1. COT 5611 Operating Systems Design Principles Spring 2010 Dan C. Marinescu Office: HEC 439 B Office hours: M-Wd 1:00-2:00 PM

2. 2 2 Names and the three basic abstractions Memory  stores named objects  write(name, value) value  READ(name)  file system: /dcm/classes/Fall09/Lectures/Lecture5.ppt Interpreters  manipulates named objects  machine instructions ADD R1,R2  modules  Variables call sort(table) Communication Links  connect named objects  HTTP protocol used by the Web and file systems Host: boticelli.cs.ucf.edu put /dcm/classes/Fall09/Lectures/Lecture5.ppt get /dcm/classes/Fall09/Lectures/Lecture5.ppt

3. 3 3 Memory Hardware memory:  Devices RAM (Random Access Memory) chip Flash memory  non-volatile memory that can be erased and reprogrammed Magnetic tape Magnetic Disk CD and DVD  Systems RAID File systems DBMS (Data Base management Systems)

4. 4 4 Attributes of the storage medium/system Durability  the time it remembers Stability  whether or not the data is changed during the storage Persistence  property of data storage system, it keeps trying to preserve the data

5. 5 5 Critical properties of a storage medium/system Read/Write Coherence  the result of a READ of a memory cell should be the same as the most recent WRITE to that cell. Before-or-after atomicity  the result of every READ or WRITE is as if that READ or WRITE occurred either completely before or completely after any other READ or WRITE

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7. 7 7 Why it is hard to guarantee the critical properties? Concurrency  multiple threads could READ/WRITE to the same cell Remote storage  The delay to reach the physical storage may not guarantee FIFO operation Optimizations  data may be buffered to increase I/O efficiency Cell size may be different than data size  data may be written to multiple cells. Replicated storage  difficult to maintain consistency.

8. 8 8 Access type; access time Sequential access  Tapes  CD/DVD Random access devices  Disk Seek Search time Read/Write time  RAM

9. 9 Physical memory organization RAM  two dimensional array. To select a flip-flop provide the x and y coordinates. Tapes  blocks of a given length and gaps (special combination of bits. Disk:  Multiple platters  Cylinders correspond to a particular position of the moving arm  Track  circular pattern of bits on a given platter and cylinder  Record  multiple records on a track 9

10. 10 Names and physical addresses Location addressed memory  the hardware maps the physical coordinates to consecutive integers, addresses Associative memory  unrestricted mapping; the hardware does not impose any constraints in mapping the physical coordinates 10 Figure 2.2 from textrbook

11. 11 RAID – Redundant Array of Inexpensive Disks The abstraction put to work to increase performance and durability. Raid 0  allows concurrent reading and writing. Increases performance but does not improve reliability. Raid 1  increases durability by replication the block of data on multiple disks (mirroring) Raid 2  Disks are synchronized and striped in very small stripes, often in single bytes/words.  Error correction calculated across corresponding bits on disks, and is stored on multiple parity disks (Hamming codes). 11

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13. 13 RAID (cont’d) Raid 3  Striped set with dedicated parity or bit interleaved parity or byte level parity. Raid 4  improves reliability, it adds error correction 13

14. 14 RAID (cont’d) Raid 5  striped disks with parity combines three or more disks to protect data against loss of any one disk.  The storage capacity of the array is reduced by one disk Raid 6  striped disks with dual parity combines four or more disks to protect against loss of any two disks.  Makes larger RAID groups more practical.  Large-capacity drives lengthen the time needed to recover from the failure of a single drive. Single parity RAID levels are vulnerable to data loss until the failed drive is rebuilt: the larger the drive, the longer the rebuild will take. Dual parity gives time to rebuild the array without the data being at risk if a (single) additional drive fails before the rebuild is complete. 14

15. 15 Interpreters The active elements of a computer system Diverse  Hardware  Processor, Disk Controller, Display controller  Software  script language: Javascropt, Pearl, Python text processing systems: Latex, Tex, Word browser : Safari, Google Chrome,Thunderbird All share three major abstractions/components:  Instruction reference  tells the system where to find the next instruction  Repertoire  the set of actions (instructions) the interpreter is able to perform  Environment reference  tells the interpreter where to find the its environment, the state in which it should be to execute the next instruction 15

16. 16 An abstract interpreter The three elements allow us to describe the functioning of an interpreter regardless of its physical realization. Interrupt  mechanism allowing an interpreter to deal with the transfer of control. Once an instruction is executed the control is passed to an interrupt handler which may change the environment for the next instruction. More than a single interpreter may be present. 16

17. 17 Figure 2.5 from the textbook

18. 18 Processors Can execute instructions from a specific instruction set Architecture  PC, IR, SP, GPR, ALU, FPR, FPU  State is saved on a stack by the interrupt handler to transfer control to a different virtual processor, thread. 18

19. 19 Interpreters are organized in layers Each layer issues instructions/requests for the next. A lower layer generally carries out multiple instruction for each request from the upper layer. 19

20. 20 Communication Links Two operations  SEND (link_name, outgoing_message_Buffer)  RECEIVE (link_name, incoming_message_Buffer) Message  an array of bits Physical implementation in hardware  Wires  Networks Ethernet Internet The phone system 20

21. 21 The Internet – an extreme example of what hides behind the communication link abstraction Internet Core and Edge The hardware  Router  Network adaptor Hourglass communication model Protocol stack It’s along way to Tipperary – the way a message squizes through protocol layers

22. 22 Naming The tree abstractions manipulate objects identified by name. How could object A access object B:  Make a copy of object B and include it in A  use by value Safe  there is a single copy of B How to implement sharing of object B?  Pass to A the means to access B using its name  use by reference Not inherently safe  both A and C may attempt to modify B at the same time. Need some form of concurrency control.

23. 23 Binding and indirection Indirection  decoupling objects from their physical realization through names. Names allow the system designer to: 1. organize the modules of a system and to define communication patterns among them 2. defer for a later time to create object B referred to by object A select the specific object A wishes to use Binding  linking the object to names. Examples:  A compiler constructs a table of variables and their relative address in the data section of the memory map of the process a list of unsatisfied external references  A linker binds the external references to modules from libraries

24. 24 Generic naming model Naming scheme  strategy for naming. Consists of:  Name space  the set of acceptable names; the alphabet used to select the symbols from and the syntax rules.  Universe of values  set of objects/values to be named  Name mapping algorithm  resolves the names, establishes a correspondence between a name and an object/value  Context  the environment in which the model operates. Example: searching for John Smith in the White Pages in Orlando (one context) or in Tampa (another context). Sometimes there is only one context  universal name space; e.g., the SSNs. Default context

25. 25 Figure 2.10 from the textbook

26. 26 Operations on names in the abstract model Simple models: value  RESOLVE (name, context)  The interpreter: Determines the version of the RESOLVE (which naming scheme is used) Identifies the context Locates the object  Example: the processor Complex models support:  creation of new bindings: status  BIND(name, value, context)  deletion of old bindings: status  UNBIND(name, value)  enumeration of name space: list  ENUMERATE(context)  comparing names status: result  COMPARE(name1,name2)

27. 27 Name mapping Name to value mapping  One-to-One  the name identifies a single object  Many-to-One  multiple names identify one objects (aliasing)  One-to-Many  multiple objects have the same name even in the same context. Stable bindings  the mapping never change. Examples:  Social Security Numbers  CustomerId for customer billing systems

28. 28 Name-mapping algorithms 1. Table lookup 1. Phone book 2. Port numbers  a port the end point of a network connection 2. Recursive lookup: 1. File systems – path names 2. Host names – DNS (Domain Name Server) 3. Names for Web objects - URL – (Universal Resource Locator) 3. Multiple lookup  searching through multiple contexts 1. Libraries 2. Example: the classpath is the path that the Java runtime environment searches for classes and other resource files

29. 29 1. Table lookup Figure 2.11 from the textbook

30. 30 How to determine the context Context references:  Default  supplied by the name resolver Constant  built-in by the name resolver  Processor registers (hardwired)  Virtual memory (the page table register of an address space) Variable  supplied by the current environment  File name (the working directory)  Explicit  supplied by the object requesting the name resolution Per object  Looking up a name in the phone book Per name  each name is loaded with its own context reference (qualified name).  URL  Host names used by DNS

31. 31 Dynamic and multiple contexts Context reference static/dynamic.  Example: the context of the “help” command is dynamic, it depends where you are the time of the command. A message is encapsulated (added a new header, ) as flows down the protocol stack:  Application layer (application header understood only in application context)  Transport layer (transport header understood only in the transport context)  Network layer (network header understood only in the network context)  Data link layer (data link header understood only in the data link context)

32. 32 2. Recursive name resolution Contexts are structured and a recursion is needed for name resolution. Root  a special context reference - a universal name space Path name  name which includes an explicit reference to the context in which the name is to be resolved.  Example: first paragraph of page 3 in part 4 of section 10 of chapter 1 of book “Alice in Wonderland.”  The path name includes multiple components known to the user of the name and to name solver  The least element of the path name must be an explicit context reference Absolute path name  the recursion ends at the root context. Relative path name  path name that is resolved by looking up its mot significant component of the path name

33. 33 Example AliceInWonderland.Chapter1.Section10.Part4.Page3.FirstParagraph Most significant   Least significant

34. 34 3. Multiple lookup Search path  a list of contexts to be searched Example: the classpath is the path that the Java runtime environment searches for classes and other resource files User-specfic search paths  user-specific binding The contexts can be in concentric layers. If the resolver fails in a inner layer it moves automatically to the outer layer. Scope of a name  the range of layers in which a name is bound to the same object.

35. 35 Comparing names Questions  Are two names the same?  easy to answer  Are two names referring to the same object (bound to the same value)?  harder; we need the contexts of the two names.  If the objects are memory cells are the contents of these cells the same?

36. 36 Name discovery Two actors:  The exporter  advertizes the existence of the name.  The prospective user  searches for the proper advertisement. Example: the creator of a math library advertizes the functions. Methods  Well-known names  Broadcasting  Directed query  Broadcast query  Introduction  Physical randezvoue

37. Computer System Organization Operating Systems (OS)  software used to  Control the allocation of resources (hardware and software)  Support user applications  Sandwiched between the hardware layer and the application layer OS-bypass: the OS does not hide completely the hardware from applications. It only hides dangerous functions such as  I/O operations  Management function Names  modularization 37

38. 38 Figure 2.16 from the textbook

39. The hardware layer Modules representing each of the three abstractions (memory, interpreter, communication link) are interconnected by a bus. The bus  a broadcast communication channel, each module hears every transmission.  Control lines  Data lines  Address lines Each module  is identified by a unique address  has a bus interface Modules other than processors need a controller. 39

40. 40 Figure 2.17 from the textbook

41. 41 Bus sharing and optimization Communication  broadcast Arbitration protocol  decide which module has the control of the bus. Supported by hardware:  a bus arbiter circuit  distributed among interfaces – each module has a priority  daisy chaining Split-transaction  a module uses the arbitration protocol to acquire control of the bus Optimization:  hide the latency of I/O devices Channels  dedicated processors capable to execute a channel program (IBM) DMA (Direct Memory Access)  Support transparent access to files: Memory Mapped I/O

42. 42 Optimization Direct Memory Access (DMA):  supports direct communication between processor and memory; the processor provides the disk address of a block in memory where data is to be read into or written from.  hides the disk latency; it allows the processor to execute a process while data is transferred Memory Mapped I/O:  LOAD and STORE instructions access the registers and buffers of an I/O module bus addresses are assigned to control registers and buffers of the I/O module the processor maps bus addresses to its own address space (registers)  Supports software functions such as UNIX mmap which map an entire file. Swap area: disk image of the virtual memory of a process.

43. 43 DMA Transfer 43

44. 44 B. The software layer: the file abstraction File: memory abstraction used by the application and OS layers  linear array of bits/bytes  properties: durable  information will not be changed in time has a name  allows access to individual bits/bytes  has a cursor which defines the current position in the file. The OS provides an API (Application Programming Interface) supporting a range of file manipulation operations. A user must first OPEN a file before accessing it and CLOSE it after it has finished with it. This strategy:  allows different access rights (READ, WRITE, READ-WRITE)  coordinate concurrent access to the file Some file systems  use OPEN and CLOSE to enforce before-or-after atomicity  support all-or-nothing atomicity  e.g., ensure that if the system crashes before a CLOSE either all or none of WRITEs are carried out

45. 45 Open and Read operations 45