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Computer Science 61 and E61
Systems Programming and Machine Organization
This is the 2013 version of the course. Main site

Assignment 2: Caching and File I/O

In this assignment, you’ll gain experience with caching by writing your own buffered I/O library.

  • Assigned Fri 9/27
  • Due Fri 10/11 at 11:59pm for college students (1 day later for extension)
  • This assignment may be completed in pairs

Get the code

Our code is released in the cs61-psets repository. To update it, run

git remote show handout

If this reports an error, run

git remote add handout git://code.seas.harvard.edu/cs61/cs61-psets.git

Then run git pull handout master. This will merge our Assignment 2 code with your previous work. If you have any “conflicts” from Assignment 1, resolve them before continuing further. Run git push to save this merge back to code.seas.

Please use an explicit merge to create your repository. If you copy code by hand, our automated scripts will have trouble analyzing your code, and it’ll be harder for you to incorporate our updates.

You may also create a new cs61-psets repository for this assignment. Make sure you update the grading server.


We’ve released a very simple IO61 I/O library, in io61.c. Our version of IO61 uses character-at-a-time system call I/O and is thus quite slow. Your goal is simple: speed it up using caching.

We also distribute several programs that use IO61.

  • cat61 reads a file sequentially one character at a time and writes it to standard output.
  • blockcat61 is like cat61, but reads the file sequentially one block at a time. Its -b BLOCKSIZE argument sets the block size.
  • randomcat61 is like blockcat61, but it chooses a random block size for each read and write. It still reads and writes sequentially (only the block sizes are random). The -b argument is the maximum possible block size. This will test your buffering package.
  • stride61 reads its input file using a strided access pattern, and outputs a transformed file. The -b and -s arguments set the block size and stride length, respectively.
  • reordercat61 reads its input file one block at a time in random order and writes each block to the output file at the correct position. The result is a copy of the input file, but created non-sequentially.
  • reverse61 reads a file a character at a time in reverse and writes the result to standard output.

Introduce buffers (that is, caches) to the io61_file abstraction and use them to speed up IO61 operations.

We’re giving you tons of freedom to implement the file caches as you like. You may even use memory-mapped I/O, prefetching system calls like madvise or posix_fadvise, or multiple threads or processes (although none of these are required). But you may not use another buffered I/O library or caching I/O library.

For full credit your library should:

  • Return correct results for all access patterns and file types, including very large files, non-seekable files, mixtures of calls (for instance, alternating io61_read and io61_readc), and random accesses.
  • Perform as well as stdio on sequential reads and writes for any files, including very large files (such as files that don’t fit in memory) and non-seekable files.
  • Improve on stdio’s performance for some non-sequential access patterns, such as simple strides or reverse access.

We will grade your work based on how much you speed up sample executions of the handout programs—and other secret tests—with points deducted for incorrect results.

All your code should fit in io61.c.


We will evaluate you based on your code’s performance relative to a version of IO61 that uses stdio. That version is provided for you for testing in stdio-io61.c (and the makefile builds stdio-cat61, stdio-blockcat61, and so forth).

Run make check to see how your current implementation is doing on a battery of tests. This will also print summary statistics at the end. Run make check-TESTNUMBERS (e.g., make check-9 or make check-5-10 or make check-1,2,3) to run selected tests.

We will also measure your code’s performance against that of other students in class. :)

We may update the tests during the pset release period.


  • Read our file descriptors notes.
  • Write code to handle sequential I/O first. This isn’t so hard and covers many important cases.
  • You will likely need to change every function in io61.c (except io61_open_check and io61_filesize).
  • To handle other cases, such as reordered I/O and particularly stride I/O, you will need to be clever and implement more complex cache algorithms. For instance, might stride I/O benefit from an associative cache?
  • Some of our tests, particularly on shorter files, are noisy, and can produce different timing results when run multiple times.
  • Stdio is a well-written package! It is OK if you can’t always beat it.
  • Write your own tests! This will help you shake out bugs in your code, particularly correctness bugs. What diabolical things can you think of to try? You can add new tests pretty easily; just edit GNUmakefile and check.pl.
  • You can run the tests on their own, too. make check creates some useful input files for testing and puts them in files/.


The build process for this pset may be different than you’re used to. We’ve changed the Makefile to generate less noise, so errors will stand out more. We’ve also changed the default compiler to clang. If you want to see the actual compilation commands, run make V=1. If you want to compile with gcc, run make CC=gcc. If you want to compile without optimization (which might help you debug), run make O=0 or edit the GNUmakefile to set O = 0 by default.

Extra credit: Deadlock

We also release a program, pipeexchange61, that demonstrates a problem with conventional blocking I/O. The pipeexchange61 program forks two copies of itself. One copy, the requester, sends requests to the other copy, the responder. The requester can send many requests back-to-back and only then wait for responses (a form of batching). When you run pipeexchange61, the program appears to get stuck! Why? Can you construct an I/O library that unsticks pipeexchange61, without modifying pipeexchange61.c?


You will turn in your code by pushing your git repository to code.seas.harvard.edu. Inform us on the grading server if you have changed partner or repository from pset 1.

Don't forget to fill out README.txt.

This pset was originally created for CS61.