Strange performance effects

[Roland]

Hi!

I am working on a project in which i implement a mathematical
optimization algorithm. One of the requirements on the code is very
good performance. So i started to optimize a bit. Now i do have a very
strange effect which i do not understand. Maybe somebody knows about:

I have the following class

template<class T> class SimplexL2Penalty : public Function<T,T>
{
public:

ASIString GetTypeInfo() const;
SimplexL2Penalty(const Simplex<T>& simplex);
SimplexL2Penalty(const Simplex<T>& simplex,const T& weight);
SimplexL2Penalty(const Simplex<T>& simplex,const Vector<T>&
w);
SimplexL2Penalty(const SimplexL2Penalty& toBeCopied);
virtual ~SimplexL2Penalty();
T Evaluate(const Vector<T>& point) const;
void operator*= (const T& factor);
ASIString ToXML() const;
Function<T,T>* ConstructCopy() const;
ASI_FLOGIC LargerOrEqual(const T& value) const;
const VectorSet<T>* GetZeroSet();

protected:

Vector<T> ComputePerturbation(const Vector<T>& point) const;
Simplex<T> m_Simplex;
T* m_WeightVector;
T m_Factor;

//Vector<T> *m_PerturbationVector1;
};

This class is used in an iterative algorithm. Performance is about 500
iterations per second.
When i add the pointer m_PerturbationVector1 to the class (without
using it at all) performance drops to 250 iterations per second.

What is going on? What can i do to overcome this effect.

Thanks a lot,
Roland

PS: I am using Microsoft Visual C++ 6.0.

 

[Victor Bazarov]

I am working on a project in which i implement a mathematical
optimization algorithm. One of the requirements on the code is very
good performance. So i started to optimize a bit. Now i do have a very
strange effect which i do not understand. Maybe somebody knows about:

I have the following class

Actually, it's a template.

template<class T> class SimplexL2Penalty : public Function<T,T>
{
public:

ASIString GetTypeInfo() const;
SimplexL2Penalty(const Simplex<T>& simplex);
SimplexL2Penalty(const Simplex<T>& simplex,const T& weight);
SimplexL2Penalty(const Simplex<T>& simplex,const Vector<T>&
w);
SimplexL2Penalty(const SimplexL2Penalty& toBeCopied);
virtual ~SimplexL2Penalty();
T Evaluate(const Vector<T>& point) const;
void operator*= (const T& factor);
ASIString ToXML() const;
Function<T,T>* ConstructCopy() const;
ASI_FLOGIC LargerOrEqual(const T& value) const;
const VectorSet<T>* GetZeroSet();

protected:

Vector<T> ComputePerturbation(const Vector<T>& point) const;
Simplex<T> m_Simplex;
T* m_WeightVector;
T m_Factor;

//Vector<T> *m_PerturbationVector1;
};

This class is used in an iterative algorithm.

How? Don't you think it might matter?

Performance is about 500
iterations per second.
When i add the pointer m_PerturbationVector1 to the class (without
using it at all) performance drops to 250 iterations per second.

What is going on?

Who the hell can tell? Without seeing how the template is used, your
guess is just as good as ours. It could be that the size causes some
kind of processor cache to be blown away more often, it could be that
allocating a bunch of them now requires more OS involvement... There
is no way to tell for certain.

What can i do to overcome this effect.

Don't declare that member, since you're not using it.

Why don't you take a profiler and run your process through it with and
without the member. See if you can spot the difference. Then analyse.
Then come back and ask for assistance if you still need it.

Victor

 

[Roland]

Actually, it's a template.


How? Don't you think it might matter?


Who the hell can tell? Without seeing how the template is used, your
guess is just as good as ours. It could be that the size causes some
kind of processor cache to be blown away more often, it could be that
allocating a bunch of them now requires more OS involvement... There
is no way to tell for certain.


Don't declare that member, since you're not using it.

Why don't you take a profiler and run your process through it with and
without the member. See if you can spot the difference. Then analyse.
Then come back and ask for assistance if you still need it.

Victor

Thank you. Clearly i will not declare a member when not using it. On
the long run i will use it.

Since i am a mathematican i do not know much about technical aspects.
Obviously nobody can answer the question in detail using the
information provided. But some may guess what it could be. Just as you
did. And then i migth be able to look for a book or something to learn
about. That is what i hoped for.

Roland

 

[Chris Theis]

[SNIP]

Thank you. Clearly i will not declare a member when not using it. On
the long run i will use it.

Since i am a mathematican i do not know much about technical aspects.
Obviously nobody can answer the question in detail using the
information provided. But some may guess what it could be. Just as you
did. And then i migth be able to look for a book or something to learn
about. That is what i hoped for.

Sorry to disappoint you but guessing and optimization are two terms that
don't match very well. The whole problem starts with the term "optimizing a
bit". I'd recommend to profile your program first to determine where the
real bottleneck is before starting to fiddle around with the code. Keep in
mind that premature optimization is something that might even fire back!

Regards
Chris