Working with the Unix OS

Buffer Cache
"getblk" to allocate a buffer for a disk block, the kernel:
(1) finds the block on its hash queue, and its buffer is free.
(2) cannot find the block on the hash queue, it allocates a buffer from the free list.
(3) same as (2), but finds a buffer on free list marked "delayed-write", must write to disk and allocate another.
(4) same as (2), but free list is empty
(5) finds the block on its hash queue, but its buffer is busy.
In (1) the kernel marks the buffer busy and
removes it from the free list. If other
processes attempt to access the block, they
sleep until it is released.
Figure 66. First Scenario in Finding a Buffer:
Buffer on Hash Queue
algorithm brelse
input: locked buffer
output: none
{
wakeup all procs: event, waiting for any buffer to become free;
wakeup all procs: event, waiting for this buffer to become free;
raise processor execution level to block interrupts;
if (buffer contents valid and buffer not old)
enqueue buffer at end of free list
else
enqueue buffer at beginning of free list lower processor
execution level to allow interrupts;
unlock(buffer);
}
"brelse" to release buffer when
kernel is finished using it.
Figure 3.6. Algorithm for Releasing a
Buffer
It wakes up processes that had fallen asleep because the buffer was busy, and processes that fallen asleep because
no buffers remained on the free list.
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