Operating SystemsCMPSC 473Processes (3)

September 17 2008 - Lecture 9

Instructor: Bhuvan Urgaonkar

Announcements

• Suggested Reading for this lecture: Sections 3.1-3.3

• Quiz 1: Will have several questions that came up during previous lectures

Overview of Process-related

Topics• How a process is born

– Parent/child relationship– fork, clone, …

• How it leads its life– Loaded: Later in the course– Executed

• CPU scheduling• Context switching

• Where a process “lives”: Address space– OS maintains some info. for each process: PCB– Process = Address Space + PCB

• How processes request services from the OS– System calls

• How processes communicate• Some variants of processes: LWPs and threads• How processes die

Overview of Process-related

Topics• How a process is born

– Parent/child relationship– fork, clone, …

• How it leads its life– Loaded: Later in the course– Executed

• CPU scheduling• Context switching

• Where a process “lives”: Address space– OS maintains some info. for each process: PCB– Process = Address Space + PCB

• How processes request services from the OS– System calls

• How processes communicate• Some variants of processes: LWPs and threads• How processes die

more today

The Process/Kernel Model

• Transitions between User and Kernel modes: An example

Process 1

USER MODE

KERNEL MODE

Process 1 Process 2 Process 2

Time

Sys call handler

SchedulerInterrupthandler

Re-entrant Kernels

• Note: Not showing scheduler invocations• Re-entrant kernel: Several processes may be in Kernel Mode at the

same time– A re-entrant kernel is able to suspend the current running process

even if it is in the Kernel Mode• Note: Traps are a type of exceptions. We will encounter more

types later.

Process 1

USER MODE

KERNEL MODE

Process 1 Process 2

Time

Excp Intr

Intr

Intr

Re-entrant Kernels

• Note: Not showing scheduler invocations• Re-entrant kernel: Several processes may be in Kernel Mode

at the same time– A re-entrant kernel is able to suspend the current running

process even if it is in the Kernel Mode

Process 1

USER MODE

KERNEL MODE

Process 1 Process 2

Time

Excp Intr

Intr

Intr

Kernel control paths

Re-entrant Kernels

• Note: Not showing scheduler invocations• Re-entrant kernel: Several processes may be in Kernel Mode at the same time

– A re-entrant kernel is able to suspend the current running process even if it is in the Kernel Mode

• A kernel control path denotes the sequence of instructions executed by the kernel to handle a system call, an exception, or an interrupt

Process 1

USER MODE

KERNEL MODE

Process 1 Process 2

Time

Excp Intr

Intr

Intr

Kernel control paths

Re-entrant Kernels

• Note: Not showing scheduler invocations

• Why re-entrancy?

Process 1

USER MODE

KERNEL MODE

Process 1 Process 2

Time

Excp Intr

Intr

Intr

Kernel control paths

Re-entrant Kernels

• Note: Not showing scheduler invocations

• Why re-entrancy?– Improves throughput of devices controllers that raise interrupts– Allows priorities among interrupts

Process 1

USER MODE

KERNEL MODE

Process 1 Process 2

Time

Excp Intr

Intr

Intr

Kernel control paths

Realizingre-entrancy: Take

1• Write kernel functions that only modify local variables and do not alter global variables– Re-entrant functions

• Pros and Cons?

• Take 1: Write kernel functions that only modify local variables and do not alter global variables– Re-entrant functions

• Pros/Cons– Simplifies/complicates kernel programming (?)

Realizingre-entrancy: Take

1

Realizing re-entrancy: Take 2

• Kernel Mode Stacks• We know: a process running in User Model refers to its private stack

• To allow re-entrancy, there is a Kernel Mode Stack for each process

• Each kernel control path uses its own private kernel stack

• Kept in part of RAM reserved for the kernel

Kernel Mode Stack

PCB(task_struct)

Stack • KM stack and PCB need to be able to find each other

• KM stack must have access to a pointer to the PCB

esp

Kernel Mode Stack

thread_infostructure

PCB(task_struct)

esp

curent

Stack• KM stack and PCB need to be able to find each other

• KM stack must have access to a pointer to the PCB– Linux: thread_info

• PCB must have access to KM stack

task

Kernel Mode Stack

thread_infostructure

PCB(task_struct)

esp

curent

Stack• KM stack and PCB need to be able to find each other

• KM stack must have access to a pointer to the PCB– Linux: thread_info

• PCB must have access to KM stackthread_info

task

Kernel Mode Stack

thread_infostructure

PCB(task_struct)

esp

curent

Stack• Since KM stacks make

little use of the stack, only a few thousand bytes suffice– An example of “Design for

the most common case”, we’ll see more

– Linux: 8KB , thread_info 52 bytes

thread_info

task

Kernel Mode Stack

thread_infostructure

PCB(task_struct)

esp

curent

Stack• Since KM stacks make

little use of the stack, only a few thousand bytes suffice– An example of “Design for

the most common case”, we’ll see more

– Linux: 8KB• Why combine KM stack and

thread_info into a union?

thread_info

union thread_union { struct thread_info thread_info; unsigned long stack[2048];};

task

Kernel Mode Stack

thread_infostructure

PCB(task_struct)

esp

curent

Stack• Since KM stacks make little use of

the stack, only a few thousand bytes suffice– An example of “Design for the most

common case”, we’ll see more– Linux: KM Stack 8KB, thread_info 52

bytes• Why combine KM stack and

thread_info into a union?– You might think spatial locality– The kernel can easily obtain the

address of the thread_info structure of the process currently running on the CPU from the value of the esp register

– task field is at offset 0– Other benefits apply to multi-

processors: makes it easy to efficiently find the current process on each processor• Earlier approach: Have an array of current pointers

thread_info

task

union thread_union { struct thread_info thread_info; unsigned long stack[2048];};

Enumerating # Possible CPU Multiplexing

Between Processes• Consider two processes P1 and P2

– P1: n instructions– P2: m instructions– No jump instrictions => No loops

• How many unique executions are possible?