MOSTLY PARALLEL GARBAGE COLLECTION

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MOSTLY PARALLEL GARBAGE COLLECTION

• Authors Hans J. Boehm
• Alan J. Demers
• Scott Shenker
• XEROX PARC
• Presented byREVITAL SHABTAI

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Introduction

• A method for adapting garbage collectors designed
  to run sequentially with the client.

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Overview

• Problem introduction
• Basic idea
• Method components
• Implementation choices
• Empirical results
• Conclusions

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Garbage Collectors Styles

• Two basic styles of garbage collection
  algorithms 1.Reference-counting
  collector 2.Tracing collector

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Traditional Tracing

• A set of root memory objects -always accessible
• Mark - performing reachability from the roots
  and marking the accessible objects
• Sweep - reclaiming unmarked objects

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The Problem

• A straightforward implementation prevents any
  client action while tracing (stop-the-world)
• Sometimes the tracing is an intolerable long
  pause
• Two approaches to reduce pause times of tracing
  collectors 1.Generational collectors
  2.Parallel collectors

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The Basic Idea

• Most of the tracing algorithm runs in parallel
  with the client
• Small portion of the tracing is done during
  stop-the-world action
• Relying on virtual memory information about pages
  that have been updates during a given period of
  time

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The Parallel Method

• Define a set of dirty bits for a set of virtual
  pages
• Clear all virtual dirty bits
• Perform a parallel tracing
• Update the dirty bits to reflect client writes
• Stop-the-world and trace from all marked objects
  that lie on dirty pages
• Reclaim unreachable (unmarked) objects

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Method - Comments

• The parallel tracing phase provides an
  approximation to the true reachable set
• The stop-the-world tracing phase provides a full
  complete set of the reachable objects
• The duration of the final stop-the-world phase is
  related to the number of pages written during the
  parallel collection

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Method -Comments 2

• Not all unreachable objects are reclaimed, but
  will be reclaimed by a subsequent collection
• Copying collectors (moving objects) require a
  more careful treatment

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The Mark Phase

• Split the heap into blocks
• In each block objects of the same size
• A block size is the a physical page
• Set a bit for each accessible object
• Queue pages for sweeping in a separate queue for
  each object size

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The Sweep Phase

• Done with object allocation (not during a
  stop-the-world)
• The allocator keeps a free lists for each object
  size
• If an empty free list is found
• Sweep the first page in the queue
• Remove the page from the queue
• Restore unreachable objects from this page to the
  free list

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Consequences

• Worst case of mark phase duration is as in the
  non-parallel tracing
• Worst case of allocating objects is when full
  pages are scanned in order to find free
  objects
• In practice both are not noticeable

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Partial Collector - Formal Definitions

• Partial Collector - a collector that reclaims
  only a subset of all unreachable objects
• A set T (threatened) - potentially collectible
  objects
• A set I (immune) - the non-collectible objects

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Formal Statements

• C Every object in I is marked and every object
  pointed to by a marked object is also marked
• TR Mark all objects in I and trace from them
• F Trace from all marked objects on dirty pages

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Stop the mostly world
parallel

• Stop the world
• Clear all mark bits
• Perform TR
• Restart the world

• Clear all mark bits
• Clear all virtual dirty bits
• Perform TR
• Stop the world
• Perform F
• Restart the world

Condition C holds
Condition C does not hold
C Every object in I is marked and every object
pointed to by a marked object on a clean page is
also marked
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Reducing the stop-the-world Delay

• M - Atomically clear the virtual dirty bits from
  the pages P, and trace from the marked objects on
  the dirty pages of P.
• A Partial Generational Mostly Parallel
  Collection 1.Perform M on the heap 2.Stop
  the world 3.Perform F 4.Restart the
  world

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Reducing the stop-the-world Delay- Version 2

• M 1.Atomically clear the dirty bits from
  the pages P 2.For all marked objects
  pointed to by marked objects on dirty pages of P
  mark them and dirty the pages on which they
  reside

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Implementation Choices

• Repeated applications of M instead of a single
  application of M
• Run M more that once before a partial collection
• Further variants of M are possible (such as M
  mark from other sets)
• When to run a full collection? When to expand
  the heap?
• How to control the scheduling of the client
  and collector threads

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Empirical Results
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Mostly Parallel Copying Collectors

• For each object, additional space is required for
  forwarding links
• The collector copies from from-space to to-space
  and update the forwarding links

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Mostly Parallel Copying Collector Implementation

• Additions to the copying collector 1.Clear the
  forwarding links 2.Update dirty pages
  bits 3.Update forwarding links
• In the stop-the-world phase for any object O that
  lie on dirty page 1.Copy any object O points
  at 2.Update pointers in copies 3.Copy O and
  update its fields

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Conclusion

• Mostly parallel partial generational tracing
  collector that run sequentially with the client
• Various implementation choices for various cases
• Reduced the stop-the-world phase time duration

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THE END