Working with the Unix OS

Buffer Cache
Process B will find the locked block on the hash queue. Process B marks the buffer "in demand" and the sleeps.
Another process C, may have been waiting for the same buffer, if C is scheduled before B, B must check the block
is free.
Process C may allocate the buffer to another block, so when process B executes it must search for the block again.
With contention for a locked buffer need to start search again.
Figure 72. Race for a Locked Buffer.
The kernel guarantees that all
processes waiting for buffers will wake
up, because it allocates buffers during
the execution of system calls and frees
them before returning.
Processes in user mode do not control the allocation of kernel buffers directly, so they cannot purposely "hog"
buffers.
The kernel does not guarantee that a process get a buffer in the order that they requested one.
Reading and Writing Disk Blocks
algorithm bread * block read */
input: file system block number
output: buffer containing data
{
get buffer for block (algorithm
getblk);
if (buffer data valid)
return buffer;
initiate disk read;
sleep(event disk read complete);
return (buffer);
Figure 73. Reading a Disk Block Bach, "bread".
If the disk block is not in cache, the kernel calls the disk driver to "schedule" a read request and goes to sleep
awaiting the event that the I/O completes.
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