David Zhang
Operating Systems - xv6 - Implement System Call

Description

Background on xv6

Part 1 - Memtop system call (returns the amount of memory available)

Main approach

xv6 holds a linked list of free memory managed by kalloc() and kfree(). The total available memory is just from the beginning of the list to the end.

Related files

• <sysproc.c> implementation of int sys_memtop(void)
• <defs.h> declare int pagesleft(void)
• <usys.S> add to SYSCALL(memtop)
• <user.h> declare int memtop(void)
• <syscall.h> define SYS_memtop as call 22
• <syscall.c> extern sys_memtop, add to vector [SYS_memtop] sys_memtop
• <kalloc.c> Implementation of int pagesleft(void)

Part 2 - Print memory stats for each process running in the system

Main approach

The total memory for a process consists of 4 parts (depends on how you count it): kernel stack, user stack, program memory, and overh- eads. We know the kernel stack is just 1 page, the user's memory can be easily computed by calling proc->sz. Overhead, the place to log where the actual memory are, is the hardest part.

To get the overhead memory for each process, first is to get all running process. We can do so by checking the highest pid number by fork a process. Then, we can check the process table - ptable with the pids from 1 to that number. After that, we just need to walk the page directory, p->pgdir, by calling walkleaves(), defined in proc.c. To see how many pages are allocated, we can check the PTE_P flag.

Related files

• <usys.S> SYSCALL(getmeminfo)
• <syscall.h> define SYS_getmeminfo 23
• <syscall.c> extern and add to vector
• <user.h> declaration
• <sysproc.c> pass parameters and invoke getprocinfo()
• <proc.c> implementation of getprocinfo() and walkleaves()
• <defs.h> declare getprocinfo()
• <mtop.c> call getmeminfo()

Program Usage

   ...                                    
   $ cd xv6-public/                       
   $ make clean && make && make qemu-nox  
   ...                                    
   $ mtop                                 
   ...                                    

Result:

available memory: 232607744
pid: 1	name: init	mem: 286720
pid: 2	name: sh	mem: 290816
pid: 3	name: mtop	mem: 286720

POSIX Threads and locks, Synchronization

Implementation

• <thread.c> main test template
• <threadlock.c> define spinlocks and mutex for threads
• <threadlock.h> header file for threadlock
• <proc.c> implementations of thread_create(), thread_join, and thread_exit()
• <proc.h> add thread flag into struct proc

linking

• <syscall.c> add system call vector
• <syscall.h> add system call number
• <user.h> connect thread functions to user
• <usys.S> add SYSCALL
• <sysproc.c> connect thread functions to system call
• <defs.h> declare thread functions