Why do we study Operating System?

• Every computer system has an operating system, which manages the resources of the computer.

• IoT or IT Services can’t work well without resource managements.

• To handle it, we have to understand the API of OS.

• To understand the API, it is better to see the implementations.

• For example, API for tuning priorities of the processes.

Xv6 Operating System as an educational tool

• A Unix-like operating system developed in MIT, based on V6 kernel

• xv6 is written in ANSI C ( not in C++ )

• xv6 is small and simple (9628 lines of codes)

• It can run on Raspberry Pi ( Small PC board )

• Or on ARM emulator such as QEMU

Xv6 development environment with QEMU

• QEMU emulates ARM CPU with virtual memory

• Xv6 operating sytem itself is running on QEMU

• gdb debugger can be attached to the emulated program (both in user space and system space)

• We can step and trace inside xv6 kernel

  ... but it requires complex development environments setup
  ... very time consuming
  

Our development environments

• homebrew (it simplifies installation of software in macOS)

• Mercurial repository of the lab where we saved everything using ssh

• Cross Compilers

• ARM libraries for C language

• QEMU (ARM Emulator)

Introducing Docker

• What is Docker? It is a container.

• What is Container? An isolated envirornment such as file system.

• It has DockerFile, which describes how to setup the envrionment in the container.

• Docker can run on any computer using an emulator such as QEMU.

• Complex development environment can be build using DockerFile and docker built command.

The DockerFile

FROM phitek/qemu-arm
RUN apt-get update -y
RUN apt-get remove gdb -y
RUN apt-get install gcc-arm-none-eabi gdb-arm-none-eabi qemu-system-arm -y
WORKDIR /code
COPY ["src", "."]
RUN make
CMD /bin/bash

• FROM line specifies the shared image in Docker repository.

• apt-get installs Linux cross compiling tools.

• Then it creates working directory, copies our sources to it, and executes make.

Docker commands

• Create docker image and environment, and build xv6 kernel

  docker build –tag xv6-docker

• Run xv6 kernel on QEMU in the container

  docker run –rm –privileged -it xv6-docker ./run-debug.sh

• Run gdb to connect the xv6 kernel running in the QEMU

  docker exec -it xv6-docker ./debug.sh

Exercise : Adding priorities to the xv6 scheduler

• Added getpriority() and setpriority()

• get_highest_priority() functions in proc.c file of xv6

• Implemented it in schelduler()

• Added a test function testpriority() in usertests.c file of xv6 to implement our algorithm

Function used in our exercise to find highest priority process

int get_highest_priority_proc(void)
{
  int highest_priority;
  int pid = 0;
  int hpid = -1;
  int rpid = 0;
  highest_priority=100;
 
 struct proc *p;
  for(p = ptable.proc; p < &ptable.proc[NPROC]; p++, pid++){
    if(p->state != RUNNABLE) {
      continue;    }
    rpid = pid;
    if (highest_priority > p->priority) {
       highest_priority=p->priority;
       hpid = pid; } }
  return hpid > 0?hpid:rpid;
}

• It assumes at least one process to be RUNNABLE.

• It may allow no highest priority process.

• The correctness of the function is important.

Docker Limitation

• If repository breaks, Docker poses a problem (a cloud problem).

• Cloud may be used conveniently but we must not rely on it.

• We can use a direct environment without Docker, since we have a Linux server.

• Docker images can easily eat up our disk spaces.

in ~/.zsh_history

: 1561014922:0;docker system prune
: 1561014973:0;docker container prune
: 1561015350:0;docker image prune
: 1561015362:0;docker volume prune

Conclusion

• Docker environment poses some problems, some of which are difficult to comprehend.

• Yet, it gives a good insight of the underlying important concepts of operating system because it is well documented, fast and uses less resources.