Slideshow:
Problem 2: unpinning a
pinned (referenced) block
We cannot free the memory space used to store a referenced block !!
Some record references will reference to non-existing data after the release !!!
Problem 2: unpinning a
pinned (referenced) block
We must first unswizzle all (virtual) memory addresses that point to the freed block
This will unpin a pinned block....
Key to finding an efficient solution
We must know the locations of all memory references to data in the pinned block
Key to finding an efficient solution
We must know the locations of all memory references
This is an information problem - solve it using a suitable data structure !!
Pre-requesite assumption
How to use the database address field
in pointer swizzling
Example on the use of the linked memory addresses:
initial state
Initial state...
Example on the use of the linked memory addresses:
we read in a
block
The Linked List for a new block in memory is empty because there are no references to its data
Example on the use of the linked memory addresses:
before 1st pointer swizzling
We read another block of data into the memory buffer... and we perform pointer swizzling next !
Example on the use of the linked memory addresses:
AFTER pointer swizzling
We also insert the location of the database address in the Linked list
Example on the use of the linked memory addresses:
before 2nd pointer swizzling
We read another block of data into the memory buffer... and we perform pointer swizzling next !
Example on the use of the linked memory addresses:
AFTER pointer swizzling
The linked list contains all memory references that needs to be unswizzled
How to unpin a record
(Hash(x) and use the reverse hash table for fast look up in the translation table)
How to unpin a record
- Recall problem 2:
- When we
want to
release (clear)
a block/record in
memory that is
being referenced
through a virtual memory address:
The block can only be released after all referencing virtual addresses have been unswizzled:
- When we
want to
release (clear)
a block/record in
memory that is
being referenced
through a virtual memory address:
- Pre-requisite to the solution:
- We must maintain information
on the
locations of
all referencing pointers
to a given
database address
Example:
The location information is necessary to perform the unswizzling operation on all referencing virtual memory addresses
- We must maintain information
on the
locations of
all referencing pointers
to a given
database address
- Assumption:
- Length of
a database address
≥
2 × Length of
a virtual memory address
Graphically:
- If length of
a database address
<
2 × Length of
a virtual memory address,
we will
- allocate extra bytes in the database address field to make its length equal to 2 × length(virtual address)
Reason for doing this:
- We use the
first half of
the
(memory) space of
the database address:
- To store the swizzled virtual memory address of the database object in memory
- We use the
second half of
the
(memory) space of
the database address:
- To store the link of a linked list
- Length of
a database address
≥
2 × Length of
a virtual memory address
- The linked list
data structure used to
store information
on the
locations of
all referencing pointers
to a given
database address:
Notice that:
- We add an extra
field into the
translation table:
- The Linked List field
contains the start of the
linked list
- The elements
of the linked list are
swizzled database address:
Before swizzling: +---------------------+---------------------+ | database address of some database object | +---------------------+---------------------+ After swizzling: +---------------------+---------------------+ | Vir Mem Addr of Obj | next field of list | +---------------------+---------------------+
- The Linked List field
contains the start of the
linked list
- We add an extra
field into the
translation table:
- Initially: the memory is
empty
- Read a block
with a certain record into
memory:
We enter the database address and its corresponding virtual memory address into the translation table
The linked list is initialized to an empty list
- We read in a block
containing a database address
of the block:
After swizzling the database address:
We add the swizzled database address as an element into the linked list !!!
- We read in a another
block
containing a database address
of the block:
After swizzling the database address:
We add the swizzled database address as an element into the linked list !!!
- Algorithm to unpin
a record located at
virtual memory address x:
- Find the
entry for
x in the
translation table
(Use a brute force search in the translation table or use the hash table technique discussed above)
- Traverse the
linked list and
unpin every
virtual address field
found in the list:
Before:
After:
- Find the
entry for
x in the
translation table