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

[Rik]

Update of bug #54100 (project octave):

                  Status:                    None => Confirmed              
                 Release:                   4.4.0 => dev                    
        Operating System:       Microsoft Windows => Any                    

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Follow-up Comment #10:

I took the first of the most obvious suggestions, calculating the size of the
file just once and storing eof_pos in a variable.  This produces a 2.5X
speed-up, and no regressions during testing, so I committed that change here
(https://hg.savannah.gnu.org/hgweb/octave/rev/336267b16a3d).

Test results were


-------------------------------------------------------------------------
Before
-------------------------------------------------------------------------
octave:18> tic; xt = fread (fid, [1000, 60], 'single', 0); toc
Elapsed time is 0.00138497 seconds.

octave:21> tic; xt = fread (fid, [1000, 60], 'single', 4); toc
Elapsed time is 0.252382 seconds.

-------------------------------------------------------------------------
After
-------------------------------------------------------------------------
octave:27> tic; xt = fread (fid, [1000, 60], 'single'); toc
Elapsed time is 0.00123215 seconds.

octave:34> tic; xt = fread (fid, [1000, 60], 'single', 4); toc
Elapsed time is 0.110174 seconds.


It is still the case that turning on 'skip' is approximately 100X slower
(0.110 vs. .0012).  Maybe the next low-hanging optimization would be to just
read SKIP bytes from 'is' and discard them rather than using fseek itself.  It
would require benchmarking.


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