Task: Copy a File with mmap()

Navigate to file-descriptors/drills/tasks/mmap_cp and run make to generate support. As you know mmap() can map files in memory, perform operations on them, and then write them back to the disk. Let’s check how well it performs by comparing it to the cp command. The benchmarking is automated by benchmark_cp.sh so focus on completing mmap_cp.c for now.

Quiz: Checkout what syscalls cp uses

  1. Inside the tests/ directory, you will need to run checker.sh. The output for a successful implementation should look like this:
./checker.sh
make: Nothing to be done for 'all'.

Test PASSED (File copies are identical)

  1. Open mmap_cp.c and complete the TODOs to map the files in memory and copy the contents. Do not forget to clean up by unmapping and closing the files.

    To test, run make test-file to generate a 1MB file with random data, and then run mmap_cp test-file output.txt. Ensure they have the same content with a simple diff: diff test-file.txt output.txt.

  2. Compare your implementation to the cp command. Run make large-file to generate a 1GB file with random data, and then run ./benchmark_cp.sh.

    Quiz: Debunk why cp is winning

    If you want a more generic answer, checkout this guide on mmap vs read()-write().

  3. This demo would not be complete without some live analysis. Uncomment the calls to wait_for_input() and rerun the program. In another terminal, run cat /proc/$(pidof mmap_cp)/maps to see mapped files, and ps -o pid,vsz,rss <PID> to see how demand paging happens.