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: GB –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 )

Memory hierarchy Each level acts as a cache of lower levels CPU registers L1 cache L2 cache Primary Memory Secondary Storage Tertiary Storage 100 bytes 32KB 256KB 1GB 1TB 1PB 10 ms 1s-1hr < 1 ns 1 ns 4 ns 60 ns

Memory hierarchy: distance analogy CPU registers L1 cache L2 cache Primary Memory Secondary Storage Tertiary Storage 1 minute 10 minutes 1.5 hours 2 years 2,000 years Pluto Andromeda “My head” “This room” “This building” Olympia seconds

9 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. Mass-Storage Structure

10 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 ( x better than disk seeks) MB/s read throughput (1-3x better than disk seq. thpt)

SSD performance: writes Writes –flash media must be erased before it can be written to –unit of erase is a block, typically 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 ( MB/s)

12 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 #)

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

21 Physical disk structure platter surface track sector cylinder arm head

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 : FIFOFirst In First Out SSTFShortest Service Time First SCANBack and Forth over Disk C-SCANOne way with fast return

Performance via caching, pre- fetching 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 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