List Processing in Real Time on a Serial

ComputerHenry G. Baker, Jr.

CS395T: Hadi EsmaeilzadehFebruary 2009

Outline

List Processing and Real-Time Garbage CollectionThe Minsky-Feinchel-Yochelson-Cheney-Arnborg (MFYCA) Copying GC MethodThe Serial Real-Time (SRT) GC MethodDiscussionCitation Statistics

List Processing and Real-Time Garbage CollectionInterpretation instead of compilation

Significantly increases the running timeGood compilers and new languages (SIMULA)

Inefficient storage structureCompilersCompact list representation CDR-coding

Garbage collectionTrace and mark all the accessible cellsBaker’s SRT System

Serial Real-Time GC System (Baker’s Algorithm)

Time required by each of the elementary operations

Bounded by a constant factor Independent of the number of live cellsPseudo real-time

Based on MFYCA’s copying GC algorithm (SIMULA)Basic idea: copy a small portion during each cons rather than copying a lot infrequentlySingle-processor solutionSpace efficiency

MFYCA Algorithm: The Setup

QuickTime™ and a decompressor

are needed to see this picture.

Before Garbage Collection

Flip is the first step of garbage collection.

QuickTime™ and a decompressor

are needed to see this picture.

MFYCA Algorithm: Copying The First Step

The cells pointed by registers are the first copied cells.

QuickTime™ and a decompressor

are needed to see this picture.

MFYCA Algorithm: Copying the Roots

Now the registers contain the forwarding addresses.

QuickTime™ and a decompressor

are needed to see this picture.

MFYCA Algorithm: Copying …

The dashed arrows show the forwarding addresses.

QuickTime™ and a decompressor

are needed to see this picture.

MFYCA Algorithm: Copying … Done!

The fromspace is now contains garbage.

QuickTime™ and a decompressor

are needed to see this picture.

MFYCA is Simple and ElegantOne path algorithm instead of 3 paths of

MarkSweepRelocate

No collector stack

Baker’s Algorithm and kKey idea: perform k iterations of GC before each allocationBoth semispaces contain accessible cells

User program is tricked to believe the GC is finished at the last flip time

All the registers are updated right after flipNew allocations are performed at the top of tospaceEach car and cdr checks for a forwarding address and updates if found

Baker’s Algorithm: Before GC (k=2)

The cells pointed to by registers are moved at the flip time.

QuickTime™ and a decompressor

are needed to see this picture.

Baker’s Algorithm: Copy the Roots (k=2)

Both of semispaces contain live cells.

QuickTime™ and a decompressor

are needed to see this picture.

Baker’s Algorithm: CONSing (k=2)

New nodes are allocated at the top of tospace.

QuickTime™ and a decompressor

are needed to see this picture.

Discussion

Discussion: Time-Space Trade offAll operation required bounded time

Independent of the number of live cells o(1)CONSing: c + k

(1 + 1/k) as much storage as needed by MFCYA

Trading space for timek=4 -> k=8 => 10% space saving

Would we need such a mathematical analysis?

Discussion: Space

Paper is very concerned about space (?!!)

With large k’s MFYCA and SRT use the same amount of storageMax MKSMW storage <= N(1.5+2m)Max SRT storage < N(2 + 2m)

Do we care about space?What do we care most about?How about embedded systems and portable devices?

Discussion: Unsupported CasesNo bounded-time operations for

Virtual memory machines !??Array structures

Copy and trace time is proportional to the length of array

What about memory hierarchy and caches?How about working set size?

What if the whole working set could have fit in the cache?

Memory trafficPower

Discussion: Breadth-First OrderThe graph of live objects is traversed in breadth first order

MFYCA and Baker

Would we care?Locality

Discussion: Proof by InductionIncremental GC is proved to be correct by induction

How important is the poof of correctness?

Discussion: Multi-core EraIs Baker’s algorithm extendible to multi-core systems?

Most of them use coherent shared memory systems

Coherence and Caching!?

Discussion: Imperative LanguagesHow well Baker’s algorithm applies to operations other than the list operations?

Program stackVectors and arraysHash tables

What about imperative languages?

Conclusions

Baker’s SRT Algorithm based on MFYCA copying GC algorithmContributions

Real-time: all operations require bounded timeSerial and single threadedSpace efficient and flexible

Trade off between space and time (through adjusting k)

Proven to be correctIt doesn’t run out of space when it should not

Citation Statistics

ACM portal: 183Google scholar: 606Guy E. Blelloch , Perry Cheng, On bounding time and space for multiprocessor garbage collection, ACM SIGPLAN Notices, v.39 n.4, April 2004A. M. Cheadle , A. J. Field , S. Marlow , S. L. Peyton Jones , R. L. While, Exploring the barrier to entry: incremental generational garbage collection for Haskell, Proceedings of the 4th international symposium on Memory management, October 24-25, 2004, Vancouver, BC, CanadaAndrew W. Appel, Real-time concurrent collection on stock multiprocessors, ACM SIGPLAN Notices, v.39 n.4, April 2004David F. Bacon , Perry Cheng , David Grove , Michael Hind , V. T. Rajan , Eran Yahav , Matthias Hauswirth , Christoph M. Kirsch , Daniel Spoonhower , Martin T. Vechev, High-level real-time programming in Java, Proceedings of the 5th ACM international conference on Embedded software, September 18-22, 2005, Jersey City, NJ, USADanny Dubé , Marc Feeley, BIT: A Very Compact Scheme System for Microcontrollers, Higher-Order and Symbolic Computation, v.18 n.3-4, p.271-298, December 2005David Siegwart , Martin Hirzel, Improving locality with parallel hierarchical copying GC, Proceedings of the 2006 international symposium on Memory management, June 10-11, 2006, Ottawa, Ontario, CanadaSylvain Stanchina , Matthias Meyer, Mark-sweep or copying?: a "best of both worlds" algorithm and a hardware-supported real-time implementation, Proceedings of the 6th international symposium on Memory management, October 21-22, 2007, Montreal, Quebec, CanadaFilip Pizlo , Erez Petrank , Bjarne Steensgaard, A study of concurrent real-time garbage collectors, ACM SIGPLAN Notices, v.43 n.6, June 2008Wolfgang Puffitsch, Decoupled root scanning in multi-processor systems, Proceedings of the 2008 international conference on Compilers, architectures and synthesis for embedded systems, October 19-24, 2008, Atlanta, GA, USAPascal Cuoq , Damien Doligez, Hashconsing in an incrementally garbage-collected system: a story of weak pointers and hashconsing in ocaml 3.10.2, Proceedings of the 2008 ACM SIGPLAN workshop on ML, September 21-21, 2008, Victoria, BC, CanadaWolfgang Puffitsch , Martin Schoeberl, Non-blocking root scanning for real-time garbage collection, Proceedings of the 6th international workshop on Java technologies for real-time and embedded systems, September 24-26, 2008, Santa Clara, CaliforniaFilip Pizlo , Antony L. Hosking , Jan Vitek, Hierarchical real-time garbage collection, ACM SIGPLAN Notices, v.42 n.7, July 2007

Resources

Michael Bond’s presentation in 2003 class