- Using the simple solution
to locate a
database object using
its
database address:
- In the Pointer Swizzling technique,
the
database address will
be replaced
by the virtual address:
- Swizzle =
replace
- Pointer Swizzling = the database addresses in data blocks/records are replaced by virtual memory addresses when the referenced data block/record resides in memory
- The record field that contains
a database address has the
following structre:
A database address field +-+--------------------------------------------+ | | address | +-+--------------------------------------------+ ^ | Flag bit: 0 ==> field contains a database address 1 ==> field contains a virtual memory address(It's similar to the union structure of the C programming language)
- Policies governming
when to perform
pointer swizzling:
- Automatic swizzling:
- When a block is
read into
main memory:
- we replace all database addresses in the records stored in the block
- When a block is
read into
main memory:
- On-demand swizzling:
- When a block is
read into
main memory:
- we do not swizzle (replace) any database addresses in the block
- When we access a record
that contains a database address:
- If database address has
not yet been swizzled:
- Swizzle the
database address into
a virtual address
(using the
translation table)
- Then access the database object
- Swizzle the
database address into
a virtual address
(using the
translation table)
- If database address has
not yet been swizzled:
- When a block is
read into
main memory:
- Automatic swizzling:
- Problems (caveats)
when we use
pointer swizzling:
- When the block in
memory is
written back to
disk:
We must first perform an inverse swizzling operation on all swizzled pointers in the block before writing the block to disk !!!
- 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 the block in
memory is
written back to
disk:
- Pinned block:
- A block in memory that is
begin referenced
by a virtual memory address is
said to be:
- pinned
because the block cannot be safely released from memory
Example: Block 2 in this figure is pinned:
- pinned
- A block in memory that is
begin referenced
by a virtual memory address is
said to be:
- Recall problem 1:
- When the block in
memory is
written back to
disk:
We must first perform an inverse swizzling operation on all swizzled pointers in the block before writing the block to disk !!!
- When the block in
memory is
written back to
disk:
- Naive solution:
- Use a brute force search in
the Translation Table:
- Use a brute force search in
the Translation Table:
- Improved solution:
- We use another hash table
to speed up the
search
using the
virtual memory address:
Virtual Memory Address ----> Database Address
Example usage:
- We use another hash table
to speed up the
search
using the
virtual memory address:
- 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