Why would you expect improved performance using a double buffer rather
than a single buffer for I/O

Hello Sumavind,

Based on your question, I will assume that the "double buffer" refers
to the area of memory used by an application to transmit information,
and not memory used by the operating system. I will also assume you
are not doing extra data copies when implementing the double buffer -
that can often hurt performance.

The main concept with "buffered I/O" is to allow the application
program to continue to do useful work while previous I/O is in
progress. Using a display as an example...
  1 application draws the image for frame 1 into buffer A
  2 application starts display card to display buffer A
  3a application draws the image for frame 2 into buffer B
  3b display card draws the image with buffer A
  4 application starts display card to display buffer B
  5a application draws the image for frame 3 into buffer A
  5b display card draws the image with buffer B
and the last few steps are repeated for each pair of display images.
Note that the steps 3a and 3b are done at the same time; the same for
steps 5a and 5b.

The same type of processing can take place for network I/O, disk I/O
and other devices. Note that many applications are not written this
way. They use I/O operations that block the application until the I/O
is complete. In this case, more than one buffer will not improve
performance of that application.

A few pages that describe related concepts include:

Open GL FAQ
  http://www.bergen.org/AAST/ComputerAnimation/Help_FAQs_OpenGL3.html
(has a description of switching between single and double buffered
modes)

Reusable Software Double Buffer
  http://research.compaq.com/SRC/juno-2/papers/dblbuffer/
(see also caution about performance penalty with copying data)

Mac OS X Quickdraw Performance
  http://developer.apple.com/technotes/tn/tn2051.html
(notes that the OS does double buffering for you, don't put it into
the application)

Advanced Circual Buffer
  http://www.ueidaq.com/appstories/appstories.asp?type=1&MM_recordId=7
(describes single and multiple buffer methods for data acquisition)

Some good search phrases include:
  "double buffer" performance

  --Maniac