Lecture 3: Dynamic Memory Allocation and Garbage Collection

Background Material

Review memory allocation in C/C++ (new, delete, malloc, free)
Review list processing in scheme (cons, car, cdr, null?)

Reading

Chapters 5 (sec. 7) and 8 (sec. 5) of the text
Additional [optional] reading if desired - Chapter 12 of Aho, Hopcroft, and Ullman, "Data Structures and Algorithms".

Theme

In functional programming there is no notion of memory: computation proceeds be returing values and names are just used as shorthand for the values of expressions since there is no assignment. Thus the computation of a functional program will produce many values which when implemented must be stored somewhere. Since values are needed dynamically as the program is executed memory must be allocated dynamically. If many values are required memory requirements may become an issue. However, values that are no longer being used become garbage and in principle could be used again. The process of collecting memory that is no longer being used is called garbage collection.

In this lecture we present an overview of techniques for dynamic memory allocation and garbage collection.

Topics

cons cells and list allocation (uniform size)
extent and overlap
garbage collection (mark and sweep)
variable size allocation
- First-fit
- Next-fit
- Best-fit
reference counting

Sample Functions

extent.scm - recursive function to the extent of an object. (extent '(1 (2 3) 4)) => 5.

slides

lec3summary.ppt or lec3summary.pdf - Lecture overview slides.
gc.ppt or gc.pdf - cons cell allocation and GC example.
memory.ppt or memory.pdf - lecture on memory allocation.

Exercises

Assume a heap with room of 8 cons cells with the available cell list initialized to contain all cells. Trace through memory allocation and garbage collection for (begin (define L '(1 2 3)) (define M '(4 5)) (define N '(6 7 8)) (set-car! L M) (define M 0) (define N 0) (define N '(1)))
Given a heap with two available cells of size 1300 and 1200 respectively. Show a sequence of memory request sizes for which first-fit does not satisfy the request and best-fit does satisfy the request. Give another sequence where the opposite is true. I.E. first-fit satisfies the request and best-fit does not satisfy the request.

Created: April 10, 2008 (revised April 16, 2009) by jjohnson AT cs DOT drexel DOT edu