in a nut shell 1. goals a little history system components threads & cpu scheduling virtual...

In a nut shell

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Goals A little history System components Threads & CPU scheduling Virtual memory Environmental subsystems File System: NTFS

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Preemptive multitasking Goals:

◦ Security, reliability, ease-of-use, Windows & POSIX application compatibility, high-performance, extensibility, portability, international language support

◦ Commercial “the layered architecture of the system

… makes it so easy to use”

Windows NT◦Adopted Windows 95 user interface and incorporated web-server & web-browser

◦User-interface routines and all graphics code were moved into the kernel to improve performance Side effect: Decrease in system

reliability

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Windows XP (Oct 2001)◦ Successor to Windows NT/2000, replacement

for Windows 95/98

◦ Reliability requirement for Windows XP more stringent than Windows 2000 (which was the most reliable, stable system released by Microsoft)

◦ “extensive manual and automatic code review to identify over 63,000 lines in the source [code] that might contain issues not detected by testing” and then set about a review & correction process

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System Components

Microkernel: Executes in protected mode◦ HAL: Hardware abstraction layer

Some hardware independence HAL provides memory mapping, configuring I/O buses,

setting up DMA, motherboard specific facilities Device drivers (I/O manager) can still work directly with

hardware◦ Kernel

thread scheduling, interrupt & exception handling, CPU synchronization, power failure recovery

Never paged out, never preempted

User mode processes◦ Environmental subsystems

User mode operating systems (e.g., MS-DOS, OS/2, Win32, POSIX)

Logon systems◦ User applications

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Processes◦ Virtual memory address space◦ Base priority◦ “affinity” (assignment) to one or more

processors◦ One or more threads

Threads◦ Units of execution; dispatched by kernel◦ States: ready, standby, running, waiting,

transition, terminated

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Priority-based, preemptive scheduling 32 priority levels; each has a queue of

threads◦ Variable class: 0-15◦ Real-time: 16-31◦ Higher numbers indicate higher priority

Scheduler traverses from highest to lowest priority queues◦ Round robin, combined with priority scheme

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Variation on standard round-robin CPU scheduling

When thread’s time quantum runs out◦ Variable class: priority lowered

unless already base priority◦ Returned to relevant priority level in queue, in

ready state◦ Why lower the thread priority? Are there

conditions under which a variable class thread will not have its priority lowered?

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When variable priority thread released from wait state◦ Dispatcher boosts priority, depending on type of

wait◦ High boost:

Waiting for keyboard I/O Thread associated with user’s active GUI window

◦ Low boost: waiting for disk I/O

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32-bit processors◦ 4K page size

12 bit page offset◦ 4GB virtual address space◦ Upper 1-2 GB of all processes, used by

operating system in kernel mode

64 bit processors ◦ 8K page size

13 bit page offset◦ 8 TB virtual address space

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Demand paging with clustering◦ Bring in neighboring pages on a page fault: “Prefetching”

Max/min working set size per process◦ Processes have initial working set size of 50 frames

If a process is at its working set maximum and a page fault occurs◦ Local page replacement

When number of free frames drops below a certain value◦ Automatic working-set trimming◦ memory manager removes pages from processes until

processes at working set minimum◦ FIFO or variation of clock algorithm depending on

hardware (p. 363)

2-level page tables Each process has a page directory (outer page

table)◦ 1024 page-directory entries (PDE’s)◦ Size 4 bytes for each PDE◦ Each PDE points to a page table

Page table (inner page table)◦ 1024 page-table entries (PTE’s)◦ Size 4 bytes for each PTE◦ Each PTE points to a 4 KB frame of physical memory◦ Page tables are swapped out to disk when necessary

Total size of all page tables, per process: 4 MB

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Logical Address structure

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20 bits for frame number 12 bits remain to describe state of page

◦ Accessed or written◦ Caching attributes◦ Access mode◦ Global◦ PTE valid

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Some implement user-level operating systems

Some implement services crucial to all user-level operating systems◦ E.g., GUI, security management

Win32◦ Executes in unprotected mode◦ Provides all keyboard, mouse, graphical display

Other environmental subsystems use this◦ Separate processes with own input queues◦ Window manager dispatches input to input

queue of appropriate process

A user-level operating system◦ Executes in unprotected mode

Instruction execution unit: Emulates Intel 486 instructions

Routines to emulate DOS ROM BIOS & “Int 21” software interrupt services

Virtual device drivers: Screen, keyboard, comm. Ports◦ Partial support only: No direct h/w access

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User-level operating systems considered to be “clients”

Kernel considered to be “server” Communication between client and

server provided by message passing “Local procedure call” (LPC) facility

◦ like Remote Procedure Calls Message passing within a single

computer Used to implement system calls

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Server (e.g., kernel)◦ Publishes a globally visible connection port object

Client (e.g., Win32), to obtain services provided by server◦ Opens a handle to server connection port

Sends connection request◦ Server creates channel & returns handle to client◦ Channel: pair of private communication ports

Client-to-server messages Server-to-client messages

Some specifics dependent on message sizes◦ Small messages (e.g., up to a few hundred bytes)

are copied from sender to receiver◦ Shared memory is used for larger messages

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Goals◦data recovery, security, fault tolerance, large file & file system sizes, multiple data streams, UNICODE names, compression

NTFS is a journaling file system◦ It provides recovery of structure of file system

(metadata) should there be a system failure NTFS volumes can occupy a portion of a

disk, an entire disk, or can span disks

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In NTFS a file is not a simple stream of bytes as it is in some operating systems (e.g., Unix)

Rather, files are structured objects comprised of typed attributes

Some types of attributes◦ Conventional data of a file◦ Standard attributes: name, creation time,

security descriptor Examples of other uses of these typed

attributes◦ Mac file resource fork on a Windows XP file

server◦ Thumbnail of an image

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Cluster: Unit of disk allocation Allocates sectors on disk in contiguous

groups◦ A number of 512 byte disk sectors (power of

two size) Example

◦ Cluster size is 4096 bytes with disks > 4GB◦ If the (block) sector size on disk is 512 bytes,

this means each cluster is going to be 8 (blocks) sectors

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“How NTFS Works”, www.microsoft.com

MFT (Master file table)◦ Describes all files; file control block information

(analogous to Unix inode information) Partial copy of MFT Log file

◦ Temporary record of all metadata updates Volume file

◦ Name of volume, NTFS version, consistency bit Attribute-definition table

◦ Types of attributes used in volume & operations on types

Root directory: top-level in hierarchy Bitmap file: free/used clusters on disk Boot file: startup code for Windows XP Bad-cluster file: Bad areas on volume

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MFT: Master file table◦ One or more records per file◦ Each record is 1-4 KB in size◦ Describes attributes of file◦ Resident attributes: small sized

data stored in MFT record◦ Nonresident attributes: larger sized

data stored in contiguous extents on disk For small files, even the data of the file may

be stored in the MFT record Each file in NTFS volume has an ID-- its file

reference◦ 48 bit file number, 16 bit sequence number

Directories: particular kinds of files; B+ tree rep’n

If a system failure (e.g., power failure) occurs when a file system is being written, can loose meta data (organization info) and/or file data◦ Metadata loss tends to be more difficult

In NTFS, all file-system data-structure (metadata) updates are performed in log file transactions◦ Before a data-structure is altered, transaction

first writes a log record that contains redo and undo information

◦ After the data structure has been changed, transaction writes a commit record to log to signify that transaction succeeded

After a crash, log file transactions that had not been committed can be undone

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