Generic Memory Management

Question 1

What is a cell? What does it mean to be live?

Answer 1

• any data item in the heap
• it is live if something else if pointing at it (in a root) not live would be collected by Java’s garbage collector

Question 2

What is a root?

Answer 2

registeres, stack locations, global/static variables

Question 3

What is garbage?

Answer 3

a block of heap memory that cannot be accessed by the program (not live and nothing points to it, we can’t access it)

Question 4

What is garbage collection?

Answer 4

• automatic management of dynamically allocated storage
• it is an ADT we can use to allocate memory for objects and clean up that memory when we are done with it

Question 5

When do you want to call garbage collection?

Answer 5

• Whenever a pointer is set to NULL
• when there is no memory left
• at predetermined times

It needs to know which objects are still in use and which are no longer in use

Question 6

What is a buffer?

Answer 6

Is a block of “generic” memory, that can be used to store any kind of object. It is up to the programmer to cast the buffer to the required type

Question 7

What are the generic memory management routines and what do they do?

Answer 7

• memcpy: copy memory from one buffer to another
• memcmp: compare two buffers of memory
• memset: set a buffer to a specified value

Question 8

What is reference counting and why is it a bad garbage collecting technique?

Answer 8

• it counts the nubmer of references to a cell.
• the problem is that two cells can reference each other and not have a root(meaning they are garbage) and still be kept by the garbage collector

Question 9

What is the mark-sweep algorithm and in gerneral, how does it work?

Answer 9

• Each cell has a mark bit
• GC goes to work when the heap is used up, then the program is suspended

Two phases

1. Marking phase
   
   • starting from the roots, set the mark bit on all live cells
2. Sweep phase
   
   • return all unmarked cells to the free list
   • reset the mark bit on all marked cells