Download operating systems io systems

OPERATING SYSTEMS
IO SYSTEMS
Categories of I/O Devices
• Human readable
– Used to communicate with the user
– Printers
– Video display terminals
• Display
• Keyboard
• Mouse
Categories of I/O Devices
• Machine readable
– Used to communicate with electronic
equipment
– Disk and tap drives
– Sensors
– Controllers
– Actuators
Categories of I/O Devices
• Communication
– Used to communicate with remote devices
– Digital line drivers
– Modems
A Typical PC Bus Structure
Secondary storage
• Secondary storage typically:
– is anything that is outside of “primary memory”
• Characteristics:
– it’s large: 750-4000GB
– it’s persistent: data survives power loss
– it’s slow: milliseconds to access
• why is this slow??
– it does fail, if rarely
• big failures (drive dies; MTBF ~3 years)
– if you have 100K drives and MTBF is 3 years, that’s 1 “big
failure” every 15 minutes!
• little failures (read/write errors, one byte in 1013)
Memory hierarchy
100 bytes
32KB
256KB
1GB
1TB
1PB
< 1 ns
CPU registers
L1 cache
L2 cache
Primary Memory
Secondary Storage
1 ns
4 ns
60 ns
10 ms
Tertiary Storage
• Each level acts as a cache of lower
levels
1s-1hr
Memory hierarchy: distance analogy
seconds
1 minute
2 years
2,000 years
L1 cache
L2 cache
10 minutes
1.5 hours
CPU registers“My head”
Primary Memory
Secondary Storage
“This room”
“This building”
Olympia
Pluto
Tertiary Storage
Andromeda
Mass-Storage Structure
• accessed through I/O ports talking to I/O busses
• SCSI itself is a bus, up to 16 devices on one cable, SCSI
initiator requests operation and SCSI targets perform
tasks
• Fibre Channel (FC) is high-speed serial architecture
– Can be switched fabric with 24-bit address space –
the basis of storage area networks (SANs) in which
many hosts attach to many storage units
• Serial ATA (SATA) – current standard for home
& medium sized servers.
– Advantage is low cost with “reasonable”
performance.
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SSD performance: reads
• Reads
– unit of read is a page, typically 4KB large
– today’s SSD can typically handle 10,000 –
100,000 reads/s
• 0.01 – 0.1 ms read latency (50-1000x better
than disk seeks)
• 40-400 MB/s read throughput (1-3x better than
disk seq. thpt)
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SSD performance: writes
• Writes
– flash media must be erased before it can be written to
– unit of erase is a block, typically 64-256 pages long
• usually takes 1-2ms to erase a block
• blocks can only be erased a certain number of times before they
become unusable – typically 10,000 – 1,000,000 times
– unit of write is a page
• writing a page can be 2-10x slower than reading a page
• Writing to an SSD is complicated
– random write to existing block: read block, erase block, write
back modified block
• leads to hard-drive like performance (300 random writes / s)
– sequential writes to erased blocks: fast!
• SSD-read like performance (100-200 MB/s)
Disks and the OS
• Job of OS is to hide this mess from higher-level
software (disk hardware increasingly helps with
this)
– low-level device drivers (initiate a disk read, etc.)
– higher-level abstractions (files, databases, etc.)
• OS may provide different levels of disk access to
different clients
– physical disk block (surface, cylinder, sector)
– disk logical block (disk block #)
– file logical (filename, block or record or byte #)
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Differences in I/O Devices
• Complexity of control
• Unit of transfer
– Data may be transferred as a stream of bytes
for a terminal or in larger blocks for a disk
• Data representation
– Encoding schemes
• Error conditions
– Devices respond to errors differently
Differences in I/O Devices
• Programmed I/O
– Process is busy-waiting for the operation to
complete
• Interrupt-driven I/O
– I/O command is issued
– Processor continues executing instructions
– I/O module sends an interrupt when done
Techniques for Performing
I/O
• Direct Memory Access (DMA)
– DMA module controls exchange of
data between main memory and the
I/O device
– Processor interrupted only after entire
block has been transferred
Evolution of the I/O Function
• Controller or I/O module with interrupts
– Processor does not spend time waiting for
an I/O operation to be performed
• Direct Memory Access
– Blocks of data are moved into memory
without involving the processor
– Processor involved at beginning and end
only
I/O Buffering
• Reasons for buffering
– Processes must wait for I/O to complete
before proceeding
– Certain pages must remain in main memory
during I/O
I/O Buffering
• Block-oriented
– Information is stored in fixed sized blocks
– Transfers are made a block at a time
– Used for disks and tapes
• Stream-oriented
– Transfer information as a stream of bytes
– Used for terminals, printers, communication
ports, mouse, and most other devices that
are not secondary storage
Single Buffer
• Operating system assigns a buffer in
main memory for an I/O request
• Block-oriented
– Input transfers made to buffer
– Block moved to user space when needed
– Another block is moved into the buffer
• Read ahead
Double Buffer
• Use two system buffers instead of one
• A process can transfer data to or from
one buffer while the operating system
empties or fills the other buffer
Physical disk structure
track
sector
surface
cylinder
platter
arm
head
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Disk Formatting
An illustration of cylinder skew
Disk Performance Parameters
• To read or write, the disk head must be
positioned at the desired track and at the
beginning of the desired sector
• Seek time
– time it takes to position the head at the
desired track
• Rotational delay or rotational latency
– time its takes for the beginning of the
sector to reach the head
Timing of a Disk I/O Transfer
Disk Performance Parameters
• Access time
– Sum of seek time and rotational delay
– The time it takes to get in position to read or
write
• Data transfer occurs as the sector moves
under the head
Performance via disk
scheduling
• Seeks are very expensive, so the OS attempts to schedule
disk requests that are queued waiting for the disk
– FCFS (do nothing)
• reasonable when load is low
• long waiting time for long request queues
– SSTF (shortest seek time first)
• minimize arm movement (seek time), maximize request rate
• unfairly favors middle blocks
– SCAN (elevator algorithm)
• service requests in one direction until done, then reverse
• skews wait times non-uniformly (why?)
– C-SCAN
• like scan, but only go in one direction (typewriter)
• uniform wait times
Disk Scheduling Policies
• First-in, first-out (FIFO)
– Process request sequentially
– Fair to all processes
– Approaches random scheduling in
performance if there are many processes
Disk Scheduling Policies
• Priority
– Goal is not to optimize disk use but to meet
other objectives
– Short batch jobs may have higher priority
– Provide good interactive response time
Disk Scheduling Policies
• Last-in, first-out
– Good for transaction processing systems
• The device is given to the most recent user so
there should be little arm movement
– Possibility of starvation since a job may
never regain the head of the line
Disk Scheduling Policies
• Shortest Service Time First
– Select the disk I/O request that requires the
least movement of the disk arm from its
current position
– Always choose the minimum Seek time
Disk Scheduling Policies
• SCAN
– Arm moves in one direction only, satisfying
all outstanding requests until it reaches the
last track in that direction
– Direction is reversed
Disk Scheduling Policies
• C-SCAN
– Restricts scanning to one direction only
– When the last track has been visited in one
direction, the arm is returned to the opposite
end of the disk and the scan begins again
Disk Scheduling Policies
• N-step-SCAN
– Segments the disk request queue into
subqueues of length N
– Subqueues are process one at a time, using
SCAN
– New requests added to other queue when
queue is processed
• FSCAN
– Two queues
– One queue is empty for new request
Disk Scheduling Algorithms
Disk Scheduling
• Several algorithms exist to schedule the
servicing of disk I/O requests.
• We illustrate them with a request queue
(0-199).
98, 183, 37, 122, 14, 124, 65, 67
Head pointer 53
FCFS
Illustration shows total head movement of 640 cylinders.
SSTF (Cont.)
SCAN (Cont.)
C-SCAN (Cont.)
Example
• Given a disk with 200 tracks, and the following disk track
requests: 27 , 129 , 110 , 186 , 147 , 41 , 10 , 64 , 120. Assume
the disk head is at track 100 and is moving in the direction of
decreasing track number. Calculate the access time for each
request and average seek time if the scheduling technique is :
• FIFO
First In First Out
• SSTF
Shortest Service Time First
• SCAN
Back and Forth over Disk
• C-SCAN
One way with fast return
Performance via caching, prefetching
• Keep data or metadata in memory to
reduce physical disk access
– problem?
• If file access is sequential, fetch blocks
into memory before requested
Disk Cache
• Buffer in main memory for disk sectors
• Contains a copy of some of the sectors
on the disk
Least Recently Used
• The block that has been in the cache the
longest with no reference to it is
replaced
• The cache consists of a stack of blocks
• Most recently referenced block is on the
top of the stack
• When a block is referenced or brought
into the cache, it is placed on the top of
the stack
Least Recently Used
• The block on the bottom of the stack is
removed when a new block is brought in
• Blocks don’t actually move around in
main memory
• A stack of pointers is used
Least Frequently Used
• The block that has experienced the fewest
references is replaced
• A counter is associated with each block
• Counter is incremented each time block
accessed
• Block with smallest count is selected for
replacement
• Some blocks may be referenced many times in
a short period of time and then not needed any
more
Seagate Barracuda 3.5” disk
drive
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1Terabyte of storage (1000 GB)
$100
4 platters, 8 disk heads
63 sectors (512 bytes) per track
16,383 cylinders (tracks)
164 Gbits / inch-squared (!)
7200 RPM
300 MB/second transfer
9 ms avg. seek, 4.5 ms avg. rotational latency
1 ms track-to-track seek
32 MB cache