The structure of process
- 2. 2 Contents Process States and Transitions Layout of System Memory The Context of a Process Saving the Context of a Process Manipulation of the Process Address Space
- 3. 3 Process States and Transitions 1 2 9 7 34 6 5 8 User Running Preempted Zombie Asleep in Memory Sleep, Swapped Ready to Run, Swapped fork Created Ready to Run in Memory Kernel Running not enough mem (swapping system only) swap out swap in wakeup swap out wakeup enough mem sleep reschedule process preempt return return to user system call, interruptinterrupt, interrupt return exit
- 4. 4 1. User Running: The process is executing in user mode. 2. Kernel Running: The process is executing in kernel mode. 3. Ready to Run in Memory: The process is not executing but is ready to run as soon as the kernel schedules it. 4. Asleep in Memory: The process is sleeping and resides in main memory. 5. Ready to Run, Swapped: The process is ready to run, but the swapper (process 0) must swap the process into main memory before the kernel can schedule it to execute. 6. Sleep, Swapped: The process is sleeping, and the swapper has swapped the process to secondary storage to make room for other processes in main memory. Process States and Transitions
- 5. 5 7. Preempted: The process is returning from the kernel to user mode, but the kernel preempts it and does a context switch to schedule another process. 8. Created: The process is newly created and is in a transition state; the process exists, but it is not ready to run, nor is it sleeping. This state is the start state for all processes except process 0. 9. Zombie: The process executed the exit system call and is in the zombie state. The process no longer exists, but it leaves a record containing an exit code and some timing statistics for its parent process to collect. The zombie state is the final state of a process. Process States and Transitions
- 6. 6 Process Structure Text Data Stack Process consists of 3 regions. Region is a contiguous area of the virtual address space.
- 7. 7 State of a Process – Process table entry • Contains general fields of processes that must be always be accessible to the kernel. – U area • Contains fields that only need to be accessible to the running process.
- 8. 8 Data Structures for a Process U Area Process table Per process region table allows independent processes to share regions. text data stack Per Process Region table Region Table Main Memory
- 9. 9 User Area - U Text Data Stack •Each process has a user area. • User area (U) has a fixed virtual address; it is mapped to different physical address. •Each user area is mapped to a physical memory when process is loaded to memory. U Area Text Data Stack Process A Process B U Area Same virtual address Physical memory Physical memory
- 10. 10 Process table fields • State field: Identifies Process state. e.g. user running, kernel running, etc. • Fields that allow the kernel to locate the process and u area in Main Memory and Secondary Storage. (Requires while context switch.) – Process size: kernel know how much space to allocate for the process. • User ID • Process ID
- 11. 11 Process table fields (cont…) • Event descriptor: – Used when the process is in the "sleep" state. (Sleep/Wakeup). • Scheduling parameters: – Allow the kernel to determine the order in which processes move to the states "kernel running" and "user running”. • A Signal field: – keeps the signals sent to a process but not yet handled. • Various timers: process execution time, resource utilization, etc.
- 12. 12 U Area fields • A pointer to the process table entry. • Real and Effective User IDs • Timer fields: – Execution time in user mode – Execution Time in kernel mode • An error field: keeps error during system call • Return value field: result of system call. • I/O parameters: – Amount of data transfer – Address of source and target etc.
- 13. 13 U Area (cont...) • The current directory and current root • User file descriptor table • Limit fields – Restrict process size – Restrict size of the file it can write • The control terminal field: – login terminal associated with the process, if one exists • An array – indicates how the process wishes to react to signal. • A permission modes field.
- 14. 14 Layout of System Memory Text Data Stack •Process consists of 3 regions. •Region is a contiguous area of the virtual address space of a process. Set of instructions, includes text addresses. Data addresses (to access global data variables.) Stack addresses (to access data structures local to subroutine)
- 15. 15 Regions • Divides virtual address space of a process into logical regions. • Region : Contiguous area of the virtual address space of a process
- 16. 16 Per Process Region Table (Pregion) • Pregion : region = File table : inode • Each pregion entry points to the kernel region table • Starting virtual (absolute) address of the region • Permission filed: – read-only, read-write, read-execute
- 17. 17 Kernel Region table • Kernel region table contains the pointer to the page table which keeps the physical memory address
- 18. 18 Regions 8K 16K 32K b a d c e 4K 8K 32K Text Data Stack Text Data Stack Process A Process B Per Proc Region Tables (Virtual Addresses) Region <Processes and Regions> • A -> 8K B -> 4K reading, region 'a’ • Data and stack region for two processes are private.
- 19. 19 Pages and Page Tables • Memory Management polices are Paging Segmentation • Pages and page table Divides Physical Memory in to set of equal sized blocks called pages. Page size range: 512 bytes to 4KB Every memory location is addressed by a pair of (page number, byte offset in page)
- 20. 20 Pages and Page Tables Logical Page Number Physical Page Number 0 177 1 54 2 209 3 17 Logical page numbers -> Physical page numbers Virtual addresses of a region -> Physical machine addresses
- 21. 21 Pages and Page Tables • Region : A contiguous range of virtual addresses in a program. – logical page number : index into an array of physical page number • Region table contains the pointer to a table of physical page no. called page table. • Page table contain m/c dependent information such as permission bits(to allow reading & writing page).
- 22. 22 Pages and Page Tables (contd.) 8K 32K 64K Text Data Stack 541K 783K 986K 897K 87K 552K 727K 941K empty 137K 852K 764K . . . . . . . . 1096K 2001K 433K 333K . . . . . . . . . . . . . . . . Per Proc Region Table Page Tables(Physical Addresses) Virtual Addresses Fig: Mapping Virtual Addresses to Physical Address
- 23. 23 Layout of Kernel • Kernel executes in the context of a process – virtual memory mapping associated with the kernel is independent of all processes. • Interrupt or system call – user mode -> kernel mode ; OS – user mode OS
- 24. 24 Layout of Kernel 856K 917K 564K 444K . . . . . . . . 0 1M 2M 4M Kernel Reg Triple 1 Kernel Reg Triple 2 Kernel Reg Triple 3 User Reg Triple 1 User Reg Triple 2 User Reg Triple 3 Address of Page Table No. of Pages in Page TableVirtual Addr 747K 950K 333K . . . . . . . . . . . . 556K 997K 458K 632K . . . . . . . . 0K 4K 3K 17K . . . . . . . . 128K 97K 135K 139K . . . . . . . . 256K 292K 304K 279K . . . . . . . . Process(Region) Page Tables Kernel Page Tables Fig: Changing Mode from User to Kernel
- 25. 25 U-area (User) • Every process has a private u area. • The two physical address represent the u areas of two processes, – but the kernel accesses them via the same virtual address • A process can access its u area when it executes in kernel mode.(not user mode)
- 26. 26 U-area (User) 856K 917K 564K 444K Reg Triple 1 2M 4 Address of Page Table Page Tables for U Areas No. of Pages in Page Table Virtual Addr in Process Reg Triple 2 Reg Triple 3(U Area) Proc B 856K 917K 564K 444K Proc A 856K 917K 564K 444K Proc C 856K 917K 564K 444K Proc D Fig: Memory Map of U Area in the Kernel data text
- 27. 27 Context of a Process • Context of a process – Contents of user address space (user-level context) – Contents of hardware registers (register context) – kernel data structure that relate to the process (system-level context) • User-level context – text, data, user stack, shared memory • register context • Program Counter (PC) – specifies address of the next instruction the CPU will execute. • Processor Status register (PS) – Specifies h/w status of machine. • User Stack Pointer (SP) – contains current address of next entry in kernel or user stack. • General purpose registers – contain data generated by the process during execution.
- 28. 28 Context of a Process • System-level context • Static Part- – Process table entry, – U area of a Process, – Pregion entries , region tables , page tables • Dynamic Part- – Kernel Stack - Stack Frames, – Set of layers.
- 29. 29 Context of a Process Dynamic Portion of Context Static Part of System Level context Process Text Data Stack Shared Data Process Table Entry U Area Per Process Region Table User Level context Static Portion of Context Layer 3 Layer 2 Layer 1 Layer 0 Kernel Context (User Level) Kernel Stack for Layer 3 Saved Register Context for Layer2 . . . . . Kernel Stack for Layer 2 Saved Register Context for Layer1 Kernel Stack for Layer 1 Saved Register Context for Layer0 logical pointer to current context layer Fig: Components of the Context of a Process>
- 30. 30 Saving the Context of a Process • Kernel saves the context of a process – Whenever it pushes a new system context layer. • Interrupts and Exception • System call Interface • Context switch.
- 31. 31 Interrupts and Exception • System is responsible for handling interrupts – result from hardware (clock, peripheral) – programmed interrupt (software interrupt) – exceptions (page faults) • Integrity of kernel data structure
- 32. 32 Interrupts and Exception • Sequence of interrupt operation 1. Saves the current register context of the executing process and creates(push) a new context layer. 2. Determine interrupt source and cause. • find interrupt vector. 1. Kernel invokes interrupt handler. 2. Interrupt handler completes it work and return. • Restores previous context layer.
- 34. 34 System Call Interface • Usual calling sequence – cannot change the mode of a process form user to kernel • Library : have the system call name – User program system call • OS trap • Same as Interrupt Handler
- 35. 35 Context Switch • Kernel permits a context switch – When a process puts itself to sleep – When it exits – When it returns from a system call to user mode but is not the most eligible process to run – When it returns to user mode after the kernel completes handling an interrupt but is not the most eligible process to run.
- 36. 36 Context Switch • Ensure integrity and consistency. • Procedure for a context switch. – similar to handling interrupts and system call • Steps for a Context Switch 1. Decide whether to do a context switch, and whether a context switch is permissible now. 2. Save the context of the "old" process. 3. Find the "best" process to schedule for execution, using the process scheduling algorithm. 4. Restore its context.
- 37. 37 Manipulation of Process Address Space • System calls Manipulate the virtual address space of a process. • The region table entry contains the information necessary to describe a region.
- 38. 38 Manipulation of Process Address Space • Region Table contains the following entries: – A pointer to the Inode of the file whose contents were originally loaded into the region. – Region type (text, shared memory, private data or stack). – Size of the region. – The location of the region in physical memory. – The status of a region, which may be a combination of • Locked • In demand • In the process of being loaded into memory • Valid, loaded into memory – The reference count, giving the number of processes that reference the region.
- 39. 39 Manipulation of Process Address Space • Operations on Region: – Locking and unlocking a Region. – Allocating a Region. – Attaching a Region to a Process. – Changing the Size of a Region. – Loading a Region. – Freeing a Region. – Detaching a region from a Process. – Duplicating a Region.
- 40. 40 Locking and unlocking a Region • Kernel has operations to lock and unlock a region – independent of the operations to allocate and free a region. • Thus the kernel can lock and allocate a region. • Later can unlock it without having to free the region.
- 41. 41 Allocating a Region • Allocate a new region during fork, exec, shmget (shared memory) system calls.
- 42. 42 Attaching a Region to a Process • Attach a region to a process during fork, Exec, and shmat system calls. • It connects the region to the process address space.
- 43. 43 Changing the size of a Region • Kernel invoke growreg to change the size of the region. 1. Process expands size by executing sbrk system call. 2. Stack expand explicitly according to the depth of nested procedure call. • The virtual space of the expanded region shouldn't overlap with others. • The process size shouldn’t exceed the maximum size. • The shared region never increase in size if it is attached to other processes.
- 44. 44 Changing the size of a Region
- 45. 45 Loading a Region• Allocate memory to load a file (growreg). • Load a file on demand if on demand paging is supported.
- 46. 46 Freeing a Region • The region will be freed when it is not attached to any process (ref count =0). • Free inode associated with region using iput.
- 47. 47 Detaching a Region from a Process • The kernel detaches regions in the exec, exit, and shmdt (detach shared memory). • Decrement process size. • Decrement region reference count..
- 48. 48 Duplicating a Region • Fork requires the kernel to duplicate data and stack regions. • The Region reference count is incremented in case of shared text & memory, allowing the parent & child processes to share regions. • In case of stack & data regions are copied: 1. Allocate a new region entry. 2. Allocate page map table. 3. Allocate physical memory for the region.