[Octave-bug-tracker] [bug #54100] fread using SKIP larger than zero is extremely slow

[Rik]

Update of bug #54100 (project octave):

                Priority:              5 - Normal => 3 - Low                

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Follow-up Comment #13:

I used arithmetic and the number of bytes read to eliminate one of the calls
to tellg.  I also removed the unneccessary check on (! is) at the end of the
skip block code as suggested in comment #6.  See
https://hg.savannah.gnu.org/hgweb/octave/rev/0812413a0bb7.

Altogether it produced a 40% savings.  Test results are now


skip = 0: Elapsed time is 0.00130677 seconds.
skip = 4: Elapsed time is 0.033772 seconds.


So using skip is now 26X slower than not using skip, but it started out at
182X.

The next step would be harder so I am going to stop.  The problem now is that
when skip != 0 the stream::read function is called in units of blocksize which
for 'single' is just 4 bytes.  When skip is zero the function reads in very
large block sizes which is much more efficient.




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