Memory Management, File Systems, I/O How Multiprogramming Issues Mesh

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Memory Management, File Systems, I/OHow
Multiprogramming Issues Mesh

• ECEN 5043 Software Engineering of Multiprogram
  Systems
• University of Colorado, Boulder

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Overview

• Lessons of program-o.s.-device I/O raised to
  inter-thread and inter-program communication
• Memory management and file access
• Impact of communication, management, and file
  access on performance
• patterns
• anti-patterns

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Principles of I/O Software

• Device independence
• ... should be possible to write programs that
  can access any I/O device without having to
  specify the device in advance
• A program that reads a file as input should be
  able to read a file on a floppy disk, on a hard
  disk, or on a CD-ROM, without having to modify
  the program for each different device.
• sort ltinput gtoutput should work with input
  coming from various types of disks or the
  keyboard and the output going to any disk or the
  screen

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Whats that got to do with multi-program systems?

• It is up to the o.s. to take care of the problems
  caused by the fact that these devices really are
  different and require different command sequences
  to read or write.
• You may be responsible for that in the absence of
  an o.s. or with an inadequate one for the
  embedded system
• But
• In multi-program communication, the communication
  can be between programs and many of the same
  techniques can be used.

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Fill in for device I/O managed by CPU
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Fill in analogous concept for inter-program or
thread communication
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I/O Software System Layers
User-level I/O software Device-independent
operating system software Device
drivers Interrupt handlers
Hardware
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How might a program communicate with another?

• O.S. device-independent software starts an I/O
  operation, blocks until the I/O has completed and
  the interrupt occurs.
• A processes communicating with shared data or
  another process synchronize by setting a
  semaphore, a wait on a condition variable, a
  receive on a message or something similar.
• When the interrupt happens, the O.S.
  device-independent software does whatever it has
  to do in order to handle the interrupt.
• The synchronizing software unblocks the process
  (resets the semaphore, executes an interrupt
  return to the previous process)

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Device driver analogy

• In a simple program that writes to a device, it
  communicates with the o.s. device-independent
  software which communicates with the device via a
  device driver.
• Using a device driver is a good example of a
  layered architecture and information hiding

o.s. device independ-ent sw
Device driver
Program
Device
Interface
Program A
Program B
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Role for device driver multiprogramming
principles

• An exercise in layered software architecture
• Standard interface procedures that the rest of
  the o.s. can call
• Functions
• accept abstract read and write requests from the
  device-independent software and see that they are
  carried out
• initialize the device
• manage power requirements and log events
• check input parameters to see if valid (Meyer
  Filter-methods in Design by Contract programming)
• may need to translate parameters from abstract to
  concrete terms
• check if device is currently in use if so, queue
  the request
• list continued on next slide ...

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continued

• timing check if request can be handled now or
  if something else must be done like start the
  device
• control the device -- issue sequence of commands,
  may need to interact sequentially
• driver blocks itself until interrupt indicates
  completion or it doesnt need to
• check for errors
• may pass data
• returns some status information for error
  reporting
• Select and start another request or blocks,
  waiting for next request
• must be reentrant
• cannot make system calls may need to make
  certain kernel calls

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Think of the device driver as a shared resource ..

• Which of these forms of communication does a
  program typically use with a device driver?
• Is there an analogy in inter-program
  communication?
• Buffered
• Synchronous vs asynchronous
• Dedicated device vs shared device
• Spooling
• Queued requests
• Other?

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• Memory management and file access impact on
  multiprogramming

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Review of Cache flow
tag index byte
16
tags tags
Example of 2-way associative cache
main memory
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Cache flow
011010 1000100101 0100
16
1K
1000100101
011010
6A250
tags tags
Example of 2-way associative cache
main memory
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Impacts

• If two programs are accessing the same data in
  memory, what impact will the presence of a cache
  have?
• If two programs access different areas of same
  data object?
• What if you have a late enhancement to create a
  report which includes the execution over a large
  trio of arrays
• SumArray i ArrayOne i ArrayTwo i
• Would you be better off with three files?

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Layers of complexity

• Typically in microprocessor world
• (on-chip) cache level 1 often direct-mapped,
  often has separate caches for instructions and
  data
• (off-chip) cache -- level 2 often 2 or 4 way
  associative and unified
• main memory
• secondary memory (file on disk)
• Both caches might both be sequential or the
  second may be associative.
• Main memory, however, is probably managed with a
  paging algorithm ...

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On the one hand ...

• We have a cache because architects assume
  temporal locality.
• We transfer entire lines because we assume
  spatial locality.
• Discuss the impact on performance.
• What are other issues to consider?

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Remember types of paging used in memory
management

• Paged, segmented, paged-segmentation
• Translation Lookahead Buffer to speed up paging
  process

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Impact of Shared Memory, Memory Management, File
Systems, I/O on Performance

• From what you know now, do you see how the
  patterns and anti-patterns can be related to the
  paging algorithm, cache management, the file
  system support, or the inter-program
  communication of a given environment?

Fast path god class First Things
First Excessive dynamic alloc Coupling Circuit
ous Treasure Hunt Batching One-Lane
Bridge Alternate Routes Traffic Jam Flex
Time and others Slender Cyclic Functions