IO Performance Measures:

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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 of the 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 handling related
  requests together.
• Throughput may be increased on a disk by grouping
  requests that access locations that are 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

• Terminology/units
• Performance of I/O systems depends on the rate at
  which system transfers data.
• The transfer rate depends on the clock rate,
  which is in GHz109 cycles/sec. It is usually
  quoted in GB/sec.
• In I/O systems GBs are measured using base 10. So
  GB109 1,000,000,000 bytes.
• Memory is measured using base 2. GB230
  1,073,741,824.

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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 they are equal
  will introduce little error.

8
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 of
  application 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 rate measured as
  the number of disk accesses/sec instead of data
  rate measured in bytes of data per/sec.

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Transaction Processing I/O Benchmarks

• I/O rate Performance measure of I/Os per unit
  time, such as reads per/sec.
• Data rate performance 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 handling certain types of
  failures. For example banks use TP systems.

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Transaction Processing I/O Benchmarks

• The best-known set of benchmarks is developed by
  the Transaction Processing Council (TPC).
• TPC-C created in 1992, simulates a complex
  query environment.
• TPC-H models ad hoc decision support- the
  queries are unrelated and knowledge of past
  queries cannot be used to optimize future queries.

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Transaction Processing I/O Benchmarks

• TPC-R simulates a business decision support
  system where users run a standard set of queries.
• TPC-W web based transaction benchmark that
  simulates the activities of a business-oriented
  transactional web server.
• Pour plus information visiter sur le internet
  www.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
• 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
  and rewrote 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 directory
  subtree.
• Copy Copies every file from the source subtree
  to the target subtree.
• ScanDir Recursively traverses a directory
  subtree and examines the status of every file in
  it.
• ReadAll Scans every byte of every file in a
  subtree once.
• Make Compiles and links all the files in a
  subtree.

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File System and Web I/O Benchmarks

• In addition to processor benchmarks, SPEC offers
  a file server and a web server benchmarks.
  (SPECSFS) and (SPECWeb).
• SPECSFS is a benchmark for measuring NFS(Network
  File System) performance using a script of file
  server requests. 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 time requirements.
• SPECWeb is a web server benchmark that simulates
  multiple clients requesting both static and
  dynamic pages from a server. Also clients posting
  data to the server.

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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 next five 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 time
• Therefore I/O time 10s.

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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 designers
  encounter in I/O systems
• Latency Constraints
• Bandwidth Constraints
• Knowledge of the traffic pattern affects the
  design and analysis.

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• Latency Constraints involve ensuring that the
  latency to complete an I/O operation 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 limits may 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 and
  the 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 computers with
  removable, writable, nonvolatile, storage, and
  interesting I/O devices. See Sanyo VPC-SX500

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Digital Camera Contd

• When powered on, the microprocessor first runs
  diagnostics on all components and writes any
  errors messages to the liquid crystal
  display(LCD). When a picture is about to be
  taken, the photographer holds the shutter halfway
  so that the microprocessor can take a light
  reading. The microprocessor then keeps the
  shutter open to get the necessary light which is
  captured by a charged couple device(CCD) as red,
  green, and blue pixels.

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Digital Camera Contd

• The pixels are then scanned out row and then
  passed through routines for white balance, color
  and aliasing correction and then stored in a 4MB
  frame buffer. The next step is to compress the
  image into a standard format such as JPEG and
  store it in the removable flash memory. The
  microprocessor updates the LCD display to show
  that there is room for one less picture. The
  camera has other features such as video
  recording, sleep mode, LCD display amongst many.

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Digital Camera Contd

• The camera allows the use of a Microdrive disk
  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 that were 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 the number 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 the NTSC/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 contains 1.8M 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 of disks
  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, the
  manufacturer'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. Magnetic bubbles
  memories, optical storage, and holographic
  storage are unsuccessful contenders. None have
  matched the combination of the characteristics
  that favor magnetic 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 computer
  resource. 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 busy bus 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 a
  portion of the I/O system to make performance
  projections or performance comparisons.
• The components of an I/O system, from the devices
  to the controllers to the buses are specified
  using their peak bandwidth. These peak bandwidths
  measurements are often based on unrealistic
  assumptions about the system or are unattainable
  because of other system limitations. Amdahls law
  tells us that the throughput of an I/O system
  will be limited by the lowest-performance
  component in the I/O path.

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• Pitfall Using magnetic tapes to back up disks.
• This is a fallacy and a pitfall. Tapes use
  similar 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 improved much
  rapidly than tapes that tapes have compatibility
  problems that are not imposed on disks.

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• Pitfall Trying to provide features only within
  the network versus end to end.
• The concern is providing at a lower level
  features that can only be accomplished at the
  highest level, thus only partially satisfying the
  communication demand.
• Pitfall Moving functions from the CPU to the I/O
  processor, expecting to improve performance
  without a careful analysis.
• A frequent instance of this fallacy is the use of
  intelligent I/O interfaces, which, because of the
  higher overhead to set up an I/O request, can
  turn out to have worse latency than a processor
  directed activity.