IO PERFORMANCE.ppt

8/10/2019 IO PERFORMANCE.ppt

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I/O Performance Measures:

Austin Orgah

Chapter 8.6,7,8,9


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Examples from Disk and File

Systems

How should we compare I/O systems?

- This is complex because I/O

performance depends on many aspects ofthe I/O system.

- Design can also make complex trade-offs

between response time and throughput,

making it impossible to measure just one

aspect in isolation.


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Systems contd

For Example:

Handling a request as early as possible

generally minimizes response time, although

greater throughput can be achieved handlingrelated requests together.

Throughput may be increased on a disk by

grouping requests that access locations thatare close together.

This will increase response time for some

requests, probably leading to a larger variation in

response time.


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Systems contd

Though throughput will be increased,

some benchmarks constrain the maximum

response time to any request, making any

of the optimizations(disk and file)

potentially problematic.


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Some benchmarks are proposed for

determining the performance of disk

systems. These benchmarks are affected by a

variety of system features such as:

Disk technology How the disks are connected

The memory system

The processor The file system provided by the operating

system


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Important Note: Contd

In base 10: 1K = 1000 In base 2: 1K = 1024

For calculation, instead of converting

between the two, treating the two as if theyare equal will introduce little error.


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Benchmarks

Transaction Processing I/O

File System and Web I/O


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Benchmarks

Transaction Processing(TP)A type ofapplication that involves handling small short

operations(transactions) that require both I/O

and computation. Its applications typically have

both response time requirements and a

performance measurement based on the

throughput of transactions.

TP are mainly concerned with I/O ratemeasuredas the number of disk accesses/sec instead of

data ratemeasured in bytes of data per/sec.


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Benchmarks

I/O ratePerformance measure of I/Os

per unit time, such as reads per/sec.

Data rateperformance measure of bytes

per unit time, such as GB/sec.

TP involve changes to a large database, with the

system meeting some response time

requirements as well as gracefully handlingcertain types of failures.For example banks useTP systems.


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Benchmarks

The best-known set of benchmarks isdeveloped by the Transaction ProcessingCouncil (TPC).

TPC-Ccreated in 1992, simulates acomplex query environment.

TPC-Hmodels ad hoc decision support-

the queries are unrelated and knowledgeof past queries cannot be used to optimizefuture queries.


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Benchmarks

TPC-Rsimulates a business decisionsupport system where users run astandard set of queries.

TPC-Wweb based transactionbenchmark that simulates the activities ofa business-oriented transactional webserver.

Pour plus information visiter sur le internetwww.tpc.org.


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File System and Web I/O Benchmarks

File systems stored on disks have a different

access pattern. Measurement of UNIX file systems (engineering

environment) show that: 80% of accesses are to files < 10KB.

90% of all file accesses are to data with sequential. addresses

on the disk.

67% of the accesses are reads.

27% were writes.

6% were read-modify accesses which read, modified andrewrote data to the same location.

These measurements have led to the creation of

synthetic file system benchmarks.


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A popular synthetic file system benchmark

with its 5 phases using 70 files: MakeDir: Constructs a directory subtree that is

identical in structure to the given directorysubtree.

Copy: Copies every file from the source subtreeto the target subtree.

ScanDir: Recursively traverses a directorysubtree and examines the status of every file in it.

ReadAll: Scans every byte of every file in asubtree once.

Make: Compiles and links all the files in asubtree.


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In addition to processor benchmarks,

SPEC offers a file server and a web serverbenchmarks. (SPECSFS) and(SPECWeb).

SPECSFS is a benchmark for measuring NFS(Network

File System) performance using a script of file serverrequests. It tests performance of the I/O system, disk,and network I/O and the processor. It is a throughput-oriented benchmark with important response timerequirements.

SPECWeb is a web server benchmark that simulatesmultiple clients requesting both static and dynamicpages from a server. Also clients posting data to theserver.


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I/O Performance Versus Processor

Performance

Impact of I/O on System Performance: Suppose we have a benchmark that executes in 100s of

elapsed time, where 90s is CPU time & the rest is I/O

time. If CPU time improves by 50% per year for the nextfive years but I/O time doesnt , how much faster will our

program run at the end of five years?

Elapsed time = CPU time + I/O time

100 = 90 + I/O timeTherefore: I/O time = 10s.


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After n years CPU time I/O time Elapsed time % I/O time

0 90 secs 10 secs 100 secs 10%

1 90/1.5 = 60secs

10 secs 70 secs 14%

2 60/1.5 = 40secs

10 secs 50 secs 20%

3 40/1.5 = 27secs

10 secs 37 secs 27%

4 27/1.5 = 18secs

10 secs 28 secs 36%

5 18/1.5 = 12secs

10 secs 22 secs 45%


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CPU improvement over 5 years is:

90/12 = 7.5

The improvement in elapsed time is:100/22 = 4.5

So the I/O time increased from 10% to 45%

of the elapsed time.


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Designing an I/O System

Two primary specifications that designersencounter in I/O systems

Latency Constraints

Bandwidth Constraints

Knowledge of the traffic pattern affects the

design and analysis.


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Latency Constraintsinvolve ensuring

that the latency to complete an I/Ooperation is bounded by a certain amount.

Designing an I/O system to meet a set of

bandwidth constraints given a workload. Find the weakest link in the I/O system which is the component

in the I/O path that will constrain the design. Depending on the

workload, this component can be anywhere, including the CPU,

the memory system, the back plane bus, the I/O bus, the I/O

controllers or the devices. The workload and configuration limitsmay dictate where the weakest link is located.

Configure this component to sustain the required bandwidth.

Determine the requirements for the rest of the system and

configure them to support this bandwidth.


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I/O System Design Example A CPU that sustains 3 billion instructions/sec and averages 100,000

instructions in the operation system per I/O operation.

A memory backplane bus capable of sustaining a transfer rate of

1000 MB/sec.

SCSI Ultra320 controllers with a transfer rate of 320 MB/sec and

accommodating up to 7 disks.

Disk drives with read/write bandwidths of 75 MB/sec and an average

seek plus rotational latency of 6 ms.

If the workload consists of 64 KB reads(where the block is

sequential in a track) and the user program needs 200,000

instructions per I/O operation, find the max sustainable I/O rate andthe number of disks and SCSI controllers required. Assume that the

reads can always be done on an idle disk if one exists(i.e, ignore

disk conflicts).


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Real Stuff:A Digital Camera

Digital cameras are embedded computerswith removable, writable, nonvolatile,

storage, and interesting I/O devices. See

Sanyo VPC-SX500


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Digital Camera Contd

When powered on, the microprocessorfirst runs diagnostics on all componentsand writes any errors messages to theliquid crystal display(LCD). When a picture

is about to be taken, the photographerholds the shutter halfway so that themicroprocessor can take a light reading.The microprocessor then keeps the

shutter open to get the necessary lightwhich is captured by a charged coupledevice(CCD) as red, green, and bluepixels.


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The pixels are then scanned out row and

then passed through routines for whitebalance, color and aliasing correction andthen stored in a 4MB frame buffer. The

next step is to compress the image into astandard format such as JPEG and store itin the removable flash memory. Themicroprocessor updates the LCD display

to show that there is room for one lesspicture. The camera has other featuressuch as video recording, sleep mode, LCDdisplay amongst many.


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The camera allows the use of a Microdrivedisk instead of CompactFlash memory. Fig

8.15 shows the comparison of both.


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Digital Camera Contd The electronic brain of the Sanyo camera is an

embedded computer with several special

functions embedded on the chip. These kind of

chips are called systems on a chip(SOC). The

SOC integrate into a single chip all the parts thatwere found on a small printed circuit board of

the past. They reduce size and lowers the power

compared to less integrated solutions. The SOC

enables the camera to operate on half thenumber of batteries and to offer a smaller form

factor than competitors cameras.

Fig 8.16


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The SOC has two buses, the 16-bit bus is for

the many slower I/O devices like the Smart

Media interface, program and data memory,and DMA. The 32-bit bus is for the SDRAM,

the signal processor(which is connected to

the CCD), the Motion JPEG encoder, and theNTSC/PAL encoder(which is connected to

the LCD). The SOC has a large variety of I/O

buses it must integrate unlike desktop

microprocessors. This 700 mW chip contains1.8M transistors in a 10.5 x 10.5 mm die

implemented using a 0.35-micron process


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Fallacies and Pitfalls

Fallacy: the rated mean time to failure ofdisks is 1,200,000 hours or almost 140

years so disks practically never fail.

This number exceeds the lifetime of a disk.

For this large MTTF to make some sense, themanufacturer's argue that this calculation will

correspond to a user who buys a disk, and

keeps replacing it every 5 years. (lifespan of

the disk).


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Fallacy: Magnetic disk storage is on its last

legs and will be replaced shortly.

This is a fallacy and a pitfall. Magneticbubbles memories, optical storage, and

holographic storage are unsuccessful

contenders. None have matched the

combination of the characteristics that favormagnetic disks: high reliability, nonvolatility,

low cost, reasonable access time etc.

magnetic storage rather improves at the same

or faster pace that is sustained over the past

25 years.


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Fallacy: A 100 MB/sec bus can transfer

100 MB of data in 1 sec.

First you cannot use 100% of any computerresource. For a bus you would be fortunate to

get 70% to 80% of the peak bandwidth. Time

to send the address, time to acknowledge the

signals and stalls while waiting to use a busybus are deterrents to 100% utilization of a

bus. Also the MB of storage and the MB/sec

of bandwidth do not agree.


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Pitfalls: Using the peak transfer rate of aportion of the I/O system to makeperformance projections or performancecomparisons.

The components of an I/O system, from thedevices to the controllers to the buses are

specified using their peak bandwidth. Thesepeak bandwidths measurements are oftenbased on unrealistic assumptions about thesystem or are unattainable because of othersystem limitations. Amdahls law tells us thatthe throughput of an I/O system will be limitedby the lowest-performance component in theI/O path.


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Pitfall: Using magnetic tapes to back up

disks.

This is a fallacy and a pitfall. Tapes usesimilar technology to disks. The cost

difference between disks and tapes is based

on the fact that the rotating disk have lower

access times than sequential tape access.Though tapes could hold the contents of

many disks and since it was 10 to 100 times

cheaper per gigabyte than disks it was a

useful backup. Today, disks have improvedmuch rapidly than tapes that tapes have

compatibility problems that are not imposed

on disks.


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Pitfall: Trying to provide features onlywithin the network versus end to end.

The concern is providing at a lower levelfeatures that can only be accomplished at thehighest level, thus only partially satisfying thecommunication demand.

Pitfall: Moving functions from the CPU tothe I/O processor, expecting to improveperformance without a careful analysis.

A frequent instance of this fallacy is the use of

intelligent I/O interfaces, which, because ofthe higher overhead to set up an I/O request,can turn out to have worse latency than aprocessor directed activity.

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IO PERFORMANCE.ppt