Chapters 13-19

Chapter 13: Concurrency Preliminaries

13.1 Hardwarse

Chapters 9-12

9-12

Chapter 9: Generational Garbage Collection

Prologue

Tracing collectors have best case when heap is mostly garbage, but long-lived objects tend to exhibit pathologically bad behavior for copying and mark-sweep

• Ex: Mark-sweep has dense prefix near bottom of heap, copying moves data unnecessarily
  

Solution: Segment the heap into generations and collect the youngest generation(nursery) most frequently. Tenure into older generations after surviving a number of sweeps.

• Most of the time use copying
  

• Has good mark/cons ratio, so should be faster for young generation
  

• Can tune pause times / frequency of pauses by altering nursery size
  

Only improves average-case time. Worst case still needs to sweep whole heap.

Racket GC Tests.

To anyone doing the racket garbage collector exercises, this is the assignment page for a class doing the exact exercise. It specifies tests that must pass as well; I found this link while trying to think up tests in the face of a lack of working libraries(equals?, map, filter, fold) and a suspicion of a lack of tail call recursion.

http://www.eecs.northwestern.edu/~robby/courses/321-2014-winter/hw7.html

I recommend the assignment as it imposes upon the student how elementary mark-sweep really is.

Notes on Chapters 5-8 of Garbage Collection Handbook

Chapter 5: Reference Counting
- Invariant: An object is alive iff the number of references to it are greater than 0
- Each object has a reference count
- When references mutated, we increment or decrement the reference count. When it reaches 0, it is freed.
- Required write barrier. 
- Mutator uses barriers to keep state of object graph consistent when changes can be non-atomic to the GC

1.1 Advantages + Disadvantages:
Advantages:
- Costs are spread throughout computation time
- Operates well without much headroom
- Potentially free memory immediately
- Locality may be not worse than actual program since only actively used or modified objects referenced
- Does not need runtime assistance, doesn’t need to know roots
- Heavily used today
- Available in C++ through “smart pointers” or pointers with special logic