The Structure of Processes

What is a Process? an instance of running program Program vs process(task) Program : just a passive collection of instructions high-level program vs. binary program Process : actual execution of the instructions Several processes may be associated with one program In addition to program code, necessary resources (memory, CPU, etc) are allocated to process 2

Process in detail Program in execution having its own memory space (text, data, stack,..) One program may have many processes Independent of each other (protection) Scheduling entity Executed in CPU (having registers) Competing for system resources pid, context, priority, allocation resources 3

Memory image of a process Segment layout Text : program code Data : global variables Stack : local variables, parameters Heap : dynamically allocated space (eg. Malloc) 4 argc, argv env. variables stack heap Uninitialized data [bss] (initialized to 0 by exec) Initialized data text (code) User level context

Multi-tasking Running multiple processes in a system Requirements Which to run? (scheduling) Which memory to allocate? (virtual memory) Maintain process information (pid, state, context, …) uni-processor vs multi-processor 5

각 프로세스별 커널이 관리하여야 할 정보 task_struct Task Identification : process id, (real/effective) user-id, group-id, … Task State Task relationship : pointer to parent, siblings Scheduling information : policy, priority, counter Signal information: received signal, signal handler Memory information : virtual memory information, mm_struct File information : file descriptor tables (files_struct, fs_struct) Thread structure : CPU information(register context) Task 가 어디까지 실행했는지 기억 (PC,SP, general purpose register, 등 ) Time information : start time, CPU time Inter-Process Communication information : signal (handler), … Executable format : Resource limit 6

Task Structure 7 HW context memory context system context

Process States and Transitions Task running (ready, running) Waiting (interruptible, uninterruptible) waiting for an event or resource Stopped (task_stopped: by receiving a signal, task_traced: by debugger) Zombie Most task structures are freed. Will be dead after wait() is called. 8

Linux Scheduler select the most deserving process to run out of all of the runnable processes use simple priority based scheduling algorithm Context switch after choosing a new process to run, it saves the state of the current process (the processor specific registers and other context) being saved in the process’s task_struct data structure. It then restores the state of the new to run and gives control of the system to that process. Schduling information policy(normal/realtime,round-robin/FIFO), priority, counter 9

Process Context User-level (memory) Context Process text, data, user stack, and shared memory System level Context task structures Hardware(register) Context Program counter (PC), process status (PS) register stack pointer (SP), general-purpose registers 10

CPU execution mode place restrictions on the operations that can be performed by the process currently running in the CPU Kernel mode When the CPU is in kernel mode, it is assumed to be executing trusted software, and thus it can execute any instructions and reference any memory addresses (i.e., locations in memory). The kernel (which is the core of the operating system and has complete control over everything that occurs in the system) is trusted software, but all other programs are considered untrusted software. User mode It is a non-privileged mode in which each process (i.e., a running instance of a program) starts out. It is non-privileged in that it is forbidden for processes in this mode to access those portions of memory (i.e., RAM) that have been allocated to the kernel or to other programs.

Layout of System Memory Physical address space impossible for two processes to execute concurrently if their set of generated addresses overlapped. Virtual address space Allows many processes to share finite amount of physical memory Each process uses the same virtual addresses but reference different physical addresses Requires mechanism for translating virtual address to physical address 12

Regions Region (segment) : vm_area_struct Contiguous area of virtual address space of a process that can be treated as a distinct object to be shared or protected. Virtual address space of a process is divided into logical regions Text : a set of instructions Data : (initialized & uninitialized) data variables Stack : data structures local to a subroutine 13

Fork() concept Create a new process(child) that has the same context with the previous process(parent) 14

fork() example 15 intglob = 6; charbuf[] = “a write to stdout\n”; int main(void) { int var; pid_t pid; var = 88; write(STDOUT_FILENO, buf, sizeof(buf)-1); printf(“before fork\n”); if ((pid = fork()) == 0) {/* child */ glob++; var++; } else sleep(2);/* parent */ printf(“pid = %d, glob = %d, var = %d\n”, getpid(), glob, var); exit (0); } Source : Adv. programming in the UNIX Env., pgm 8.1)

Memory image of a process (Example) 16

Before fork() 17 text stack data Segment (vm_area_struct) task_struct pid = 11 glob, buf var, pid … movl %eax, [glob] addl %eax, 1 movl [glob], %eax...

After fork() 18 memory text stack data Segment (vm_area_struct) task_struct pid = 11 Segment (vm_area_struct) task_struct pid = 12 stack data glob, buf var, pid glob, buf

Fork with COW (Copy-On-Write) 19 after fork with COW after “glob++” operation memory text stack data Segment (vm_area_struct) task_struct pid = 11 Segment (vm_area_struct) task_struct pid = 12 text stack data Segment (vm_area_struct) task_struct pid = 11 Segment (vm_area_struct) task_struct pid = 12 data

exec concept 20 header text data bss stack a.out text stack data Segment (vm_area_struct) task_struct pid = 11 stack data text Replace memory context with new binary program (loader) and execute

exec example 6 syntaxes for exec() execl(), execv(), execlp(), execvp(), execle(), execve() : will be covered in next lectures 21 int main() { printf("before exec\n"); execl("exec_example", "exec_example", 0); printf("after exec\n"); return 0; }