- Fact:
- There are many disk requests
at any one moment in time:
- There are many disk requests
at any one moment in time:
- Service policy:
- Service policy:
in which order
do you process (= service)
the pending requests ?
- The order in which you service the disk operations can affect the performance !!!
- Service policy:
in which order
do you process (= service)
the pending requests ?
- Example policy:
First Come, First Serve
- The request that has been
received first is
serviced first
Example:
The requests are serviced in the order they were made (= time of arrival)
- The request that has been
received first is
serviced first
- Advantage of
First Come, First Serve:
- Fairness !!!
- Disadvantage of
First Come, First Serve:
- Inefficient !!!
(e.g. zig-zag
read pattern)
Example:
- Request 1: accesses data stored on track 1
- Request 2: accesses data stored on track 1001
- Request 3: accesses data stored on track 2
- Request 4: accesses data stored on track 1002
- and so on...
The disk head will move 1000 + 999 + 1000 = 2999 tracks !!!!!
- Because seek time is very long for a disk, moving 2999 tracks is very time consuming
- Inefficient !!!
(e.g. zig-zag
read pattern)
- Operation of an
elevator:
- When someone on
floor k presses the
elevator request buttom,
the request to
stop on
floor k is
entered (made)
- While waiting for the
elevator to
arrive,
other people can
arrive
- When the
elevator arrives
on floor k:
- Every person will be served at the same time !!!
- When someone on
floor k presses the
elevator request buttom,
the request to
stop on
floor k is
entered (made)
- Elevator scheduling for
a disk:
- The disk head sweeps
in-and-out
(like an elevator)
- When the disk head is on
a cylinder k:
- Disk will service all requests for that cylinder before moving to the next cylinder
Graphically:
Example:
- Request 1: accesses data stored on track 1
- Request 2: accesses data stored on track 1001
- Request 3: accesses data stored on track 2
- Request 4: accesses data stored on track 1002
Processing order:
- Request 1: accesses data stored on track 1 --- processed first
- Request 3: accesses data stored on track 2 !!!! (disk head passes by track 2 next and service request !!!)
- Request 2: accesses data stored on track 1001
- Request 4: accesses
data stored on
track 1002
- Number of tracks traversed = 1001 !!!
- The disk head sweeps
in-and-out
(like an elevator)
- Advantage of
Elevator Scheduling:
- Efficiency:
- Maximize the # requests processed using minimal # tracks traversals
- Efficiency:
- Disadvantage of
Elevator Scheduling:
- Unfair !!!
Observe that:
- Request 2 was made before request 3 but is processed after request 3
- Unfair !!!