COP 4600 Operating Systems Spring 2011

Dan C. MarinescuOffice: HEC 304Office hours: Tu-Th 5:00-6:00 PM

Last time: Midterm solutions

Today: Virtualization for the three abstractions

Threads Virtual Memory Bounded buffer

The kernel of an operating system Threads

State Thread manager Thread state Kernel and application threads

Next time Processor switching

Lecture 15 – Thursday, March 17, 2011

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Virtualization – relating physical with virtual objects Virtualization

simulating the interface to a physical object by:1. Multiplexing create

multiple physical objects from one instance of a physical object.

2. Aggregation create one virtual object from multiple physical objects

3. Emulation construct a virtual object from a different type of a physical object. Emulation in software is slow.

Method Physical Resource

VirtualResource

Multiplexing processor thread

real memory virtual memory

communication channel virtual circuit

processor server (e.g., Web server)

Aggregation disk RAID

core multi-core processor

Emulation disk RAM disk

system (e.g. Macintosh) virtual machine (e.g., Virtual PC)

Multiplexing + Emulation

real memory + disk virtual memory with paging

communication channel +processor

TCP protocol

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Virtualization of the three abstractions

We analyze virtualization for the three abstractions1. Interpreter Threads2. Communication link/channel Bounded Buffer3. Storage Virtual Memory

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(1) Virtualization of interpreter - Threads Process/Threads a virtual processor Multiplexing or processor sharing is possible because

there is a significant discrepancy between processor bandwidth and the bandwidth of memory and of I/O devices

threads spend a significant percentage of their lifetime waiting for external events. Called:

Time-sharing Processor multiplexing Multiprogramming Multitasking

Processes versus thread: Both represent a module in execution A process may consist of multiple threads A thread is a light-weight process, less overhead to create it.

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Tight coupling between interpreter and storage

We need a memory enforcement mechanism; to prevent a thread running the code of one module from overwriting the data of another module.

Address space the range of memory addresses a thread is allowed to access.

Close relationship between a thread and an address space.

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(2) Virtualization of storage - Virtual memory

Address space – the storage a thread is allowed to access. Virtual address space – an address space of a standard size regardless

of the amount of physical memory. The physical memory may be too small to fit an application; otherwise each

application would need to manage its own memory. The size of the address space is a function of the number of bits in an

address. For example, 32 bits addresses allow an address space of 232 (4 Gbytes), 64 bit addresses allow 264

Virtual Memory - a scheme to allow each thread to access only its own virtual address space (collection of virtual addresses).

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Memory map of a process

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(3) Virtualization of communication link - Bounded buffers

Bounded buffers implement the communication channel abstraction Bounded the buffer has a finite size. We assume that all messages are of

the same size and each can fit into a buffer cell. A bounded buffer will only accommodate N messages.

Threads use the SEND and RECEIVE primitives.

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1 2 N-1N-2

out

in

Read from the bufferlocation

pointed by out

Write to the bufferlocation

pointed by out

shared structure buffer message instance message[N] integer in initially 0 integer out initially 0

procedure SEND (buffer reference p, message instance msg) while p.in – p.out = N do nothing /* if buffer full wait p.message [p.in modulo N] ß msg /* insert message into buffer cell p.in ß p.in + 1 /* increment pointer to next free cell

procedure RECEIVE (buffer reference p) while p.in = p.out do nothing /* if buffer empty wait for message msgß p.message [p.in modulo N] /* copy message from buffer cell p.out ß p.out + 1 /* increment pointer to next message return msg

0 1

Basic concepts Principle of least astonishment - study and understand simple phenomena

or facts before moving to complex ones. For example: Concurrency - an application requires multiple threads that run at the same time.

Tricky. Understand sequential processing first. Examine a simple operating system interface to the three abstractions

At the same time we need concepts that can be extended; Recall the extension of the UNIX file systems to NFS!!

Create a framework which can allow us to deal with more complex phenomena. Example: how to deal with concurrency

Serialization Critical sections code that must be executed by a single process to completion. Locks mechanisms to ensure serialization

Interrupts – external events which require suspension of the current thread, identification of the cause of the interrupt, and reaction to the interrupt.

State – critical concepts which allows us to interrupt the execution of a thread and restart it at a later point in time.

Event – a change of the state of an interpreter (thread, processor)

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Side effects of concurrency

Race condition error that occurs when a device or system attempts to perform two or more operations at the same time, but because of the nature of the device or system, the operations must be done in the proper sequence in order to be done correctly.

Race conditions depend on the exact timing of events thus are not reproducible. A slight variation of the timing could either remove a race condition or

create. Very hard to debug such errors.

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Lock

Lock a mechanism to guarantee that a program works correctly when multiple threads execute concurrently a multi-step operation protected by a lock behaves like a single operation can be used to implement before-or after atomicity shared variable acting as a flag (traffic light) to coordinate access to a

shared variable works only if all threads follow the rule check the lock before accessing a

shared variable.

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The kernel implements the three abstractions

The kernel is responsible for the management of all system resources including the

processor performs operations on behalf of users in response to system calls; in other

words extends the instruction set of a processor with a number of operations. Two modes of operation of a computing system

kernel/privileged mode user mode

Multiple functions Scheduling and thread management Event handling Memory management Communication management

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Thread and VM management – virtual computer The kernel supports thread and virtual memory management Thread management:

Creation and destruction of threads Allocation of the processor to a ready to run thread Handling of interrupts Scheduling – deciding which one of the ready to run threads should be allocated

the processor Virtual memory management maps virtual address space of a

thread to physical memory. Each module runs in own address space; if one module runs

multiple threads all share one address space. Thread + virtual memory virtual computer for each module.

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Threads and the Thread Manager

Thread virtual processor - multiplexes a physical processor a module in execution; a module may have several threads. sequence of operations:

Load the module’s text Create a thread and lunch the execution of the module in that thread.

Scheduler system component which chooses the thread to run next

Thread manager implements the thread abstraction. Interrupts processed by the interrupt handler which interacts with

the thread manager Exception interrupts caused by the running thread and processed by

exception handlers Interrupt handlers run in the context of the OS while exception

handlers run in the context of interrupted thread.

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The state of a thread; kernel versus application threads

Thread state: Thread Id unique identifier of a thread Program Counter (PC) -the reference to the next computational step Stack Pointer (SP) PMAR – Page Table Memory Address Register Other registers

Application threads – threads running on behalf of users Kernel threads – threads running on behalf of the kernel

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Virtual versus real; the state of a processor

Virtual objects need a physical support. In addition to threads we should be concerned with the processor or

cores running a thread. A system may have multiple processors and each processor may

have multiple cores The state of the processor or core:

Processor Id/Core Id unique identifier of a processor /core Program Counter (PC) -the reference to the next computational step Stack Pointer (SP) PMAR – Page Table Memory Address Register Other registers

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Thread Layer

Processor Layer

Thread 1 Thread 2 Thread 7

Processor A Processor B

IDSPPC

PMAR

IDSPPC

PMAR

IDSPPC

PMAR

IDSPPC

PMAR

IDSPPC

PMAR

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Virtual machines

Allow multiple operating systems on the same processor. a processor with a certain instruction set to emulate another one with a

different instruction set e.g., the Java Virtual Machines runs under Window, Linux, Mac OS, etc.

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VM Monitor

Windows GNU/Linux

Kernel Mode

User Mode

FirefoxInternetExplorer X WindowsWord

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Basic primitives for processor virtualization

Memory CREATE/DELETE_ADDRESS SPACEALLOCATE/FREE_BLOCKMAP/UNMAPUNMAP

Interpreter ALLOCATE_THREAD DESTROY_THREADEXIT_THREAD YIELDAWAIT ADVANCETICKETACQUIRE RELEASE

Communication channel

ALLOCATE/DEALLOCATE_BOUNDED_BUFFERSEND/RECEIVE

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Switching the processor from one thread to another

Thread creation: thread_id ALLOCATE_THREAD(starting_address_of_procedure, address_space_id); YIELD function implemented by the kernel to allow a thread to wait for an

event. Save the state of the current thread Schedule another thread Start running the new thread – dispatch the processor to the new thread

YIELD cannot be implemented in a high level language, must be implemented in the machine

language. can be called from the environment of the thread, e.g., C, C++, Java allows several threads running on the same processor to wait for a lock. It replaces the

busy wait we have used before.

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Thread states and state transitions

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The state of a thread and its associated virtual address space

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