32. LA: Hints for Computer System Design

Question 1

What are 8 hints that Butler Lampson gave for Computer System Design?

Answer 1

1. Separate normal and worst case
2. Plan to throw one away
3. Use brute force
4. Do one thing well
5. Shed load
6. Use hints
7. Compute in the background
8. Cache answers

Question 2

Describe the hint “Separate normal and worst case” and give an example of how to use it

Answer 2

The MIPS R3000 provides separate TLB (normal case) and general (worst case) exception handlers

Question 3

**Describe the hint “Plan to throw one away” and give an example of how to use it

Answer 3

Might as well make it (whatever system you’re designing) simple. Before implementing two-level page tables, try a linked list, or a list of segments. Don’t over-design the first draft.

Question 4

**Describe the hint “Use brute force” and give an example of how to use it

Answer 4

Particularly for the first implementation. Don’t make things fast until they need to be.

Apply Amdahl’s Law. Unsorted linked list page tables are easier to maintain and not a performance problem until you can demonstrate that the overhead of page translation is dominating.

Question 5

Describe the hint “Do one thing well” and give an example of how to use it

Answer 5

Write an interface just for manipulating your page tables. Test it. Then use it as part of a complete ASST3 solution

Question 6

Describe the hint “Shed load” and give an example of how to use it

Answer 6

When you run out of swap, start failing calls to fork or exec

Question 7

Describe the hint “Use hints” and give an example of how to use it

Answer 7

A page being mapped in the TLB is a hint that it isn’t safe to evict.

Without holding a lock, a page looking available in the coremap is a hint that it may be after the coremap lock is acquired

Question 8

**Describe the hint “Compute in the background” and give an example of how to use it

Answer 8

Writing pages to disk when the system is idle makes swap out much faster

Question 9

**Describe the hint “Cache answers” and give an example of how to use it

Answer 9

Use a cache. Cache’s are everywhere. The TLB is a cache. The buffer is a cache. Saving process TLB entries across context switches is an example of using a cache