Midterm Exam – EECS 211



Midterm Exam – EECS 211
Process Scheduling

In this midterm exam you will modify a tick-based kernel to make the tick period dynamic and
under the kernel’s control. Your goal is to modify the kernel so that tick period is not fixed. You
should have some policy for how the tick period changes based on the behavior of the processes.
You are required to include a short description of your adaptive tick interval algorithm.
1 Background
For this assignment you will use the xv6 kernel ( for the
RISC-V 64 bit architecture. This includes all of the core components of an operating system kernel
while still being accessible to hack on.
The kernel runs on QEMU (, a CPU emulator (similar to the LC3 VM,
but more elaborate and can emulate more complicated processor architectures).
1.1 xv6 Kernel
The xv6 kernel includes the various components you would expect to be in an operating system
kernel. The actual scheduler is in proc.c. You should be able to verify that the scheduler by
default uses a round-robin scheduler.
The code for handling interrupts and exceptions is in trap.c. This is, for example, where the code
that handles when a userspace process calls a syscall and the system traps to the kernel resides, as
well as where interrupts are handled.
Userspace applications have a very standard set of syscalls, include read, write, fork, etc.
1.2 RISC-V
RISC-V is a new risc architecture with substantial documentation available online. Here are some
basics which should be useful.
RISC-V cores have three privilege level modes: M (machine), S (supervisor), and U (user). A
specific instruction is used to switch privilege modes. For example, the mret instruction switches
from M mode to S mode, and the sret instruction switches from S mode to U mode. To go the
other way, the ecall instruction traps to a higher privilege level.
The core boots into M mode. Some initial setup is done, and then the core switches to S mode
where the kernel starts running. Of course applications execute in U mode.
The tick in xv6 is implemented using the machine-mode timer and configured initially in start.c.
This is a simple timer which is just a continuously running clock and single compare register. When
the clock value matches the compare register an interrupt is generated. Since the machine timer
operates in M mode, this interrupt traps the CPU into M mode. The interrupt handler is specified
by the mtvec register, which is set to the timervec function in the kernelvec.S file. The timervec
handler adds the tick interval to the compare register to reset the timer for the next tick event. It
then generates a software interrupt to notify the kernel in S mode that the tick occurred. These
trap events are handled by the kernel in the trap.c file.
The different privilege levels can store state in the mscratch, sscratch, and uscratch registers.
For example, machine mode can store 64 bits of state in the mscratch register. The xv6 kernel
uses the mscratch register to store a pointer to some state reserved for the machine timer code.
You can see in the timervec function how that is used. Note that the interval used for the tick is
stored in the array pointed to by mscratch.
2 Setup
First you have to install some dependencies.
1. Clone the xv6 Kernel source.
1 $ git clone https :// github . com / mit – pdos / xv6 – riscv
2. Install the RISC-V compiler.
1 $ sudo apt – get install gcc – riscv64 – linux – gnu
1 $ brew tap riscv / riscv
2 $ brew install riscv – tools
3. Install QEMU
1 $ sudo apt – get install qemu
1 $ brew install qemu
4. Test
1 $ cd xv6 – riscv
2 $ make qemu
That will load the emulator with the kernel, and by default the kernel loads the shell process
(sh). This is a basic shell, but should be reasonably familiar.
To exit QEMU you can enter ctrl+a and then x to exit QEMU.
2.1 Troubleshooting
Here are some of the problems that you may face (depending on the version of your Linux and
1. error: unrecognized command line option ‘-mno-relax’; did you mean ‘-Wno-vla’?
This error is due to mismatch between the GCC versions. you may find the solution in this
post useful:
2. make: qemu-system-riscv64: command not found. This means there is a problem with
either (i) installing the QEMU or (ii) the PATH variable of your shell. One solution is to
install QEMU from its source code as follows:
1 $ git clone https :// github . com / qemu / qemu
2 $ cd qemu
3 $ ./ configure — target – list = riscv64 – softmmu
4 $ make -j $( nproc )
5 $ sudo make install
Now you will see that the file qemu-system-riscv64 is located under the qemu/build/riscv64-softmmu
folder. Now you can modify the Makefile used by the xv6-riscv kernel (STEP 4 above) by
changing the QEMU variable and hardcode the path to the compiled qemu-system-riscv64
2.2 How to modify the kernel:
To modify the kernel you can directly edit the source and then re-run make qemu. You can also
add new applications in the user folder and then add them to the UPROGS variable in the Makefile
and they will be available in the kernel.
To simplify the project, you should edit the CPUS variable in the Makefile and set it to 1. Run
make clean to ensure the change propagates followed by make qemu to compile.
3 Deliverable
The objective of this project is to implement a dynamic tick interval policy. This should respond
to the operation of processes in the system with the goal of reducing the number of ticks while
preventing a process from monopolizing the CPU.
You should devise a scheme for verifying that the tick interval is being dynamically adjusted.
Certainly one option is to add a printf() when the tick occurs and inspect that the frequency
of the print messages changes. You can also write an application which calls fork() to create
multiple processes that the OS has to schedule. If one is intermittently CPU bound (i.e. it has
periods of high computation (say while(1)) and periods of waiting) and the other is IO bound,
both processes should make progress but the tick interval should change.


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