MOSTLY PARALLEL GARBAGE COLLECTION

Authors : Hans J. Boehm

Alan J. Demers

Scott Shenker

XEROX PARC

Presented by:REVITAL SHABTAI

Introduction

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

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

Garbage Collectors Styles

Two basic styles of garbage collection algorithms:

1.Reference-counting collector

2.Tracing collector

Traditional Tracing A set of root memory objects -always

accessible MarkMark - performing reachability from

the roots and marking the accessible objects

SweepSweep - reclaiming unmarked objects

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

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

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

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

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

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

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

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

Partial Collector - Formal Definitions Partial CollectorPartial Collector - a collector that reclaims

only a subset of all unreachable objects A set TT (threatened) - potentially collectible

objects A set I I (immune) - the non-collectible

objects

Formal Statements

CC : Every object in I I is marked and every object pointed to by a marked object is also marked

TRTR : Mark all objects in I I and trace from them FF : Trace from all marked objects on dirty pages

Stop the mostly world parallel Stop the world Clear all mark bits Perform TRTR Restart the world

Clear all mark bits Clear all virtual

dirty bits Perform TR TR Stop the world Perform FF Restart the world

Condition C holds Condition C

does not hold

C’C’ : Every object in I I is marked and every object pointed to by a marked object on a clean page is also marked

Reducing the stop-the-world Delay MM - 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 M on the heap 2.Stop the world 3.Perform FF 4.Restart the world

Reducing the stop-the-world Delay- Version 2 M’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

Implementation Choices Repeated applications of M’M’ instead of a single

application of MM Run MM more that once before a partial collection Further variants of M M are possible (such as MM

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

Empirical ResultsAve Pause Times Max Pause

Times No. of

Collections (full) Total Time In

Sec10MBalloc20meg

46323 51350 3(3) 332.2 Full 125 870 20(0) 24.4 Gen 102 350 11(0) 32.0 Gen.Par

Ave Pause Times Max Pause

Times No. of

Collections (full) Total Time In

Sec10MB Boyer

56548 63380 4(4) 512.6 Full 9291 41840 22(5) 360.6 Gen 169 329 12(2) 259.6 Gen.Par

Ave Pause Times Max Pause

Times No. of

Collections (full) Total Time In

Sec36MBalloc20meg

1037 1040 3(3) 16.4 Full 81 200 17(0) 15.5 Gen 76 100 11(0) 17.2 Gen.Par

Ave Pause Times Max Pause

Times No. of

Collections (full) Total Time In

Sec36MB Boyer

1368 1610 4(4) 51.6 Full 528 1471 22(5) 60.4 Gen 134 159 13(2) 66.1 Gen.Par

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

Mostly Parallel Copying Collector Implementation

Additions to the copying collector:1.Clear the forwarding links2.Update dirty pages bits3.Update forwarding links

In the stop-the-world phase for any object OO that lie on dirty page:

1.Copy any object OO points at2.Update pointers in copies3.Copy O O and update its fields

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

THE END