CS202: HW 6: virtual memory, WeensyOS

This question is open-ended; there are many possible answers. You don't have to type very much, and you can think about these questions even when you are not in front of a computer.

There are many metrics that a scheduler has to balance: turnaround time, response time, throughput, and various definitions of fairness. We will consider the following types of systems in this question:

• A high-performance computer used for large number crunching tasks (genetic sequencing, graphics rendering...)
• A multimedia computer (video games, movies...)
• A smartphone
• A general-purpose computer on a corporate desktop

1. For each of the system types above, which metrics do you think are the most important, and why? Some things to consider: How many users does the system have? Are user processes typically interactive? How long is a user willing to wait? What frustrations will users have if certain metrics are ignored? (There are many possible answers to this question.)
2. Taking into consideration your answers above, propose a scheduling policy (FIFO, etc.) for each system above.
3. For each of the systems above, what is the scheduling policy that would most completely undermine the purpose of the system?

Complete the following table, filling in the missing entries and replacing each question mark with the appropriate integer. Use the following units: K = 2¹⁰ (kilo), M = 2²⁰ (mega), G = 2³⁰ (giga), T = 2⁴⁰ (tera), P = 2⁵⁰ (peta), or E = 2⁶⁰ (exa).

Determine the number of page table entries (PTEs) that are needed for the following combinations of virtual address size (n) and page size (P).

Given a 32-bit virtual address space and a 24-bit physical address, determine the number of bits in the VPN, PPN (your book, OSTEP, calls this PFN), and offset, for the following page sizes P:

Begin Weensy OS:

• Read the lab 4 description. Seriously, read it. (As opposed to skimming it.)

• Fetch and build the code.

• Now answer:

  • Where in physical memory—which pages or addresses—does the kernel’s code and data live? Give the range in hex. (Hint: you do not need to look at the code; you can simply make run or make run-console and read it off the graphical memory maps.)

  • What physical address does the kernel’s stack live on?

• Same two question as above, for virtual memory: what are the virtual addresses of the kernel’s code and data, and at what virtual address does the kernel’s stack live in virtual memory?

• Use the constants and macros given in the assignment (in the sections “Memory system layout” and “Writing expressions for addresses”) to write an expression for the number of physical pages in memory. This will be handy to have as you begin coding the lab.

Consider the x86-64 architecture. Below we are asking about the physical pages consumed by a process, including the page tables themselves. As you answer the question, assume that any allocated memory consumes physical pages in RAM; that is, there is no swapping or demand paging. Note that it may be helpful for you to draw pictures (but you don’t have to).

As a reminder, the x86-64 imposes a multi-level page table structure: pages are 4KB, each page table entry is 8 bytes, and each individual page table (a node in the “tree”) occupies one page. Thus, each page table holds 4 KB / 8 B = 512 = 2⁹ entries. Recall that the structure is four levels; each level is indexed by 9 bits of the virtual address.

• What is the minimum number of physical pages consumed by a process that allocates 12KB (for example, 1 page each for code, stack, and data)?

• What is the minimum number of physical pages consumed by a process that makes 2⁹ + 1 allocations of size 4KB each? You can leave your answer in terms of powers of 2, and sums thereof.

• What is the minimum number of physical pages consumed by a process that makes 2¹⁸ + 1 allocations of size 4KB each? You can leave your answer in terms of powers of 2, and sums thereof.