Download VIII. Input/Output

VIII.
Input/Output
Operating Systems • Prof. Dr. Marc H. Scholl • DBIS • U KN • Summer Term 2009
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Intended Schedule
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Date
20.04.
27.04.
04.05.
11.05.
18.05.
25.05.
01.06.
08.06.
15.06.
22.06.
29.06.
06.07.
13.07.
20.07.
27.07.
12.10.
Lecture
Introduction to Operating Systems
Systems Programming using C (File Subsystem)
Systems Programming using C (Process Control)
Process Scheduling
Process Synchronization
Inter Process Communication
Pfingstmontag
Deadlocks
Memory Management
Hand out
Submission
Course registration
1. Assignment
2. Assignment
1. Assignment
3. Assignment
2. Assignment
4. Assignment
3. Assignment
5. Assignment
4. Assignment
6. Assignment
5. Assignment
7. Assignment
6. Assignment
8. Assignment
7. Assignment
9. Assignment
8. Assignment
10. Assignment
9. Assignment
10. Assignment
Input / Output
Filesystems
Special subject: XQuery your Filesystem
Wrap up session
First examination date
Second examination date
Operating Systems • Prof. Dr. Marc H. Scholl • DBIS • U KN • Summer Term 2009
2
I/O Hardware
Operating Systems • Prof. Dr. Marc H. Scholl • DBIS • U KN • Summer Term 2009
3
I/O Hardware
Control
Unit
Input /
Output
(I/O)
Different Peripherals
Mass Storage Devices
Processing
Unit
Main
Memory
Operating Systems • Prof. Dr. Marc H. Scholl • DBIS • U KN • Summer Term 2009
Glatz ©2005
4
I/O Hardware
 Incredible variety of I/O devices
 Common concepts

Port

Bus (daisy chain or shared direct access)

Controller (host adapter)
 I/O instructions control devices
 Devices have addresses, used by

Direct I/O instructions

Memory-mapped I/O
Operating Systems • Prof. Dr. Marc H. Scholl • DBIS • U KN • Summer Term 2009
Silberschatz, Galvin and Gagne ©2005
5
A Typical PC Bus Structure
Operating Systems • Prof. Dr. Marc H. Scholl • DBIS • U KN • Summer Term 2009
Silberschatz, Galvin and Gagne ©2005
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Device I/O Port Locations on PCs (partial)
Operating Systems • Prof. Dr. Marc H. Scholl • DBIS • U KN • Summer Term 2009
Silberschatz, Galvin and Gagne ©2005
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CPU Device
Interaction
Operating Systems • Prof. Dr. Marc H. Scholl • DBIS • U KN • Summer Term 2009
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CPU
Device Interaction
• Principal Choices
• Programmed I/O (Polling)
• CPU waits (busy wait) for peripheral
• Interrupt-Driven I/O
• Peripheral signals „ready“ to CPU by (hardware) interrupt
• Direct Memory Access (DMA)
• Steal cycles from CPU to transfer data from/to main memory w/o CPU
interaction
• needs special DMA controller to execute I/O independently from CPU
• DMA controller signals „ready“ to CPU via (hardware) interrupt
Operating Systems • Prof. Dr. Marc H. Scholl • DBIS • U KN • Summer Term 2009
9
CPU↔Device Interaction by Polling
 CPU determines state of device by repeatedly checking busy flag.

... prepares data in data-out register (for write operation)

... sets command register to “write”

... sets command-ready bit
 Once controller sees command-ready bit
6.2.1 Programmgesteuerte E/A
Beispiel: serielle Schnittstelle

... sets busy bit

... sees command=write and reads data-in register

... performs write-I/O

... clears command-ready, error & busy bits
Example © Glatz: Serial I/F
1 Zeichen senden
 Interface


Command-ready bit
ja
Busy bit

Error bit

Command register

Data-in, data-out registers
 Busy-wait cycle to wait for I/O from device
NB: S
sende
Ablau
Statusregister lesen
bereit?
nein
1 Zeichen senden
Ende
kap61-3_Ein-Ausgabe.fm
Operating Systems • Prof. Dr. Marc H. Scholl • DBIS • U KN • Summer Term 2009
Silberschatz, Galvin and Gagne ©2005
10
CPU↔Device Interaction by Interrupts
Betriebssys
 CPU Interrupt-request line (HW wire)

Triggered by I/O devices

Checked by CPU after each instruction
Erkennen einer Unterbrechungsanforderung
Zwei grundsätzliche Fälle:
(1) Externes Hardware-Signal
(2) Unterbrechung als direkte Folge der Instruktionsausführung
■
CPU-instruction cycle © Glatz
■
 Interrupt handler receives interrupts
Initialisierung
loop
 Maskable to ignore or delay some interrupts
FETCH (Instruktion holen)
EXECUTE (Instruktion ausführen)
 Interrupt vector to dispatch interrupt to correct handler

Based on priorities

Some nonmaskable

INTERRUPT CHECK
kap61-3_Ein-Ausgabe.fm
F
Using interrupt vector (and chaining)
 Interrupt mechanism can also used for

Exceptions

Virtual memory paging

System calls, ...
Operating Systems • Prof. Dr. Marc H. Scholl • DBIS • U KN • Summer Term 2009
Silberschatz, Galvin and Gagne ©2005
11
Interrupt-Driven I/O Cycle
Operating Systems • Prof. Dr. Marc H. Scholl • DBIS • U KN • Summer Term 2009
Silberschatz, Galvin and Gagne ©2005
12
Intel Pentium Processor Event-Vector Table
Operating Systems • Prof. Dr. Marc H. Scholl • DBIS • U KN • Summer Term 2009
Silberschatz, Galvin and Gagne ©2005
13
Direct Memory Access
 Used to avoid programmed I/O (PIO, byte-wise data transfer between
memory and devices) for large data movement
 Requires DMA controller
 Bypasses CPU to transfer data directly between I/O device and memory

During DMA access to memory, CPU cannot access memory (“cycle
stealing”)
 Signals completion by a CPU interrupt
 DMA may be implemented into real or into virtual memory
Operating Systems • Prof. Dr. Marc H. Scholl • DBIS • U KN • Summer Term 2009
Silberschatz, Galvin and Gagne ©2005
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Six Step Process to Perform DMA Transfer
Operating Systems • Prof. Dr. Marc H. Scholl • DBIS • U KN • Summer Term 2009
Silberschatz, Galvin and Gagne ©2005
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Betriebssysteme
6.2.3 Ein-/Ausgabe mittels DMA (Direct Memory Access)
DMA-Betriebsarten
DMA: More details
Cycle Stealing
in Single vs. BlockBlocktransfer
Transfer
Einzeltransfer
DMA-Anforderung
DMA-Anforderung
DMAZyklen
DMAZyklen
Betriebssysteme
ProzessorPrinzipaufbau
eines DMA-Kontrollers
ProzessorZyklen
Beispiel für indirekten Transfer:
Zeit
Zyklen
Zeit
DMA: Direct vs. Indirect Data Transfer
kap61-3_Ein-Ausgabe.fm
(A) Programmierung DMA-Kontr.
Hauptspeicher
DMAKontroller
CPU
Peripheriegerät
(B) Direkter Transfer
Hauptspeicher
DMAKontroller
CPU
Peripheriegerät
Operating Systems • Prof. Dr. Marc H. Scholl • DBIS • U KN • Summer Term 2009
Folie 24
(C) Indirekter Transfer
Hauptspeicher
DMAKontroller
CPU
Peripheriegerät
Glatz ©2005
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Application
I/O-Interface
Operating Systems • Prof. Dr. Marc H. Scholl • DBIS • U KN • Summer Term 2009
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Application I/O Interface
 I/O system calls encapsulate device behaviors in generic classes
 Device-driver layer hides differences among I/O controllers from kernel
 Devices vary in many dimensions

Character-stream or block

Sequential or random-access

Sharable or dedicated
Treiberhierarchie
Betriebssystemteil
© Glatz
Anwenderprozess
Geräteverwaltung (I/O manager)
logischer Treiber
Software
Klassentreiber (class driver)


Speed of operation
physischer Treiber
read-write, read only, or write only
Gerätetreiber (device/port driver)
Peripheriekontroller (interface HW)
Hardware
Gerät (device)
Operating Systems • Prof. Dr. Marc H. Scholl • DBIS • U KN • Summer Term 2009
Silberschatz, Galvin and Gagne ©2005
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A Kernel I/O Structure
Operating Systems • Prof. Dr. Marc H. Scholl • DBIS • U KN • Summer Term 2009
Silberschatz, Galvin and Gagne ©2005
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Characteristics of I/O Devices
Operating Systems • Prof. Dr. Marc H. Scholl • DBIS • U KN • Summer Term 2009
Silberschatz, Galvin and Gagne ©2005
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Block and Character Devices
 Block devices include disk drives

Commands include read, write, seek

Raw I/O or file-system access

Memory-mapped file access possible
 Character devices include keyboards, mice, serial ports

Commands include get, put

Libraries layered on top allow line editing
Operating Systems • Prof. Dr. Marc H. Scholl • DBIS • U KN • Summer Term 2009
Silberschatz, Galvin and Gagne ©2005
21
Network Devices
 Varying enough from block and character to have own interface
 Unix and Windows NT/9x/2000 include socket interface

Separates network protocol from network operation

Includes select functionality
 Approaches vary widely (pipes, FIFOs, streams, queues, mailboxes)
Operating Systems • Prof. Dr. Marc H. Scholl • DBIS • U KN • Summer Term 2009
Silberschatz, Galvin and Gagne ©2005
22
Clocks and Timers
 Provide current time, elapsed time, timer
 Programmable interval timer used for timings, periodic interrupts
 ioctl (on UNIX) covers odd aspects of I/O such as clocks and timers
Operating Systems • Prof. Dr. Marc H. Scholl • DBIS • U KN • Summer Term 2009
Silberschatz, Galvin and Gagne ©2005
23
Blocking and Nonblocking I/O
 Blocking - process suspended until I/O completed

Easy to use and understand

Insufficient for some needs
 Nonblocking - I/O call returns as much as available

User interface, data copy (buffered I/O)

Implemented via multi-threading

Returns quickly with count of bytes read or written
 Asynchronous - process runs while I/O executes

Difficult to use

I/O subsystem signals process when I/O completed
Operating Systems • Prof. Dr. Marc H. Scholl • DBIS • U KN • Summer Term 2009
Silberschatz, Galvin and Gagne ©2005
24
Two I/O Methods
Synchronous
Operating Systems • Prof. Dr. Marc H. Scholl • DBIS • U KN • Summer Term 2009
Asynchronous
Silberschatz, Galvin and Gagne ©2005
25
Kernel I/O Subsystem
 Scheduling — which I/O request to serve next?

Some I/O request ordering via per-device queue

Some OSs try fairness
 Buffering — store data in memory while transferring between devices

To cope with device speed mismatch

To cope with device transfer size mismatch

To maintain “copy semantics”
 Caching — fast memory holding copy of data

Always just a copy

Key to performance
 Spooling — hold output for a device

If device can serve only one request at a time, e.g., Printing
 Device reservation — provides exclusive access to a device

System calls for allocation and deallocation

Watch out for deadlock!
Operating Systems • Prof. Dr. Marc H. Scholl • DBIS • U KN • Summer Term 2009
Silberschatz, Galvin and Gagne ©2005
26
Transforming I/O Requests to Hardware Operations
 Consider reading a file from disk for a process:

Determine device holding file

Translate name to device representation

Physically read data from disk into buffer

Make data available to requesting process

Return control to process
Software
Anwenderprozess 1
Anwenderprozess 2
Anwenderprozess n
© Glatz
Hardware
Treiber A
Kontroller A
Gerät A
Treiber B
Kontroller B
Gerät B
Treiber X
Kontroller X
Gerät X
Geräteverwaltung
(I/O manager)
Treiber-Schnittstelle (driver interface)
Operating Systems • Prof. Dr. Marc H. Scholl • DBIS • U KN • Summer Term 2009
Silberschatz, Galvin and Gagne ©2005
27
Performance
 I/O a major factor in system performance:

Demands CPU to execute device driver, kernel I/O code

Context switches due to interrupts

Data copying

Network traffic especially stressful
 Improving Performance:

Reduce number of context switches

Reduce data copying in memory between application and device

Reduce frequency of interrupts by using large transfers, smart
controllers, polling

Use DMA to increase concurrency (offload CPU)

Move processing into device controllers (offload CPU & bus)

Balance CPU, memory, bus, and I/O performance for highest
throughput
Overload
in one of these will leave others idle!
Operating Systems • Prof. Dr. Marc H. Scholl • DBIS • U KN • Summer Term 2009
Silberschatz, Galvin and Gagne ©2005
28
Mass Storage
Operating Systems • Prof. Dr. Marc H. Scholl • DBIS • U KN • Summer Term 2009
29
Mass Storage Overview
• Secondary Storage
• Magnetic disks provide bulk of secondary storage of modern computers
• Disks can be removable
• Drives attached to computer via I/O bus (e.h., EIDE,ATA, SATA, USB, FC,
SCSI, ...)
• Host controller (in computer) talks to disk controller (in device) via this
bus
• Tertiary Storage
• Low cost is defining characteristic
• Typically uses removable media
• Considered „off-line“ storage; robot-machinery can turn it into“near-line“
• Magnetic tapes, floppy, CD, DVD, ...
Operating Systems • Prof. Dr. Marc H. Scholl • DBIS • U KN • Summer Term 2009
30
Moving-head Disk Machanism
Disk access time = queue waiting + seek + rotational delay + transfer
Operating Systems • Prof. Dr. Marc H. Scholl • DBIS • U KN • Summer Term 2009
Silberschatz, Galvin and Gagne ©2005
31
Disk Structure
 Disk drives are addressed as large 1-dimensional arrays of logical
blocks, where the logical block is the smallest unit of transfer.
 The 1-dimensional array of logical blocks is mapped into the sectors of
the disk sequentially.

Sector 0 is the first sector of the first track on the outermost cylinder.

Mapping proceeds in order through that track, then the rest of the
tracks in that cylinder, and then through the rest of the cylinders from
outermost to innermost.
Seite 0
© Glatz
Sektor 8
7
23
Sektor 7
Seite 1
Sektor 1
Zylinder 0
0
Zylinder 1
16
Zylinder 2
32
6 22
5
33 17
1
18 2
21
Sektor 6
20
4
Sektor 5
Sektor 2
Sektor 3
15
31
Sektor 7
14 30
Sektor 6
13
41
10
Sektor 3
28
12
Sektor 5
Sektor 2
25 9
26
29
19
3
Sektor 4
Sektor 1
Zylinder 0
8
Zylinder 1
24
Zylinder 2
40
Sektor 8
27
11
Sektor 4
Nummerierung: Zylinder 0.. (zero based), Seiten 0.. (zero based), Sektoren 1.. (one based)
Operating Systems • Prof. Dr. Marc H. Scholl • DBIS • U KN • Summer Term 2009
Silberschatz, Galvin and Gagne ©2005
32
Disk Attachment
 Host-attached storage accessed through I/O ports talking to I/O busses
 SCSI itself is a bus, up to 16 devices on one cable, SCSI initiator
requests operation and SCSI targets perform tasks

Each target can have up to 8 logical units (disks attached to device
controller
 FC is high-speed serial architecture

Can be switched fabric with 24-bit address space – the basis of
storage area networks (SANs) in which many hosts attach to many
storage units

Can be arbitrated loop (FC-AL) of 126 devices
Operating Systems • Prof. Dr. Marc H. Scholl • DBIS • U KN • Summer Term 2009
Silberschatz, Galvin and Gagne ©2005
33
Network-Attached Storage
 Network-attached storage (NAS) is storage made available over a
network rather than over a local connection (such as a bus)
 NFS and CIFS are common protocols
 Implemented via remote procedure calls (RPCs) between host and
storage
 New iSCSI protocol uses IP network to carry the SCSI protocol
Operating Systems • Prof. Dr. Marc H. Scholl • DBIS • U KN • Summer Term 2009
Silberschatz, Galvin and Gagne ©2005
34
Storage Area Network
 Common in large storage environments (and becoming more common)
 Multiple hosts attached to multiple storage arrays - flexible
Operating Systems • Prof. Dr. Marc H. Scholl • DBIS • U KN • Summer Term 2009
Silberschatz, Galvin and Gagne ©2005
35
Disk Management
 Low-level formatting, or physical formatting — Dividing a disk into sectors
that the disk controller can read and write.
 To use a disk to hold files, the operating system still needs to record its
own data structures on the disk.

Partition the disk into one or more groups of cylinders.

Logical formatting or “making a file system”.
 Boot block initializes system.

The bootstrap is stored in ROM.

Bootstrap loader program.
 Methods such as sector sparing used to handle bad blocks.
Operating Systems • Prof. Dr. Marc H. Scholl • DBIS • U KN • Summer Term 2009
Silberschatz, Galvin and Gagne ©2005
36
Disk Scheduling
Operating Systems • Prof. Dr. Marc H. Scholl • DBIS • U KN • Summer Term 2009
37
Disk Scheduling
 The operating system is responsible for using hardware efficiently — for
the disk drives, this means having a fast access time and disk bandwidth.
 Access time has two major components

Seek time is the time for the disk are to move the heads to the cylinder
containing the desired sector.

Rotational latency is the additional time waiting for the disk to rotate
the desired sector to the disk head.
 Minimize seek time
 Seek time ≈ seek distance
 Disk bandwidth is the total number of bytes transferred, divided by the
total time between the first request for service and the completion of the
last transfer.
Operating Systems • Prof. Dr. Marc H. Scholl • DBIS • U KN • Summer Term 2009
Silberschatz, Galvin and Gagne ©2005
38
Disk Scheduling (Cont.)
 Several algorithms exist to schedule the servicing of disk I/O requests.
 We illustrate them with a request queue (0-199).
98, 183, 37, 122, 14, 124, 65, 67
Head pointer 53
Operating Systems • Prof. Dr. Marc H. Scholl • DBIS • U KN • Summer Term 2009
Silberschatz, Galvin and Gagne ©2005
39
FCFS — First Come First Served
Illustration shows total head movement of 640 cylinders.
Operating Systems • Prof. Dr. Marc H. Scholl • DBIS • U KN • Summer Term 2009
Silberschatz, Galvin and Gagne ©2005
40
SSTF — Shortest Seek Time First
 Selects the request with the minimum seek time from the current head
position.
 SSTF scheduling is a form of SJF scheduling; may cause starvation of
some requests.
 Illustration shows total head movement of 236 cylinders.
Operating Systems • Prof. Dr. Marc H. Scholl • DBIS • U KN • Summer Term 2009
Silberschatz, Galvin and Gagne ©2005
41
SCAN (aka. Elevator Algorithm)
 The disk arm starts at one end of the disk, and moves toward the other
end, servicing requests until it gets to the other end of the disk, where the
head movement is reversed and servicing continues.
 Illustration shows total head movement of 208 cylinders.
Operating Systems • Prof. Dr. Marc H. Scholl • DBIS • U KN • Summer Term 2009
Silberschatz, Galvin and Gagne ©2005
42
C-SCAN
 Provides a more uniform wait time than SCAN.
 The head moves from one end of the disk to the other, servicing requests
as it goes. When it reaches the other end, however, it immediately
returns to the beginning of the disk, without servicing any requests on the
return trip.
 Treats the cylinders as a circular list that wraps around from the last
cylinder to the first one. (→“Circular Scan”)
Operating Systems • Prof. Dr. Marc H. Scholl • DBIS • U KN • Summer Term 2009
Silberschatz, Galvin and Gagne ©2005
43
C-LOOK
 Somewhat smarter version of C-SCAN
 Arm only goes as far as the last request in each direction, then reverses
direction immediately, without first going all the way to the end of the
disk.
Operating Systems • Prof. Dr. Marc H. Scholl • DBIS • U KN • Summer Term 2009
Silberschatz, Galvin and Gagne ©2005
44
Selecting a Disk-Scheduling Algorithm
 SSTF is common and has a natural appeal.
 SCAN and C-SCAN perform better for systems that place a heavy load
on the disk.
 Performance depends on the number and types of requests.
 Requests for disk service can be influenced by the file-allocation method.
 The disk-scheduling algorithm should be written as a separate module of
the operating system, allowing it to be replaced with a different algorithm,
if necessary.
 Either SSTF or LOOK is a reasonable choice for the default algorithm.
Operating Systems • Prof. Dr. Marc H. Scholl • DBIS • U KN • Summer Term 2009
Silberschatz, Galvin and Gagne ©2005
45
RAID-Systems
(Disk Arrays)
Operating Systems • Prof. Dr. Marc H. Scholl • DBIS • U KN • Summer Term 2009
46
RAID-Systems — Basic Ideas
• Idea came up in the 1980s
• „Winchester disk drives“: smaller, cheaper, less power & space/
volume consumption, no air conditioning, ...
than state-of-the-art mainframe disk drives
• But: Many disk drives largely reduced MTTF (by factor 1/n, assuming independence)
• Need to use redundancy to improve/regain reliability
• Many disks can also provide much better performance
• Can also avoid restrictions of file systems:
• files must fit onto single disk → „logical volumes“ spanning disks
• Striping: use a group of disks as a single storage unit
• Mirroring/Shadowing or Parity/ECCs improve reliability by
storing redundant data
Operating Systems • Prof. Dr. Marc H. Scholl • DBIS • U KN • Summer Term 2009
47
RAID Levels
 Large variety of exact definitions of the
levels, plus additional levels.
 Various choices as to how and where to
implement RAID functionality (OS, drive,
controller, SAN interconnect; HW/SW;...)
 Additional functionality can be
integrated, such as replication and
snapshots.
 Volume mgmt. software offers a lot of
additional functionality.
 Hot spare disks can be reserved as
„immediate“ replacement.
Operating Systems • Prof. Dr. Marc H. Scholl • DBIS • U KN • Summer Term 2009
Silberschatz, Galvin and Gagne ©2005
48
More Aspects
Operating Systems • Prof. Dr. Marc H. Scholl • DBIS • U KN • Summer Term 2009
49
Hierarchical Storage Management (HSM)
 A hierarchical storage system extends the storage hierarchy beyond
primary memory and secondary storage to incorporate tertiary storage —
usually implemented as a jukebox of tapes or removable disks.
 Usually incorporate tertiary storage by extending the file system.
 Automatically migrate data up and down the storage hierarchy

Small and frequently used files remain on disk.

Large, old, inactive files are archived to the jukebox.
 HSM is usually found in supercomputing centers and other large
installations that have enormous volumes of data.
Operating Systems • Prof. Dr. Marc H. Scholl • DBIS • U KN • Summer Term 2009
Silberschatz, Galvin and Gagne ©2005
50
Cost
 Main memory is much more expensive than disk storage
 The cost per megabyte of hard disk storage is competitive with magnetic
tape if only one tape is used per drive.
 The cheapest tape drives and the cheapest disk drives have had about
the same storage capacity over the years.
 Tertiary storage gives a cost savings only when the number of cartridges
is considerably larger than the number of drives.
Operating Systems • Prof. Dr. Marc H. Scholl • DBIS • U KN • Summer Term 2009
Silberschatz, Galvin and Gagne ©2005
51
Price per Megabyte of DRAM (1981 to 2004)
Operating Systems • Prof. Dr. Marc H. Scholl • DBIS • U KN • Summer Term 2009
Silberschatz, Galvin and Gagne ©2005
52
Price per Megabyte of Magnetic Hard Disk (1981 to 2004)
Operating Systems • Prof. Dr. Marc H. Scholl • DBIS • U KN • Summer Term 2009
Silberschatz, Galvin and Gagne ©2005
53
Price per Megabyte of a Tape Drive (1984-2000)
Operating Systems • Prof. Dr. Marc H. Scholl • DBIS • U KN • Summer Term 2009
Silberschatz, Galvin and Gagne ©2005
54
Intended Schedule
0
1
2
3
4
5
6
7
8
9
10
11
12
✏
Date
20.04.
27.04.
04.05.
11.05.
18.05.
25.05.
01.06.
08.06.
15.06.
22.06.
29.06.
06.07.
13.07.
20.07.
27.07.
12.10.
Lecture
Introduction to Operating Systems
Systems Programming using C (File Subsystem)
Systems Programming using C (Process Control)
Process Scheduling
Process Synchronization
Inter Process Communication
Pfingstmontag
Deadlocks
Memory Management
Hand out
Submission
Course registration
1. Assignment
2. Assignment
1. Assignment
3. Assignment
2. Assignment
4. Assignment
3. Assignment
5. Assignment
4. Assignment
6. Assignment
5. Assignment
7. Assignment
6. Assignment
8. Assignment
7. Assignment
9. Assignment
8. Assignment
10. Assignment
9. Assignment
10. Assignment
Input / Output
Filesystems
Special subject: XQuery your Filesystem
Wrap up session
First examination date
Second examination date
Operating Systems • Prof. Dr. Marc H. Scholl • DBIS • U KN • Summer Term 2009
55
IX.
File Systems
Operating Systems • Prof. Dr. Marc H. Scholl • DBIS • U KN • Summer Term 2009
56