UCLA CS239--Advanced Operating Systems

Fall, 1999

Midterm Exam ("take home")

Clarifications

Inquiry #4 (Tuesday, ~10:30am)

It may be already late, but I assumed until now that the file system for PDP-11/45 was some variant of UNIX with indexed file access. Was I wrong?

Response

The PDP-11/45 was indeed running a Unix operating system--back in the days when AT&T directly provided all such systems.

Inquiry #3 (Monday, ~2pm)

If we use information gained from the papers listed on the class homepage, do we need to reference/quote them? For example, if I say flash memory performance so and so, do I need to quote the flash paper? Or is it "known" that we are going to be using the papers on the class web page?

Response

For the exams (as distinct from a paper:-) you may assume that the instructor knows the context/identity of all class-related sources.

Inquiry #2 (Friday, ~4pm)

Do any of the tasks that a computer system on the satellite executes have strong real-time requirements? When I say 'strong', I mean significantly stronger than for a general purpose system. Although it seems that a satellite may have some tasks that require real-time reaction, I cannot think of any right now.

Response

For the sake of this exam, assume no real-time issues are relevant, UNLESS you have an argument that real-time requirements would affect the overall structure of the OS, etc.

Inquiry #1 (Friday, ~1pm)

In problem 2 on the test, parts 1 and 2 are asking us to support opposite positions. While I can argue both ways, is it really your desire for us to play devil's advocate? Or am I missing something?

Response

Yes, I'm explicitly asking you to argue (well) from opposite sides!

Exam Rules

You must complete the academic integrity statement prior to taking this exam. If you have not already completed and submitted the statement, please complete it and submit it along with your exam answers.

The exam answers are due at the beginning of class on Tuesday, November 9, at the usual time and place. Please submit hard copy of your answers; no handwritten submissions, please:-)

This exam is open books, notes, libraries, archives, internet, etc. You are free to consult any static resource. Fellow class members, other students (UCLA or otherwise), faculty, friends (or enemies), etc., are NOT to be involved in any way.

Please limit each answer to 500 words. It is often a good idea to prepare a "complete" answer without regard to the word limit, and then carefully edit your answer down to meet the limit.

You are required to answer each question.

Exam Questions

One

You have been hired as an outside consultant on a JPL project that will launch a geo-synchronous earth orbiting satellite with an expected lifetime of 25 years. No physical maintenance of the satellite components is expected to be possible, given it's orbital altitude of 23,000 miles (100 times higher than current shuttle flights). The in-house team has made a proposal regarding the computer system that will support all satellite functions (control, communications, other tasks). Key elements of their proposal include:
  1. Dual components, including CPU, bus, ROM, RAM, flash memory.

  2. Monolithic OS structure, with OS stored in ROM.

  3. RAM will be used for task execution, etc.

  4. Flash memory is reserved use by a standard Berkeley-style "fast file system."

Please comment on their proposal, taking special note of the importance of reliability and flexibility requirements of a system that will receive no physical maintenance, but whose functionality requirements are likely to change over its 25-year lifetime. Specific issues to address, at a bare minimum, are whether the hardware and its proposed utilization are appropriate and if the proposed OS and FS methodologies are well-matched to the needs of the system.

[Recall that flash memory has an unusual erase/write/read cycle: it takes about one second to erase a memory location (usually done in a "block" granularity) prior to writing the location (writing is done a byte at a time); read and write times are comparable to RAM--usually in the tens of nanoseconds.]

Two

You have been hired as a consultant to the senior systems architect on a project which is designing a global, continuous (24 hours/day, 7 days/week) stock market. The market supports 10,000 brokers scattered around the world. Each broker provides "sell" and "buy" offers; the system's primary task is to match sell and buy offers, and ensure that both buyer and seller are aware of the (irrevocable) match. Note that this market manages the transfer of a few trillion dollars a day, so reliability is important. Provide complete responses to each of the following three sub-tasks:
  1. Argue that a centralized solution is essential.

  2. Argue that a distributed solution is required.

  3. What specific distributed systems technologies are important in #2 above, and why?

Three

In the late 1970's in our department, a typical timesharing system was built out of a PDP-11/45 (a 300ns/cycle CPU) with 512KB of RAM, a 20MB hard drive, and a 300KB DECtape magnetic tape unit. DECtapes were unusual in that they were bi-directional, direct-block access (for both read and write) storage devices of about 600 0.5KB blocks, with a maximum seek delay (end-to-end) of about 10 seconds. A typical configuration had from 2-4 such tape units.

One day the hard drive failed (with a 3-5 day repair estimate), and ongoing use of the system was absolutely critical. A few clever programming tricks later, the DECtape units were each happily masquerading as a separate hard drive.

How would you expect a disk-tuned file system to behave under these conditions?

How would you design/tune a file system for a DECtape-based secondary storage system?


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(c)1999 Richard Guy
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