Message-ID: <3AD10B71.9019CC9@jps.net>
From: Dennis Yelle <dennis51 AT jps DOT net>
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Subject: Re: gdb still doesn't work for me
References: <3AD0B992 DOT 44D0D3C3 AT jps DOT net> <4634-Sun08Apr2001234906+0300-eliz AT is DOT elta DOT co DOT il>
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Date: Sun, 08 Apr 2001 18:08:01 -0700
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To: djgpp AT delorie DOT com
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Eli Zaretskii wrote:
[...]
> No, GDB doesn't slow down programs, unless the program does something
> very special, such as issues a lot of DPMI function calls, or
> generates lots of SIGFPE exceptions.
> 
> Normal programs don't do that.  But your program, especially its car5
> function, allocates lots of memory in very small chunks (because it
> creates lots of temporary Bset objects), which forces the library to
> requests lots of small allocations from the DPMI server.  Each such
> allocation gets caught by GDB, because it needs to be aware of all
> memory allocated by the program being debugged, to be able to
> deallocate that memory if the program is restarted without exiting the
> debugger.  This causes a huge overhead, which is why you see this
> terrible slowdown.

Here is a much smaller program that also demonstrates
a terrible slowdown under gdb.  That is, the program 
below takes much more than 10 times as long to run under gdb.

----------------------------------
#include <iostream>

int count = 0;
int i, j, k;
unsigned long ii, jj, mm;

class Paths {
public:
  static void car7()
  {
    for( i=0; i<10; i++) {
      cout << "car7, i=" << i << '\n';
      ii = i;
      for( j=0; j<100; j++) {
	jj = j | ii;
	for( k=0; k<10000; k++) {
	  mm = jj | k;
	  if ( mm == 127)
	    if ( ++count < 5)
	      cout << "car7: " << (i*100+j)*10000+k << '\n';
	}
      }
    }
  }
};

int main()
{
  cout << "Calling car7\n";
  Paths::car7();
  cout << "all done.\n";
}
-----------------------------------

I compile the program with:
    gxx -g -Os -Wall test.cpp -o test.exe
Then I run it from the command line like this:
    test
It completes in less than 1/2 second.
It writes 16 short lines.

Then I run it under gdb like this:
   gdb test.exe
   break main
   run
   n
After 20 seconds it has only produced 3 lines of output.
The program does not allocate any memory in the
function car7.

Clearly there is something wrong somewhere.
Can anyone tell me how to fix this?

Dennis Yelle
-- 
I am a computer programmer and I am looking for a job.
There is a link to my resume here:  
http://table.jps.net/~vert/