Buffer Management System

Implement a model of how buffer cache works in a system, and show how the kernel and other processes interact to get the desired effect.

How to run

Pre-requisites

• Unix Operating System
• Python - v3.6 or higher
• Node - v10 or higher
• npm - v5.6 or higher

Steps

• Run the shell script gb_run. If the shell script is not executable run chmod +x gb_run, and then run ./gb_run
• After a while a browser window will pop-up, if the window doesn't show anything then try refreshing the page.
• If you wish to see the detailed log file, then open ./GUI/public/log.

Data flow

Modules

• Config
      Stores the specification details set by the superuser.

• buffer_header
      Implements the structure of a buffer(including buffer header).

• hash_queue
      Implements the Hash Queue frame.

• free_queue
      Implements the Free Queue frame.

• buffer_cache
      Implements the buffer cache(including the free list header).

• get_block
      Implements the function to allocate a buffer for a disk block.

• brelease
      Implements the function to de-allocate a disk block from a buffer.

• request
      Contains the classes to handle threading queues.

• Kernel
      Implements the kernel that interacts with the buffer cache and various processes to service their needs.

• gb_run
      Contains shell script to run the module.

Module details

BufferHeader

BufferHeader is essentially the entire Buffer, but because of the existing buffer library in python we had to name it BufferHeader.

Variable	Type	Definition
block_number	Number	Block Number whose data is in the buffer
process_id	Number	Process ID of the process using the buffer
status	String	Status of the buffer which is a combination of the various status states defined below
data	String	Data of the Block stored in the buffer
next_hash_queue	BufferHeader	Next buffer in the Hash Queue
prev_hash_queue	BufferHeader	Previous buffer in the Hash Queue
next_free_list	BufferHeader	Next buffer in the Free List
prev_free_list	BufferHeader	Previous buffer in the Free List
lock	Threading Lock	Lock the buffer

Method	Definition
get_status	Returns the status of the buffer
set_status	Sets the status of the buffer(keeping in mind that buffer status is a combination of the status states)
remove_status	Removes the status of the buffer

  

---

  

Status of a buffer

These status states are defined in the Config file.

State Symbol (String)	State Name	State Meaning
0	BUSY	Buffer is Busy(Locked)
1	FREE	Buffer is Free(Unlocked)
2	VALID	Buffer contains Valid data
3	ACCESS_DISK	Some Process is reading or writing data from or to the disk
4	PROCESS_WAITING	Some Process is waiting for the buffer to become free
5	OLD	Buffer is Old (already in free list)
6	NOT_OLD	Buffer is not Old (just entered in free list)

  

---

  

Type of requests from a process

These requests are defined in the Config file.

Request Type
Read
Write
Delayed Write

  

---

  

Type of flags sent from get_block

These flags are defined in the Config file.

Flag Symbol (String)	Flag Name	Flag Meaning
0	NONE	Flag is null
1	DELAYED_WRITE	The block in the free list has status delayed write
2	BLOCK_BUSY	The block process want to access is busy
3	FREE_LIST_EMPTY	Free list is Empty

  

---

  

HashQueue

Variable	Type	Definition
head	BufferHeader	Buffer at the start of the hash queue
tail	BufferHeader	Buffer at the end of the hash queue

Method	Definition
add	add the buffer to this particular hash queue
remove	remove the buffer from this particular hash queue
show	returns the state of hash queue

  

---

  

FreeQueue

Variable	Type	Definition
head	BufferHeader	Buffer at the start of the free queue
tail	BufferHeader	Buffer at the end of the free queue

Method	Definition
add_to_tail	add the buffer to the tail of the free queue
add_to_head	add the buffer to the head of the free queue
is_empty	returns true if free list is empty
remove	remove the buffer from the free queue
remove_from_head	remove the buffer from the head of free queue
show	returns the state of free queue

  

---

  

BufferCache

Variable	Type	Definition
free_list	FreeQueue	Free List(initially containing all the buffers)
hash_queue_headers	List of HashQueue	Hash Queues (all of them initially empty)

Method	Definition
in_hash_queue	Returns True if block is in the corresponding HashQueue
assign_block	Returns the BufferHeader that contains the block (this function can only be called if in_hash_queue returns true)
show	returns the state of buffer cache

  

---

  

get_block

This function implements the system call getblk

  

---

  

brelease

This function implements the system call brelease

  

---

  

Request

Class	Definition
Request	To create objects for get_block requests
ReleaseRequest	To create objects for brelease requests
Reply	To create objects for reply from request manager

Class-Request

Variable	Type	Definition
process_id	Number	Process id of the process which requested the block
block_number	Number	Block number which is required
request_type	String	Type of Request(Read/ Write/ Delayed write)
return_queue	BufferHeader-Queue	Buffer containing the block is returned in the queue

Class-ReleaseRequest

Variable	Type	Definition
process_id	Number	Process id of the process which is releasing the block
block	BufferHeader	The block which is to be released

Class-Reply

Variable	Type	Definition
block	BufferHeader	Address of locked buffer
event	String	Flag message if the buffer lock is not attained

  

---

  

kernel

It is the backbone of the module. It is divided into four broad categories.

Category	Definition
Queues	So that at a moment only one process can access the resource
Daemon Threads	Threads which runs till the life of the kernel and acts as a resource allocator
Process Threads	Threads which acts like a process and asks for blocks time to time
Thread Events	Events to control sleep and wakeup of the threads, when some block is busy/free

• Queues

Name	Definition
request_queue	This queue contains get_block requests
print_queue	This queue contains print requests
release_queue	This queue contains brelease requests

• Daemon Threads

Name	Function	Definition
requests_thread	requests_manager	This thread accesses get_block algorithm
print_thread	print_manager	This thread accesses std_output
release_thread	release_manager	This thread accesses brelease algorithm

• Process Threads

Name	Function	Definition
Process	worker	This thread acts as an arbitrary process

• Thread Events

Name	Definition
any_block_free	This event is used for processes waiting for any buffer to become free
block_free[i]	This event is used for processes waiting for ith buffer to become free

• delay_func is a function used to manage asynchronous write.   

---