Buffer Cache

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Operating systems-UNIX

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Buffer Cache : 

1 Buffer Cache Chapter 3 TOPICS UNIX system Architecture Buffer Cache Buffer Pool Structure Retrieval of Buffer Release Buffer Reading and Writing Disk Blocks Reference: The Design of the UNIX Operating System by Maurice J. Bach

UNIX Kernel Architecture : 

2 UNIX Kernel Architecture

Buffer Cache : 

3 Buffer Cache When a process wants to access data from a file, the kernel brings the data into main memory, alters it and then request to save in the file system Example: copy cp one.c two.c To increase the response time and throughput, the kernel minimizes the frequency of disk access by keeping a pool of internal data buffer called buffer cache.

Buffer Cache : 

4 Buffer Cache Buffer cache contains the data in recently used disk blocks When reading data from disk, the kernel attempts to read from buffer cache. If data is already in the buffer cache, the kernel does not need to read from disk If data is not in the buffer cache, the kernel reads the data from disk and cache it

Buffer Headers : 

5 Buffer Headers A buffer consists of two parts a memory array buffer header disk block : buffer = 1 : 1

Buffer Headers : 

6 Buffer Headers device num logical file system number block num block number of the data on disk status The buffer is currently locked. The buffer contains valid data. delayed-write The kernel is currently reading or writing the contents of the disk. A process is currently waiting for the buffer to become free. kernel identifies the buffer content by examing device num and block num.

Buffer Headers : 

7 Buffer Headers struct buffer_head{ /*First cache line: */ struct buffer_head *b_next; /*Hash queue list*/ unsigned long b_blocknr; /*block number*/ unsigned long b_size; /*block size*/ kdev_t b_dev; /*device(B_FREE = free)*/ kdev_t b_rdev; /*Read device*/ unsigned long b_rsector; /*Real Buffer location on disk*/ struct buffer_head *b_this_page; /*circular list of buffers in one page*/ unsigned long b_state; /*buffer state bitmap(see above)*/ struct buffer_head *b_next_free; unsigned int b_count; /*users using this block*/ char *b_data; /*pointer to data block(1024 bytes)*/ unsigned int b_list; /*List that this buffer appears*/ unsigned long b_flushtime; /*Time when this(dirty) buffer should be written*/ struct wait_queue *b_wait; struct buffer_head **b_pprev; /*doubly linked list of hash-queue*/ struct buffer_head *b_prev_free; /* double linked list of buffers*/ struct buffer_head *b_reqnext; /*request queue*/ /*I/O completion*/ void (*b_end_io)(struct buffer_head *bh, int uptodate); void *b_dev_id; };

Buffer Headers : 

8 Buffer Headers /* buffer head state bits*/ #define BH_Uptodate 0 /*1 if the buffer contains valid data*/ #define BH_Dirty 1 /*1 if the buffer is dirty*/ #define BH_Lock 2 /*1 if the buffer is locked*/ #define BH_Req 3 /*0 if the buffer has been invalidated*/ #define BH_Protected 6 /*1 if the buffer is protected*/

Structures of the buffer pool : 

9 Structures of the buffer pool Buffer pool according to LRU The kernel maintains a free list of buffer doubly linked list take a buffer from the head of the free list. When returning a buffer, attaches the buffer to the tail.

Structures of the buffer pool : 

10 Structures of the buffer pool When the kernel accesses a disk block separate queue (doublely linked circular list) hashed as a function of the device and block num Every disk block exists on one and only on hash queue and only once on the queue

Scenarios for retrieval of a buffer : 

11 Scenarios for retrieval of a buffer Determine the logical device num and block num The algorithms for reading and writing disk blocks use the algorithm getblk The kernel finds the block on its hash queue The buffer is free. The buffer is currently busy. The kernel cannot find the block on the hash queue The kernel allocates a buffer from the free list. In attempting to allocate a buffer from the free list, finds a buffer on the free list that has been marked “delayed write”. The free list of buffers is empty.

Retrieval of a Buffer:1st Scenario (a) : 

12 Retrieval of a Buffer:1st Scenario (a) The kernel finds the block on the hash queue and its buffer is free Search for block 4

Retrieval of a Buffer:1st Scenario (b) : 

13 Retrieval of a Buffer:1st Scenario (b) Remove block 4 from free list

Retrieval of a Buffer: 2nd Scenario (a) : 

14 Retrieval of a Buffer: 2nd Scenario (a) The kernel cannot find the block on the hash queue, so it allocates a buffer from free list Search for block 18: Not in cache

Retrieval of a Buffer: 2nd Scenario (b) : 

15 Retrieval of a Buffer: 2nd Scenario (b) Remove 1st block from free list: Assign to 18

Retrieval of a Buffer: 3rd Scenario (a) : 

16 Retrieval of a Buffer: 3rd Scenario (a) The kernel cannot find the block on the hash queue, and finds delayed write buffers on hash queue Search for block 18, Delayed write blocks on free list delay delay

Retrieval of a Buffer: 3rd Scenario (b) : 

17 Retrieval of a Buffer: 3rd Scenario (b)

Retrieval of a Buffer: 4th Scenario : 

18 Retrieval of a Buffer: 4th Scenario The kernel cannot find the buffer on the hash queue, and the free list is empty Search for block 18, free list empty

Retrieval of a Buffer: 5th Scenario : 

19 Retrieval of a Buffer: 5th Scenario Kernel finds the buffer on hash queue, but it is currently busy Search for block 99, block busy busy

Algorithm: GetBlock : 

20 Algorithm: GetBlock GetBlock (file_system_no,block_no) while (buffer not found) if (buffer in hash queue) if (buffer busy) sleep (event buffer becomes free) continue mark buffer busy remove buffer from free list return buffer else if (there is no buffer on free list) sleep (event any buffer becomes free) continue remove buffer from free list if (buffer marked as delayed write) asyschronous white buffer to disk continue remove buffer from hash queue put buffer onto hash queue return buffer

Algorithm: ReleaseBlock : 

21 Algorithm: ReleaseBlock ReleaseBlock (locked buffer) wakeup all process event, waiting for any buffer to become free wakeup all process event, waiting for this buffer to become free raise processor execution level to block interrupt if (buffer content valid and buffer not old) enqueue buffer at the end of free list else enqueue buffer at the beginning of free list lower processor execution level to allow interrupt unlock (buffer)

Reading and Writing disk blocks : 

22 Reading and Writing disk blocks To read block ahead The kernel checks if the block is in the cache or not. If the block in not in the cache, it invokes the disk driver to read the block. The the process goes to sleep awaiting the event that the I/O is complete. The disk controller interrupts the processor when the I/O is complete The disk interrupt handler awakens the sleeping processes The content of disk blocks are now in the buffer When the process no longer need the buffer, it releases it so that other processes can access it

Reading and Writing disk blocks : 

23 Reading and Writing disk blocks To write a disk block The kernel informs the disk driver that it has a buffer whose contents should be output. The disk driver schedules the block for I/O. If the write is synchronous, the calling process goes the sleep awaiting I/O completion and releases the buffer when awakens. If the write is asynchronous, the kernel starts the disk write. The kernel release the buffer when the I/O completes.

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