12. Intro to Memory Management

Question 1

What is time multiplexing?

Answer 1

Sharing a resource by dividing up access to it over time.

Question 2

What are three examples of time multiplexing?

Answer 2

CPU scheduling on a single-core system

Classroom scheduling using temporal scheduling

Car share programs

Question 3

What is space multiplexing?

Answer 3

Sharing a resource by dividing it into smaller pieces.

Question 4

What are three examples of space multiplexing?

Answer 4

CPU scheduling on a multi-core system (low granularity)

Memory management (high granularity)

Splitting up a cake

Question 5

What are the two main steps of memory allocation?

Answer 5

First, a process requests large chunks of memory from the OS kernel
(is this the address space?)

Second, the process divides the available memory up using a process-level allocation library (like malloc)

Question 6

Why can’t we just divide physical memory up between processes (direct multiplexing)?

Answer 6

1. We are limited to the amount of physical memory on the machine. Too many processes means we’d run out of memory.
2. There is the potential for fragmentation, which reduces the efficiency of allocation
3. Potential for discontiguous allocations, which complicates the process memory layout

Question 7

What is the process memory layout concerned with?

Answer 7

It is concerned with how the process knows where its code and data are located.

Question 8

What is fragmentation?

Answer 8

When a request for contiguous memory fails despite the fact that there is enough unused memory available (on the system)

Question 9

What is internal fragmentation?

Answer 9

Fragmentation/unused memory inside an existing allocation

Ex: Firefox is given a block of memory but doesn’t use parts of it

Question 10

What is external fragmentation?

Answer 10

Fragmentation/unused memory between existing allocations

(Ex: There is a small piece of unused memory in between the memory allocated for Firefox and the memory allocated for iMail)

Question 11

Is it always feasible to splid data structures across multiple pieces of discontiguous memory?

Answer 11

No. For example, an int array data[10240] requires contiguous memory because of the way indexes are used to access elements within it.

Question 12

What are the 5 main problems with direct multiplexing?

Answer 12

1. We’re limited to the amount of physical memory on the machine
2. We can get stuck with discontiguous allocations
3. Potential for fragmentation, which reduces allocation efficiency
4. Enforcing allocations (making sure the right process is accessing the mem address) requires checking permission for every memory access (very slow)
5. Reclaiming unused memory leads to increased discontiguity

Question 13

What are the 4 requirements for memory multiplexing?

Answer 13

Grant, enforce, reclaim, and revoke

Question 14

What is the “grant” requirement for memory multiplexing?

Answer 14

The kernel should be able to allocate memory to processes statically (at startup) and dynamically (as needed)

Question 15

What is the “enforce” requirement for memory multiplexing?

Answer 15

The kernel should be able to enforce memory allocations efficiently

Question 16

What is the “reclaim” requirement for memory multiplexing?

Answer 16

The kernel should be able to repurpose unused memory without destroying its contents

Question 17

What is the “revoke” requirement for memory multiplexing?

Answer 17

The kernel should be able to stop a process from using memory that it was previously allocated

Question 18

What is the “grant” requirement a CPU?

Answer 18

CPU must be able to schedule a thread via a context switch

Question 19

What is the “enforce” requirement a CPU?

Answer 19

CPU must be able to interrupt a thread using a timer interrupt

Question 20

What is the “reclaim” requirement a CPU?

Answer 20

?

“This is new”

Question 21

What is the “revoke” requirement a CPU?

Answer 21

CPU must be able to reschedule a thread via a context switch

Question 22

Which abstraction is the memory management abstraction? (and how?)

Answer 22

The address space.

We provide every process with an identical view of memory that makes it appear plentiful, contiguous, uniform, and private

Question 23

What is the principle service (benefit) of the address space layout?

Answer 23

The uniformity of the address space simplifies the process layout.

It allows a process to always put things like static variables, the heap, and the stack at the same addresses

Question 24

Where is the process layout (convention) specified?

Answer 24

In the Executable and Linker Format (ELF) file

Question 25

Why don’t ELFs typically load process code at address 0x0?

Answer 25

To catch possibly the most common programming error, a NULL pointer problem.

We want to set up an environment where a pointer to 0x0 is not valid unless explicitly specified to be.