Calculation mystery

I have problem with calculations in a program produced using VC++ and
MFC. The calculation is using double floats. It loops until the result
is reduced to a target value. 

In debug mode the calculation needs 178 loops. In release mode the
calculation is not consistent and sometimes needs over 38000 loops. 

I think this must be something to do with double floats in debug mode.
I can't fathom out what could cause the loops to vary in release mode.
It shouldn't change from run to run and it doesn't change in debug
mode.

Any ideas?

Steve

-- 
EasyNN-plus. Build Neural Networks.  http://www.easynn.com 
SwingNN.     Forecast the Future.    http://www.swingnn.com
Neural Planner Software Ltd          http://www.npsl1.com
0
steve (110)
5/15/2008 12:08:00 PM
vc.mfc 33608 articles. 0 followers. Follow

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You're aware that floating point may produce different result depending on 
exact order of intermediate operations, which could be different for release 
build, aren't you?

That said, make sure you have all your data initialized. And remember there 
is no exact result in floating point calculations.

"Stephen Wolstenholme" <steve@tropheus.demon.co.uk> wrote in message 
news:fq9o241pbk7m5hd6bpu2ttnc03to9u5487@4ax.com...
>I have problem with calculations in a program produced using VC++ and
> MFC. The calculation is using double floats. It loops until the result
> is reduced to a target value.
>
> In debug mode the calculation needs 178 loops. In release mode the
> calculation is not consistent and sometimes needs over 38000 loops.
>
> I think this must be something to do with double floats in debug mode.
> I can't fathom out what could cause the loops to vary in release mode.
> It shouldn't change from run to run and it doesn't change in debug
> mode.
>
> Any ideas?
>
> Steve
>
> -- 
> EasyNN-plus. Build Neural Networks.  http://www.easynn.com
> SwingNN.     Forecast the Future.    http://www.swingnn.com
> Neural Planner Software Ltd          http://www.npsl1.com 


0
alegr (1130)
5/17/2008 1:35:17 AM
Stephen Wolstenholme wrote:
> I have problem with calculations in a program produced using VC++ and
> MFC. The calculation is using double floats. It loops until the result
> is reduced to a target value. 
> 
> In debug mode the calculation needs 178 loops. In release mode the
> calculation is not consistent and sometimes needs over 38000 loops. 
> 
> I think this must be something to do with double floats in debug mode.
> I can't fathom out what could cause the loops to vary in release mode.
> It shouldn't change from run to run and it doesn't change in debug
> mode.

Stephen:

Try turning off optimization in release mode and playing with the floating point 
model (/fp) settings .

But with the differences you are seeing, I would suspect either a bug in your 
code (uninitialized variable?) or perhaps an unstable algorithm.

-- 
David Wilkinson
Visual C++ MVP
0
no-reply8010 (1790)
5/17/2008 8:58:33 AM
On Sat, 17 May 2008 04:58:33 -0400, David Wilkinson
<no-reply@effisols.com> wrote:

>Stephen Wolstenholme wrote:
>> I have problem with calculations in a program produced using VC++ and
>> MFC. The calculation is using double floats. It loops until the result
>> is reduced to a target value. 
>> 
>> In debug mode the calculation needs 178 loops. In release mode the
>> calculation is not consistent and sometimes needs over 38000 loops. 
>> 
>> I think this must be something to do with double floats in debug mode.
>> I can't fathom out what could cause the loops to vary in release mode.
>> It shouldn't change from run to run and it doesn't change in debug
>> mode.
>
>Stephen:
>
>Try turning off optimization in release mode and playing with the floating point 
>model (/fp) settings .
>

Some of the first things I tried. They make no difference.

>But with the differences you are seeing, I would suspect either a bug in your 
>code (uninitialized variable?) or perhaps an unstable algorithm.

My post took ages to get to this newsgroup so I have been running with
the problem in the aisip.technical newsgroup. The present theory is
that there is some interaction between the four calculating threads
and the GUI thread. It proving difficult to make any progress because
no code has been changed for years. It used to work OK. 

Steve

-- 
EasyNN-plus. Build Neural Networks.  http://www.easynn.com 
SwingNN.     Forecast the Future.    http://www.swingnn.com
Neural Planner Software Ltd          http://www.npsl1.com
0
steve (110)
5/17/2008 11:24:45 AM
Stephen Wolstenholme wrote:
> My post took ages to get to this newsgroup so I have been running with
> the problem in the aisip.technical newsgroup. The present theory is
> that there is some interaction between the four calculating threads
> and the GUI thread. It proving difficult to make any progress because
> no code has been changed for years. It used to work OK. 

Steve:

Ah, threads. You didn't tell us that before...

-- 
David Wilkinson
Visual C++ MVP
0
no-reply8010 (1790)
5/17/2008 2:11:37 PM
Are the four threads working on the same data?  If so, how are they synchronized?  Are you
running on the same kind of processor?  For example, if you are now running on a
multiprocessor or multicore processor, this could uncover bugs that have been lurking for
years but simply never noticed.  Floating point is tricky, and concurrency is tricky, and
you might have hit a combination of these in a multiprocessor environment that results in
erroneous computations.  For example, memory barrier issues could arise if there is no
synchronization.
				joe

On Sat, 17 May 2008 12:24:45 +0100, Stephen Wolstenholme <steve@tropheus.demon.co.uk>
wrote:

>On Sat, 17 May 2008 04:58:33 -0400, David Wilkinson
><no-reply@effisols.com> wrote:
>
>>Stephen Wolstenholme wrote:
>>> I have problem with calculations in a program produced using VC++ and
>>> MFC. The calculation is using double floats. It loops until the result
>>> is reduced to a target value. 
>>> 
>>> In debug mode the calculation needs 178 loops. In release mode the
>>> calculation is not consistent and sometimes needs over 38000 loops. 
>>> 
>>> I think this must be something to do with double floats in debug mode.
>>> I can't fathom out what could cause the loops to vary in release mode.
>>> It shouldn't change from run to run and it doesn't change in debug
>>> mode.
>>
>>Stephen:
>>
>>Try turning off optimization in release mode and playing with the floating point 
>>model (/fp) settings .
>>
>
>Some of the first things I tried. They make no difference.
>
>>But with the differences you are seeing, I would suspect either a bug in your 
>>code (uninitialized variable?) or perhaps an unstable algorithm.
>
>My post took ages to get to this newsgroup so I have been running with
>the problem in the aisip.technical newsgroup. The present theory is
>that there is some interaction between the four calculating threads
>and the GUI thread. It proving difficult to make any progress because
>no code has been changed for years. It used to work OK. 
>
>Steve
Joseph M. Newcomer [MVP]
email: newcomer@flounder.com
Web: http://www.flounder.com
MVP Tips: http://www.flounder.com/mvp_tips.htm
0
newcomer (15977)
5/17/2008 3:51:31 PM
On Sat, 17 May 2008 11:51:31 -0400, Joseph M. Newcomer
<newcomer@flounder.com> wrote:

>Are the four threads working on the same data?  If so, how are they synchronized?  Are you
>running on the same kind of processor?  For example, if you are now running on a
>multiprocessor or multicore processor, this could uncover bugs that have been lurking for
>years but simply never noticed.  Floating point is tricky, and concurrency is tricky, and
>you might have hit a combination of these in a multiprocessor environment that results in
>erroneous computations.  For example, memory barrier issues could arise if there is no
>synchronization.
>				joe
>

The threads read a link list. The results are written to four
different lists that are merged when the calculating has finished. The
four threads are to take advantage of multi's. However, I am testing
on a single core machine to keep things simple. 

Steve


>On Sat, 17 May 2008 12:24:45 +0100, Stephen Wolstenholme <steve@tropheus.demon.co.uk>
>wrote:
>
>>On Sat, 17 May 2008 04:58:33 -0400, David Wilkinson
>><no-reply@effisols.com> wrote:
>>
>>>Stephen Wolstenholme wrote:
>>>> I have problem with calculations in a program produced using VC++ and
>>>> MFC. The calculation is using double floats. It loops until the result
>>>> is reduced to a target value. 
>>>> 
>>>> In debug mode the calculation needs 178 loops. In release mode the
>>>> calculation is not consistent and sometimes needs over 38000 loops. 
>>>> 
>>>> I think this must be something to do with double floats in debug mode.
>>>> I can't fathom out what could cause the loops to vary in release mode.
>>>> It shouldn't change from run to run and it doesn't change in debug
>>>> mode.
>>>
>>>Stephen:
>>>
>>>Try turning off optimization in release mode and playing with the floating point 
>>>model (/fp) settings .
>>>
>>
>>Some of the first things I tried. They make no difference.
>>
>>>But with the differences you are seeing, I would suspect either a bug in your 
>>>code (uninitialized variable?) or perhaps an unstable algorithm.
>>
>>My post took ages to get to this newsgroup so I have been running with
>>the problem in the aisip.technical newsgroup. The present theory is
>>that there is some interaction between the four calculating threads
>>and the GUI thread. It proving difficult to make any progress because
>>no code has been changed for years. It used to work OK. 
>>
>>Steve
>Joseph M. Newcomer [MVP]
>email: newcomer@flounder.com
>Web: http://www.flounder.com
>MVP Tips: http://www.flounder.com/mvp_tips.htm
0
steve (110)
5/17/2008 5:05:26 PM
On Thu, 15 May 2008 13:08:00 +0100, Stephen Wolstenholme
<steve@tropheus.demon.co.uk> wrote:

>I have problem with calculations in a program produced using VC++ and
>MFC. The calculation is using double floats. It loops until the result
>is reduced to a target value. 
>
>In debug mode the calculation needs 178 loops. In release mode the
>calculation is not consistent and sometimes needs over 38000 loops. 

On Sat, 17 May 2008 18:05:26 +0100, Stephen Wolstenholme
<steve@tropheus.demon.co.uk> wrote:

>The threads read a link list. The results are written to four
>different lists that are merged when the calculating has finished. The
>four threads are to take advantage of multi's. However, I am testing
>on a single core machine to keep things simple. 

Is it possible to define the subset of the problem each thread will solve,
such that run-to-run, the results are reproducible? This could help you
discover concurrency problems, and it could help you further understand the
nature of the discrepancy between debug and release.

-- 
Doug Harrison
Visual C++ MVP
0
dsh (2499)
5/17/2008 9:14:00 PM
Cores and multiprocessors are not quite the same thing.  A single-core machine can have
hyperthreading, which can look like multicore from a logical viewpoint of the code
executing.  How many CPUs does Task Manager show you?

Also, the only way to guarantee that you are not a victim of a threading error is to
compare the solution in debug mode, computed with a single thread, vs. computed in release
mode, with a single thread.  If these exhibit the same pathology, then it is a problem of
code generation creating different computations (I was told by one of the foremost
numerical analysts, Joe Traub, when I asked him what floating point optimizations should
look like, he said "You do not touch floating point computations, ever, in an optimizing
compiler.  You perform whatever computations the programmer asked for, in the order
specified").  Since you have turned off optimization, it is not optimization, which leaves
threading.
				joe

On Sat, 17 May 2008 18:05:26 +0100, Stephen Wolstenholme <steve@tropheus.demon.co.uk>
wrote:

>On Sat, 17 May 2008 11:51:31 -0400, Joseph M. Newcomer
><newcomer@flounder.com> wrote:
>
>>Are the four threads working on the same data?  If so, how are they synchronized?  Are you
>>running on the same kind of processor?  For example, if you are now running on a
>>multiprocessor or multicore processor, this could uncover bugs that have been lurking for
>>years but simply never noticed.  Floating point is tricky, and concurrency is tricky, and
>>you might have hit a combination of these in a multiprocessor environment that results in
>>erroneous computations.  For example, memory barrier issues could arise if there is no
>>synchronization.
>>				joe
>>
>
>The threads read a link list. The results are written to four
>different lists that are merged when the calculating has finished. The
>four threads are to take advantage of multi's. However, I am testing
>on a single core machine to keep things simple. 
>
>Steve
>
>
>>On Sat, 17 May 2008 12:24:45 +0100, Stephen Wolstenholme <steve@tropheus.demon.co.uk>
>>wrote:
>>
>>>On Sat, 17 May 2008 04:58:33 -0400, David Wilkinson
>>><no-reply@effisols.com> wrote:
>>>
>>>>Stephen Wolstenholme wrote:
>>>>> I have problem with calculations in a program produced using VC++ and
>>>>> MFC. The calculation is using double floats. It loops until the result
>>>>> is reduced to a target value. 
>>>>> 
>>>>> In debug mode the calculation needs 178 loops. In release mode the
>>>>> calculation is not consistent and sometimes needs over 38000 loops. 
>>>>> 
>>>>> I think this must be something to do with double floats in debug mode.
>>>>> I can't fathom out what could cause the loops to vary in release mode.
>>>>> It shouldn't change from run to run and it doesn't change in debug
>>>>> mode.
>>>>
>>>>Stephen:
>>>>
>>>>Try turning off optimization in release mode and playing with the floating point 
>>>>model (/fp) settings .
>>>>
>>>
>>>Some of the first things I tried. They make no difference.
>>>
>>>>But with the differences you are seeing, I would suspect either a bug in your 
>>>>code (uninitialized variable?) or perhaps an unstable algorithm.
>>>
>>>My post took ages to get to this newsgroup so I have been running with
>>>the problem in the aisip.technical newsgroup. The present theory is
>>>that there is some interaction between the four calculating threads
>>>and the GUI thread. It proving difficult to make any progress because
>>>no code has been changed for years. It used to work OK. 
>>>
>>>Steve
>>Joseph M. Newcomer [MVP]
>>email: newcomer@flounder.com
>>Web: http://www.flounder.com
>>MVP Tips: http://www.flounder.com/mvp_tips.htm
Joseph M. Newcomer [MVP]
email: newcomer@flounder.com
Web: http://www.flounder.com
MVP Tips: http://www.flounder.com/mvp_tips.htm
0
newcomer (15977)
5/17/2008 11:16:09 PM
Reply:

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