CODEWITHSUNDEEP
delphifilebuffer
Thomas O
Thomas O

Reputation: 6220

Why is writing data slow very slow in Delphi?

I am designing, for my A2 Computing project, an application to simulate the response of a variety of filters. One problem I'm having is that the export data option is really slow.

Normally, when generating data to show on the display it's about 40000 - 80000 points/sec. When recording it to a file, it drops to about a fifth of that.

Initially I thought my problem was because I was calling writeln on every data point. So I wrote it such that it queued the data up into a string and every 1000 points it wrote it in one big operation. It made it slightly faster, but still around 4-5x slower as displaying it on the built in form.

Why might this be?

Here's the export code:

   for xx := 0 to npoints do
   begin
     freq := minfreq + ((xx / npoints) * maxfreq);
     ampl := GetAmplPoint(freq);
     phase := GetPhasePoint(freq);
     tempstr := tempstr + FormatFloat('#.#####', freq) + ',';
     tempstr := tempstr + FormatFloat('#.#####', ampl) + ',';
     tempstr := tempstr + FormatFloat('#.#####', phase) + sLineBreak;
     // Queue up to 1000 points, then write the data in one lump:
     // most of the time is spent in writeln waiting for IO which
     // slows down export.
     if xx mod 1000 = 0 then
     begin
       write(fileptr, tempstr);
       tempstr := '';
       ProgressBar.Position := 4 + Trunc((xx / npoints) * 96);
     end;
   end;

Upvotes: 1

Views: 1635

Answers (2)

Chris Thornton
Chris Thornton

Reputation: 15817

If I recall correctly.... you will have much better performance with binary files than text files. The contents don't matter, it's just the declaration. You don't show the declaration for fileptr. But if it's declared like this: 

var fileptr : TextFile;

it will be slower than:

var fileptr : File;

or

var fileptr : File of <some record or type>;

Give that a try and see if it speeds things up. You can also use BlockWrite(). Note that your output may lag behind the program, due to buffering. You probably want to flush the file in any error handler.

Upvotes: 2

Mad Hatter
Mad Hatter

Reputation: 772

Disk I/O is one of the slowest bottlenecks today, especially if you use slow disks (i.e. the 4200/5400 rpm disks found on many laptops).

Better perfomance can be obtained using buffered I/O (old pascal I/O functions were designed long ago, and may use small buffers, better to use one of the buffered streams available today in Delphi) or asynch I/O (you pass the buffer to write to the OS, the call returns immediately and later the OS will tell you when it had written the data).

Upvotes: 4

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