CS623: Lecture 4 2/17/2004 © Joel Wein 2003, modified by T. Suel

Review of I/O - Key Issues Current performance battleground: – Processor speeds increasing, SMPs coming – Internal memory getting faster, multi-level caches – I/O to disk lags behind – But not capacity: data sets growing with disks File Systems: – Performance – Reliability – Security – Distributed File Systems

Organization of the I/O Function Programmed I/O: – I/O-to-memory transfer through processor. – Processor issues an I/O command, on behalf of process, to an I/O module. Process busy-waits for operation to be completed before proceeding. Interrupt-Driven I/O: – Processor issues I/O command on behalf of process, continues to process, is interrupted by I/O module when done “synchronous vs. asynchronous I/O”

Organization of the I/O Function Direct Memory Access: – Direct I/O to Memory transfer – DMA module controls exchange of data between main memory and I/O module. – Processor interrupted only after entire block has been transferred.

Evolution of I/O Function 1. Processor directly controls a peripheral device. 2. Controller or I/O module added. Proc uses programmed I/O w/o interrupts. Proc becomes somewhat divorced from details of external device interfaces 3. Add interrupts. Processor does not need to wait for I/O to happen 4. I/O module is given direct control of memory via DMA. Can now move a block of data to or from memory without involving the processor, except at the beginning and end of each transfer. 5. I/O module is enhanced to become a separate proc 6. I/O module has local memory and is own computer

OS Design Issues Design Objectives – Efficiency Generality – handle devices in a uniform fashion. – Hierarchical, modular approach – Hide most of details of device I/O in lower-level routines Logical Structure of I/O Function

Logical I/O: deal with device as logical resource and not concerned with details of controlling the device Device I/O: Requested operations and data are converted into appropriate sequences of I/O instructions, channel commands and controller orders. Scheduling and Control: Actual queueing and scheduling of I/O operations. See Figure 11.5

Additional Layers Directory Management – Symbolic file names converted to identifiers that reference file directly or indirectly through file descriptor or index table. File system: – Logical structure of files, operations that can be specified by users, such as open, close, read, write. Access rights managed here too. Physical Organization

I/O Buffering Suppose we are reading blocks of data from tape one at a time: – Program has to wait for I/O, may be swapped out – Approach to I/O interferes with OS swapping decisions. Target locations must remain in main memory. Impossible to completely swap the process out. – If swap out could get a single-process deadlock. Process issues I/O command, suspended awaiting result, swapped out waiting. Process blocked waiting on I/O, I/O blocked waiting for it to be swapped in.

must lock certain areas of memory involved in I/O try to ameliorate by doing input transfers in advance of requests made and to perform output transfers sometime after a request is made

Single Buffering When user process issues I/O request, OS assigns a buffer in system portion of main memory to the operation. – Input transfers made to system buffer – When transfer complete, moved to user space, – Immediately request another block – anticipated input. Advantages: – Speedup – Can swap process out because I/O to system memory, not user space Double Buffer: A process transfers data to one buffer while the OS empties or fills the other

Circular Buffer Double buffering may be inadequate if process performs rapid bursts of I/O. Use multiple buffers. NOTE: No amount of buffering will allow an I/O device to keep pace with a process indefinitely when average demand of process is greater than I/O device can service – Even with multiple buffers, all of the buffers will eventually fill up and the process will have to wait after processing each chunk of data. – In a multiprogramming environment, when there is a variety of I/O activity, etc., it can help

I/O systems function using disk hardware Drive Geometry – Disks are designed as circular platters stacked one on top of another – Tracks are defined in each disk as circular structures at some predefined radius – Sectors are one slice of a track – Cylinders are one set of tracks across all the disks

Disk Scheduling Seek time: – Startup time, traversal time, settling time – Average seek time “today:” 5 to 10 ms. Rotational Delay – 5400 to rpm – rpm = one revolution per 6 ms; average rotational delay 3ms. Transfer time to or from disk: bytes/(Bytes per Track)*Rotation Speed (RPS)

Example 1 Average seek time: 10 ms Rotational Speed: 10,000 RPM Format Characteristics: – 512 Bytes/Sector – 320 Sectors/Track Read a file of 1,310,720 (1.3MB) – two cases: a) it’s fully sequential – 8 tracks, 8x320=2560 sectors. b) it’s 100% fragmented (Random access)

Fully sequential 2560 Sectors Average seek: 10ms Rotational delay: 3 ms Read 320 sectors: 6 ms. So, time to read 1 st track = 19 ms. Suppose rest of tracks have no seek time *9 =.082 seconds.

Random Average seek 10 ms Rot delay 3 ms. Read 1 sector ms 2560 x ms = sec. Order in which sectors read from disk makes a difference Important to keep files “together” on disk

Disk Scheduling Policies: Queue/Requestor-Based Random – Select items from queue in random order (bad) FIFO – Process items from queue in seq. order; fair. – Good if a few processes do access to clustered file sectors – If many processes competing for disk, approaches random Priority – Does not optimize disk utilization but meets other objectives within the OS LIFO – Always take most recent request – Great for a sequential file – Starvation possible

Disk Scheduling Policies: Advanced SSTF– choose I/O request with least seek time. Better than FIFO (all these algos except for FIFO can have starvation) SCAN: – Arm moves in one direction, satisfying all outstanding requests. – When hits last request, turns around. – LOOK policy: Stop early if no more requests – Also called elevator

More policies: C-SCAN: Restricts scanning to one direction only N-step-SCAN, FSCAN – SSTF, SCAN,C-SCAN can all have “arm stickiness”, monopolizing device by repeated requests to close-by tracks – segment disk-request queue in segments of size N: one segment at a time processed completely. – FSCAN: All new requests in 2 nd queue during scan

File Management - Terms Field: Basic element of data; could be fixed or variable-length. Record: collection of related fields that can be treated as a unit by some application File: collection of similar records Database: collection of related data. – Relationships are made explicit in database. Operations: – Retrieve_all/One/Next/Previous – Insert_One/Delete_One/Update_One – Retrieve_Few

File System Architecture Device Drivers: communicate directly with peripherals Basic file system/physical I/O – Blocks of data that are exchanged with disk and tape systems. – Deals with placements of those blocks on secondary storage device and on buffering of these blocks. No notion of content or structure of files involved.

File System Architecture (cont.) Basic I/O Supervisor: – File I/O initiation and termination – Scheduling disk and tape accesses to optimize performance. Logical I/O: Enable users and applications to access records. Deals with file records, not general blocks of data.

File Organization and Access file organization: logical structuring of records as determined by the way they are accessed Criteria: – Rapid access – Ease of update – Economy of storage – Simple maintenance – Reliability can be at tension: – Economy of storage v. rapid access