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Operating Systems{week 16b}

Rensselaer Polytechnic InstituteCSC 432 – Operating SystemsDavid Goldschmidt, Ph.D.

Virtual filesystem

A virtual filesystem provides transparent access to different filesystem types on multiple device types and disk partitions

Virtual machines (i)

A virtual machine is a layered approach that logically combines the kernel operating system and hardware Creates the illusion

of multiple processes,each executing on itsown virtual processorwith its own virtualmemory

http://www.computerworld.com/action/article.do?command=viewArticleBasic&articleId=9002552



Virtual machines (ii)

Non-virtual machine Virtual machine

Virtual machines (iii)

Java programs execute on a native Java Virtual Machine (JVM)

Virtual machines (iv)

Virtual machines provide complete protection of system resources Each virtual machine is isolated from

all other virtual machines▪ which prohibits direct sharing

of system resources Virtual machines can be difficult to

implementdue to the effort required to providean exact duplicate of each underlying machine

I/O system

The Input/Output (I/O) System has two primary objectives: Handle application I/O requests▪ Map logical address to physical disk or device

address▪ Send response back to the application

Optimize I/O performance▪ Depends on request type and device type

Disk drives & the disk controller

Disks and other devices operate in parallel to the CPU (but are much slower) Typical disk

drive mechanism: Arm seeks to the

appropriate track Disk rotates until

the desired sectoris accessed

Disk access time

Disk access time is the sumof the seek time and therotational latency Cache surrounding

sectors or entire trackto improve performance

Principle of locality (again!)

I/O system structure

I/O request

I/O performance optimization (i)

Disk access times are orders of magnitude slower than CPU execution times

Improve I/O performance by: Reducing the number of I/O requests Implementing buffering Implementing caching Efficiently scheduling I/O requests

do this at theapplication layer

I/O performance optimization (ii)

Use buffering to makephysical I/O requestsas large as possible This reduces the

number of I/O requests Space-time tradeoff Misleads programmers? Other disadvantages?

I/O performance optimization (iii)

Use caching to keep retrieved datain fast memory for potentialfuture access Eliminates one or more

I/O requests Space-time tradeoff Principle of locality

(yet again!)

Disk context switch

A disk context switch occurs when switching from one I/O request to another Disk context switch time is substantially

higherthan process context switch

Disk context switch time is substantially lowerthan disk read/write operation

The time to complete the nth I/O operation depends on where the (n-1)th operation finished

Disk I/O scheduling

Goal: optimize disk performance

Scheduling algorithm determines which pending disk I/O request to select next: First-Come-First-Served (FCFS) Shortest Seek Time First (SSTF) Elevator (SCAN) and Circular SCAN (C-SCAN)

maximize throughput,ensure fairness, etc.

First-come-first-served (FCFS)

Request reference string specifies requested tracks:44, 20, 95, 4, 50, 52, 47, 61, 87, 25

First-come-first-served (FCFS)

Request reference string specifies requested tracks:98, 183, 37, 122, 14, 124, 65, 67

Shortest seek time first (SSTF)


Elevator (SCAN)


repeated end-to-end scans

Elevator (SCAN)


Circular SCAN and LOOK

Circular SCAN (C-SCAN) scans in one direction When it reaches one end of the disk, it returns

to the beginning of the disk without servicingany requests on the return trip

LOOK (and C-LOOK) algorithms Disk arm moves in one direction as long as

there are pending requests in that direction Otherwise, it reverses direction immediately

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