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Fast forward-backward (write-read)

Paul Rubin no.email at nospam.invalid
Tue Oct 23 12:17:35 EDT 2012 
Virgil Stokes <vs at it.uu.se> writes:
> Finally, to my question --- What is a fast way to write these
> variables to an external file and then read them in backwards?

Seeking backwards in files works, but the performance hit is
significant.  There is also a performance hit to scanning pointers
backwards in memory, due to cache misprediction.  If it's something
you're just running a few times, seeking backwards the simplest
approach.  If you're really trying to optimize the thing, you might
buffer up large chunks (like 1 MB) before writing.  If you're writing
once and reading multiple times, you might reverse the order of records
within the chunks during the writing phase.  

You're of course taking a performance bath from writing the program in
Python to begin with (unless using scipy/numpy or the like), enough that
it might dominate any effects of how the files are written.

Of course (it should go without saying) that you want to dump in a
binary format rather than converting to decimal.
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