ConsiderConsider

Starting with 160 k of memory do:Starting with 160 k of memory do: Allocate p1 (50 k)Allocate p1 (50 k) Allocate p2 (30 k)Allocate p2 (30 k) Allocate p3 (40 k)Allocate p3 (40 k) Free p2Free p2 Allocate p4 (40 k)Allocate p4 (40 k) Free p3 Free p3 Allocate p5 (60 k)Allocate p5 (60 k) Free p1Free p1 Allocate p6 (30k)Allocate p6 (30k)

Memory Allocation Memory Allocation AlgorithmsAlgorithms

Design YOUR algorithm for allocation Design YOUR algorithm for allocation and deallocation of memoryand deallocation of memory

Memory ManagementMemory Management Dynamic (heap)Dynamic (heap) Significant issuesSignificant issues

Significant execution time (16%)Significant execution time (16%) Memory performance not uniformMemory performance not uniform Allocation policiesAllocation policies BugsBugs

Dereference problemsDereference problems Memory leaksMemory leaks

Memory Allocation Memory Allocation StrategiesStrategies

Explicit vs. Implicit Memory AllocatorExplicit vs. Implicit Memory Allocator

General purpose vs. custom allocatorGeneral purpose vs. custom allocator

Software vs. hardwareSoftware vs. hardware

Allocation ExamplesAllocation Examplesp1 = malloc(4)

p2 = malloc(5)

p3 = malloc(6)

free(p2)

p4 = malloc(2)

Goals of Good Goals of Good malloc/free malloc/free

Good execution-time performance Good execution-time performance

Good space utilizationGood space utilization

Good locality propertiesGood locality properties

FragmentationFragmentation

Poor memory utilization --- Poor memory utilization --- fragmentationfragmentation Internal – overhead associated with Internal – overhead associated with

a block of memorya block of memory External – have enough blocks of External – have enough blocks of

memory for a request, but not memory for a request, but not contiguouscontiguous

Space in use

Internal fragmentation

External External FragmentationFragmentation

p1 = malloc(4)

p2 = malloc(5)

p3 = malloc(6)

free(p2)

p4 = malloc(6)

External fragmentation depends on future requests;thus difficult to anticipate

Bidirectional Coalescing Bidirectional Coalescing

Boundary tagsBoundary tags [Knuth73] [Knuth73]

Replicate size/allocated word Replicate size/allocated word at bottom of free blocksat bottom of free blocks

Allows us to traverse the Allows us to traverse the “list” backwards, but “list” backwards, but requires extra spacerequires extra space

Important and general Important and general technique!technique!

Boundary Tags Boundary Tags

size

1 word

Format ofallocated andfree blocks

ApplicationMemory

(and padding?)

a = 1: allocated block a = 0: free block

size: total block size

Application memory(allocated blocks only)

a

size aBoundary tag (footer)

4 4 4 4 6 46 4

Header

Your turnYour turn

Using boundary tag data structure, Using boundary tag data structure, define algorithms for:define algorithms for:

AllocationAllocation FreeFree

Key Allocator PoliciesKey Allocator Policies Placement policy:Placement policy:

First fit, worst fit, best fit, etc.First fit, worst fit, best fit, etc. Trades off lower throughput for less Trades off lower throughput for less

fragmentationfragmentation Splitting policy:Splitting policy:

When do we go ahead and split free blocks?When do we go ahead and split free blocks? How much internal fragmentation are we willing How much internal fragmentation are we willing

to tolerate?to tolerate? Coalescing policy:Coalescing policy:

Immediate coalescing: coalesce adjacent blocks Immediate coalescing: coalesce adjacent blocks each time free is called each time free is called

Deferred coalescing: try to improve Deferred coalescing: try to improve performance of free by deferring coalescing performance of free by deferring coalescing until needed. e.g.,until needed. e.g.,

RefinementsRefinements

Separate listsSeparate lists Binary buddyBinary buddy Lea allocatorLea allocator Custom allocatorsCustom allocators

Lea AllocatorLea Allocator An approximate best-fit allocator with An approximate best-fit allocator with

different behavior based on object sizedifferent behavior based on object size Small Objects (<64 bytes) allocated by exact-Small Objects (<64 bytes) allocated by exact-

size quicklistssize quicklists Medium Objects (<128K) – Medium Objects (<128K) – coalesce coalesce quicklistsquicklists Large Objects – allocate and free by Large Objects – allocate and free by mmapmmap

Generally considered the best allocator Generally considered the best allocator known (as of 2000, anyway)known (as of 2000, anyway)

http://g.oswego.edu/dl/html/malloc.htmlhttp://g.oswego.edu/dl/html/malloc.html

Why programmers use Why programmers use Custom Allocators?Custom Allocators?

Improving runtime performanceImproving runtime performance Reducing memory consumptionReducing memory consumption Improving software engineering (?)Improving software engineering (?)

Alternative Memory Alternative Memory ManagementManagement

Region (arenas) Region (arenas) Reserve memory blocks for program Reserve memory blocks for program

“parts”“parts” Deallocate entire regions, not per Deallocate entire regions, not per

allocationallocation Garbage collectionGarbage collection

Programmer allocates but doesn’t freeProgrammer allocates but doesn’t free ““System” keeps track of memory System” keeps track of memory

“pointed to” locations, removes the rest“pointed to” locations, removes the rest JavaJava

Why Garbage Collect at All?Why Garbage Collect at All?

SafetySafety Memory leaksMemory leaks Continued use of freed pointersContinued use of freed pointers

SimplicitySimplicity CorrectnessCorrectness Programming easeProgramming ease

The Two-Phase AbstractionThe Two-Phase Abstraction

1. Detection1. Detection

2. Reclamation2. Reclamation

Liveness and Garbage Liveness and Garbage

There is a root set which is defined There is a root set which is defined as live.as live.

Anything reachable from a live Anything reachable from a live pointer is also livepointer is also live

Everything else is garbageEverything else is garbage

The Root SetThe Root Set

The Root SetThe Root Set Static global and module variablesStatic global and module variables Local VariablesLocal Variables Variables on any activation stack(s)Variables on any activation stack(s)

Everyone elseEveryone else Anything Reachable From a live valueAnything Reachable From a live value

Reference Counting Reference Counting

Each allocated chunk has reference Each allocated chunk has reference count that shows how many locations count that shows how many locations point (refer) to this one.point (refer) to this one.

Advantages ???Advantages ??? Disadvantages ???Disadvantages ???

Mark-Sweep CollectionMark-Sweep Collection

Starting from the root set traverse all Starting from the root set traverse all pointers via depth/breadth first pointers via depth/breadth first search.search.

Free everything that is not marked.Free everything that is not marked.

More Information/DetailMore Information/Detail

If you wish to know more:If you wish to know more:

https://www.mpi-inf.mpg.de/https://www.mpi-inf.mpg.de/departments/rg1/teaching/departments/rg1/teaching/advancedc-ws08/script/lecture09.pdfadvancedc-ws08/script/lecture09.pdf