ECS 150 Demo

• Some demo programs related to the operating systems class (ECS 150).

Demos about synchronization

sync_barrier

This is a synchronization barrier implemented using semaphores. There is an argument that can turn this program into an incorrect implementation. The program sync_barrier.py also contains a demo that shows how this synchronization barrier works.

The correct behavior of the program with n threads is to print "0" for n times, then print "1" for n times, and so on. Each thread prints one "0", then one "1", and so on.

To run the correct one, use

python3 sync_barrier.py			# 10 threads by default
python3 sync_barrier.py 100		# Specify number of threads

To run the incorrect one, use

python3 sync_barrier.py 100 1

Demos about page tables

Disclaimer

These demos are very likely to be OS and machine dependent. For example, the memory page size of 4096 bytes is hard-coded. I only tested it on a Linux 4.9.189 x86_64 machine. Using other operating systems and CPU architectures may result in error in the program.

page2frame

This demo gets a memory page of a process in user space by directly reading memory frame from the kernel space.

To run this demo, you need a Linux kernel which is compiled with CONFIG_STRICT_DEVMEM=n. Or the last step of reading /dev/mem will fail. For some hints on compiling, see here.

To run the demo, use:

sudo python3 page2frame.py | hexdump -C

The Python program, page2frame.py, will:

1. Compile the program page2frame.c, run it, and record its pid.
2. Read /proc/[pid]/maps and find index of the page with permission column -w-p (only writing permission).
3. Check and print corresponding data in /proc/kpagecount and /proc/kpageflags.
4. Read the page in /dev/mem, and print it in binary form (can be read using hexdump or xxd).

The C program, page2frame.c, will:

1. mmap a memory page with only writing permission.
2. Fill the page with some specific pattern. Namely, some integers at the beginning and "EOF" at the end.
3. Print something (so that the Python program knows it has done with step 1 and 2, and then go to sleep.

page_table

This demo displays the virtual page table of a program. This program uses the graphviz module in Python 3, which can be installed using pip. You also need to install graphviz so that the dot command is available.

To run the demo, which shows the page table for page2frame.c, use:

python3 page_table.py /output/image.pdf

You can use any other formats that graphviz supports (e.g. png, jpg)

To view page table for some existing process with pid [pid], use:

python3 page_table.py /output/image.pdf [pid]

Note: this program is still evolving.

Sample page tables

Sample page table of page2frame:

page2frame in another system: pdf

Page table of find: pdf