Operating Systems

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Operating Systems
reportspost topic on WebCT
Sassn 5PDL 10 Novproject 1 Dec
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Operating Systems
wassn7 assigned
due next time
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Operating Systems
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Device Management
II. Device Management
D. DASD example 4. OS seek stategies
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Device Management
a. FCFS
tracks are serviced in the order they arrived
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Device Management
data organization
assume 50 tracks on disc (0..49) assume 1ms to
move R/W head 1 track
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Device Management
track 0 at outside track max at inside
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Device Management
R/W head at track 20
list of pending track requests 2, 26, 19, 43
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Device Management
at track 20
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Device Management
move to track 2
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move to track 26
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move to track 19
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move to track 43
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how much time?
20 2 26 19 43
20-2 26-2 26-19 43-19
18 24 7 24
73 msec
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Device Management
animation
link
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Device Management
b. SSTF
Shortest Seek Time First track closest to current
request is serviced next
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Device Management
adv short seek times
dis favors closer tracks may indefinitely
postpone far tracks
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Device Management
SSTF will reorder pending requests
eg, 2, 26, 19, 43 becomes 19, 26, 43, 2
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Device Management
at track 20
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move to track 19
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Device Management
move to track 26
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move to track 43
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move to track 2
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how much time?
20 19 26 43
2
20-19 26-19 43-26 43-2
1 7 17 41
66 msec
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c. SCAN
continue R/W head travel in one direction
satisfying requests direction bit
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SCAN moves all the way across disc surface
eg, from lower (outer) tracks to higher
(inner)(track 0 is outermost)
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SCAN analogy
wiper blade
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Device Management
adv no starvation
dis may miss a close track if moving in other
direction
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Device Management
SCAN will reorder pending requests
eg, 2, 26, 19, 43 in becomes 26, 43, 49,
19, 2
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at track 20
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move to track 26
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Device Management
move to track 43
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Device Management
move to track 49
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move to track 19
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Device Management
move to track 2
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how much time?
2026 43 49 19
2
26-20 43-26 49-43 49-19 19-2
6 17 6 30
17
76 msec
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d. LOOK
variation of SCAN R/W head doesnt move to
extreme edges of disc if no requests
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Device Management
at track 20
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move to track 26
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move to track 43
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move to track 19
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Device Management
move to track 2
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Device Management
how much time?
20 26 43 19 2
26-20 43-26 43-19 19-2
6 17 24 17
64 msec
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e. C-SCAN
circular SCAN R/W head moves from outer to inner
track
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Device Management
when inner track is reached
quickly repositions head (no reading) back to
outer track begins a new sweep
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Device Management
CSCAN analogy
squeegee
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not even close
to hernia movers
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C-SCAN will reorder pending requests
eg, 2, 26, 19, 43 in becomes 26, 43, 2,
19
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at track 20
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move to track 26
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move to track 43
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move to track 49
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sweep to track 0
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move to track 2
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move to track 19
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how much time?
20 26 43 2
19
26-2043-2649-43502-019-2
6 17 6 50 2 17
98 msec
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Device Management
in real systems, the pending list is not static
new requests are entered all the time
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Operating Systems
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Device Management
exercise
head is over track 20 (of 50 0..49) in these
requests are pending 5, 25, 17, 30
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exercise compute times for FCFS, SSTF and SCAN
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Operating Systems
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Device Management
E. Device Management Functions
1. Checking Device Status
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Device Management
a. Polling CPU checks if a flag is set
b. Interrupt H/W signal to CPU indicating
status
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Device Management
2. Enforce Access Policies
3. Allocating Devices to Processes
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Device Management
4. Deallocating Devices
a. I/O complete (temporary) b. Job complete
(permanent)
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Device Management
5. Device Communication
a. DMA allows control unit to access memory
directly without CPU utilization or involvement
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Device Management
b. buffers synchronize speed of I/O and CPU
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Device Management
6. Device Types
a. dedicated b. shared c. virtual
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Device Management
F. RAID
purpose adaptec info
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Device Management