracing. It discusses the advantages, pitfalls, and implementation details of each technique.

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Uniprocessor Garbage Collection Techniques

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  1. Uniprocessor Garbage Collection Techniques Paul R. Wilson

  2. FromSapce/Tospace before

  3. FromSpace/ToSpace after

  4. Cheney Breadth-first copying

  5. 3mb vs. 6mb

  6. The Two-Phase Abstraction • 1. Detection • 2. Reclamation

  7. Why Garbage Collect at All? • Safety • Memory leaks • Continued use of freed pointers • Simplicity

  8. Why Garbage Collect at All? • Flexibility • Hard coded program limits • Efficiency! • Who is responsible for deletion? • Extraneous copies

  9. Liveness and Garbage • There is a root set which is defined as live. • Anything reachable from a live pointer is also live • Everything else is garbage

  10. The Root Set • The Root Set • Static global and module variables • Local Variables • Variables on any activation stack(s) • Everyone else • Anything Reachable From a live value

  11. Reference Counting Advantages • Implicitly distributes garbage collection • Real Time guarantees with deferred reclamation • Keep a list of zeroed objects not yet processed • Memory efficiency, can utilize all available memory with no work room

  12. Reference Counting Pitfalls • Conservative- needs a separate GC technique to reclaim cycles • Expensive- pointer reassignment requires: • Increment • Decrement • Zero Check • Stack Variables frequent creation/destruction • Can be optimized to some extent

  13. Reference Counting

  14. Ref counting, unreclaimable

  15. Deferred Reference Counting • Defer deletion of zero counted objects • Periodically scan the stack for pointers

  16. Mark-Sweep Collection • Starting From the root set traverse all pointers via depth/breadth first search. • Free everything that is not marked.

  17. Non-Copying issues • Same as for traditional allocators • Fragmentation • Memory block size management • Locality of reference- interleaved new/old • General issues- work proportional to heap size

  18. Copying Advantages • Memory locality preserved • Disadvantages • Lots of copying! • “Scavenging”

  19. Stop and Copy • How to update multiple pointers to the same object? • Forwarding Pointers • Mark/Sweep is proportional to the amount of live data. Assuming this stays roughly constant, increasing memeory will increase efficiency.

  20. Non Copying Version • Facts • Allocated with a color • Fragmentation • Advantages • Does not require pointer rewriting • Supports obscure pointer formats, C friendly

  21. In place collection • Conservative estimates • Useful for languages like C • Pointers can be safely passed to foreign libraries not written with Garbage Collection in mind

  22. Incremental Tracing Collectors • The ‘Mutator’ • The reachability graph may change • From the garbage collectors point of view the actual application is merely a coroutine ir cuncurrent process with an unfortunate tendency to modify data structures that the collector is trying to traverse • Floating Garbage • Can’t survive more than one extra round

  23. Real Time Garbage Collection • Incremental Tracing Collectors • In Place Collection • Many readers single writer(mutator) • As a Copying Collector • Multiple Readers Multiple Writers

  24. Tricolor Marking • White • Initial color for an object subject to collection • Black • Objects that will be retained after the current round • Gray • Object has been reached, but not its descendents • Wave front effect

  25. A violation of the Coloring Invariant

  26. Read Barrier • Detects an attempt to read a white object and immediately colors it gray

  27. Write Barrier • Traps attempts to write a pointer into an object

  28. Some algorithms • Snapshot-at-beginning write barrier • Black-only read barrier • Baker’s read barrier • Dijkstra’s write Barrier • Steele’s write Barrier

  29. Baker’s Read Barrier • Allocates Black • Grey Objects cannot be reverted to white • Immediately Invalidates fromspace • Any pointer access to fromspace causes the GC to grey the target object by copying it to tospace if necessary and updating the pointer.

  30. Baker’s Non Copying Scheme • Real Time Friendly

  31. Treadmill

  32. Black Only Read Barrier • When a white object in fromspace is touched it is scanned completely.

  33. Replication Copying Collection • Until copying from from space to to space is completed, the mutator continues to read from from space. • Write updates must be trapped to update tospace. • Single simultaneous ‘flip’ where all pointers are updated. • Expensive for standard hardware, but cheap for functional languages

  34. Real time considerations • Read Barriers add an unpredictable cost per pointer access • Nilson background scavenger, reserve only • Write barrier may be more expensive overall, but the cost per access is well bounded • Guaranteeing progress allocation clock, frees per allocation • Statically allocate troublesome objects

  35. Results • Writer barrier more efficient on standard hardware

  36. Snapshot at the Beginning • Catches pointers which try to escape from white objects • If a pointer is replace in a black object, the replaced pointer is first stored. All overwritten pointers are saved via a write barrier. • All objects that are live at the beginning of collection remain live • Allocate Black during collection round • Incremental Update • Reverts black to gray when an object is written to, or else grays they new pointed to object

  37. Incremental Update with Write-Barrier(Dijkstra)g • Catches pointers that try to hide in black objects • Reverts Black to gray • If the overwritten pointer is not pointed to elsewhere then it is garbage • Allocated white. Newly allocated objects assumed unreachable

  38. Motivation for a new Strategy • Most objects are short lived • 80% to 90% die within a few million instructions • Objects that don’t die quickly are more likely to live a while • Long lived objects are copied over and over • Excessive Paging in Scanning if the heap must exceed available physical memory

  39. Generational Garbage Collection

  40. Generational gc before

  41. Generational gc after

  42. Gc memory usage

  43. Variations of generational collection • Intergenerational references • Write barrier • Old to younger • Young to old • Collection • Advancement policies • Advance always • Advance after 2 rounds Counter in the header field? Advance always? Semispace in the last generation 3 spaces Bucket brigade Mark compact in the oldest generation for memory efficiency

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