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Date: Sat, 28 Apr 2007 08:41:26 -0400
From: Jeffrey Creem <jeff@thecreems.com>
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Subject: Re: Reading and writing a big file in Ada (GNAT) on Windows XP
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Date: 2007-04-28T08:41:26-04:00
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Fionn Mac Cumhaill wrote:
> I have written a very simple program that runs  on a Windows XP
> machine. (Pentium D , I don't remember the clock speed, it is in
> middle of Pentium D clock speeds, 2 Gb memory) It is compiled with the
> MinGW GNAT 3.4.5 Ada compiler.
> 
> All it does is read lines in a loop from a text file with
> Ada.Text_IO.Get_Line, does minor modifications on about 80% of the
> lines that it reads, and writes the lines to an output file with
> Put_Line. 
> 
> The modifications consist of replacing a slice of text at the end of a
> line with another bit of text. The biggest slice is 10 characters, and
> the replacement slice is always smaller than the original slice. An
> occasional line of text is about 6000 characters long, but most are
> about 700 haracters.  Get_Line reads them into a String variable that
> is 10,000 characters long.
> 
> 
> The problem is that the input file has more than 10 million lines of
> text in it. The program works perfectly, but takes about 5 hours to
> run. The Cygwin version of wc can count the lines in the input file in
> less than one minute.
> 
> The program that produces the file pulls data from a SQL Server 7
> running on an 800 MhZ Pentium III machine with 512 Mb and writes it to
> a file in 1-1/2 hours. Almost all (99.9%) of the lines are rows from a
> database table.
> 
> Why is this so slow? 
> Do I have an Ada problem, a GNAT problem, or a MinGW problem?

Is there any chance that the code is doing things like initializing the 
string to blanks/nulls before each getline and or scanning beyond the 
value returned for last when doing the replace?