1. Mr. M. D. Jamadar Assistant Professor
Introduction to Unix
Mr. M. D. Jamadar
Assistant Professor

2. Architecture of Unix OS

3. Unix File System The UNIX file system Characterized by
A Hierarchical structure,
Consistent treatment of file data,
The ability to create and delete file,
Dynamic growth of the file,
The protection of the data,
The treatment of peripheral devices as files

4. OS Services
Controlling the execution of process by allowing their creation, termination or suspension, and communication
Scheduling Processes fairly for execution on the CPU.
Allocating Main memory for an executing process
Allocating secondary memory for efficient storage and retrieval of user data.
Allowing processes controlled access to peripheral devices such as terminals, tape driver, disk driver, and network devices

5. Block Diagram of System Kernel

6. Overview of File Subsystem
The File System Has the following Structure
The boot block occupies the beginning of a file system, may contain the bootstarp code that is read into the machine to boot or initialize the OS
The Super Block Describes the state of a file system—How large it is , how many no. of files it an store, where to find free space, and so on
The inode List is a list of inodes. Administrators specify the size of the inode list when configuring a file system.
The data Block Start sat the end of the inode list and contain file data and administrative data.

7. Process Control SubSystem
Fields in the Process Table
A State Field
ID Indicating the owner
Event descriptor set when a process is suspended
Fields in the u-Area
A pointer to the process table slot of the current executing process
Parameters of the current system call, return value and error codes,
File descriptors for all open files,
Internal I/o Parameters,
Current directory and current root
Process and file size limit

8. Process Control SubSystem

9. Process States The process is currently executing in user mode.
The process is currently executing in kernel mode.
The process is not executing, but it is ready to run as soon as the scheduler chooses it.
The process is sleeping.

10. The Buffer Cache

11. Buffer Cache
During System Initialization, The Kernel allocates space for a number of buffers, configurable according to memory performance constraints.
A buffer consist of two parts
A memory (Data) array
Buffer Header
There is a one to one mapping of buffer headers to data array hence collectively called as “Buffer”.
The buffer is the in-memory copy of the disk block.
The kernel identifies the buffer content by examining identifier field in the buffer header.
The content of disk block map into the buffer, but mapping is temporary until the kernel decides to map another disk block into the buffer.
A disk block can never map into more than one buffer

12. Buffer Header
The Status of a buffer is a combination of the following condition
The buffer is currently locked
The buffer contains valid data
The kernel must write the buffer content to the disk before reassigning the buffer the condition is known as “Delayed Write.”
The kernel is reading or writing the content of the buffer to the disk
A process is currently waiting for the buffer to become free

13. Buffer Pool Buffer Pool Free List
The kernel caches data in the buffer pool according to a least recently used algorithm
After it allocates a buffer to a disk block, it cannot use the buffer for another block until all other buffer have been used more recently.
The kernel maintains a free list of buffers that preserves the least recently used order
The free list is a doubly linked circular list of buffers with dummy buffer header that marks its beginning and end.
Every buffer is put on the free list when the system is booted.
The kernel takes a buffer from head of the free list when it wants any free buffer, but it can take a buffer from the middle of the free list if it identifies a particular block in the buffer pool

14. Buffer Pool Cont…..
When the kernel accesses a disk block, it searches for a buffer with the appropriate device – block number combination
It organizes the buffers into separate queues, hashed as a function of the device and block number.
The kernel links the buffer on a hash queue into a circular, doubly linked list.
The kernel uses a hashing function that distributes the buffers uniformly across the set of hash queues
System administrators configure the number of hash ques when generating the OS.

15. Scenarios for Retrieval of Buffer
High-level kernel algorithm in the file subsystem invoke the algorithms for managing the buffer cache.
The high level algorithm determines the logical device number and block number to retrieve the block
The kernel checks weather the block is in the buffer pool and, if it is not there, assigns it a free buffer.
The algorithms for reading and writing disk block use the algorithm getblk to allocate buffer from the pool.
Scenarios the kernel may follow in getblk to allocate a buffer for a disk block
The kernel finds the block on its hash queue, and its buffer is free
The kernel cannot find the block on the hash queue and, so it allocates a buffer from the free list.
The kernel cannot find the block on the hash queue and, finds a buffer on the free list that has been marked “Delayed write”.
The kernel cannot find the block on the hash queue, and the free list is empty
The kernel find the block on the hash queue, but its buffer is currently busy

16. Scenarios for Retrieval of Buffer
Scenario 1: The kernel finds the block on its hash queue, and its buffer is free

17. Scenarios for Retrieval of Buffer
Scenario 2: The kernel cannot find the block on the hash queue and, so it allocates a buffer from the free list.

18. Scenarios for Retrieval of Buffer
Scenario 3: The kernel cannot find the block on the hash queue and, finds a buffer on the free list that has been marked “Delayed write”.

19. Scenarios for Retrieval of Buffer
Scenario 4: The kernel cannot find the block on the hash queue, and the free list is empty

20. Scenarios for Retrieval of Buffer
Scenario 5: The kernel find the block on the hash queue, but its buffer is currently busy

21. Algorithm for buffer allocation

22. Algorithm for releasing a buffer
Buffer is old when it is marked “Delayed Write”

23. Algorithm for Reading a Disk Block
Steps for BREAD Algorithm
Process uses algorithm getblk to search for block in the buffer cache, if it is in the cache, the kernel returns it immediately.(Without Reading from disk)
If it is not in the cache, the kernel calls the disk driver to schedule a read request and goes to sleep
The disk driver notifies the disk controller hardware to read data
The disk controller transmits the data to the buffer
The disk controller interrupts the processor when the I/O is complete and the disk interrupt handler awakens the sleeping process

24. Algorithm for Block Read Ahead
Higher level kernel module (file subsystem) may anticipate the need for a second disk block when a process read a file sequentially.
The module request the second I/O asynchronously In the hope that the data will be in memory when needed, improving performance. To do this the kernel executes the block read-ahead algorithm(breada)

25. Algorithm for Writing a Disk Block
Steps for Buffer Write
The kernel informs the disk driver that it has buffer whose content should be output(Write)
The disk driver schedules the block for I/O.
If the write is synchronous, the calling process goes to sleep awaiting I/O completion and release the buffer when it awaken
If the write is asynchronous, the kernel starts the disk write but doesn't wait for the I/O to complete.
The kernel will release the buffer when the I/O Completes

26. References
The design of Unix Operating System - Maurice J. Bach (PHI)