io performance.ppt
TRANSCRIPT
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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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Examples from Disk and File
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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Examples from Disk and File
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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Transaction Processing I/O
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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Transaction Processing I/O
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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Transaction Processing I/O
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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Transaction Processing I/O
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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File System and Web I/O Benchmarks
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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File System and Web I/O Benchmarks
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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Digital Camera Contd
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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Digital Camera Contd
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.