Uniprocessor Garbage Collection Techniques Paul R. Wilson

FromSapce/Tospace before

Ft after

Too much

3mb vs. 6mb

The Two-Phase Abstraction 1. Detection 2. Reclamation

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

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

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

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

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

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

Reference Counting

Ref counting, unreclaimable

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

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

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

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

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.

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

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

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

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

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

A violation of the Coloring Invariant

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

Write Barrier Traps attempts to write a pointer into an object

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

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.

Baker’s Non Copying Scheme Real Time Friendly

Treadmill

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

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

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

Results Writer barrier more efficient on standard hardware

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

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

Motivationfor 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

Generational Garbage Collection

Generational gc before

Generational gc after

Gc memory usage

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