[Dune] MultiLinearGeometry

[Oliver Sander]

Hi Martin,

> first of all, I'm not an expert on the stl, so there might be a way to
> put the reference counter into the object the smart pointer points to.
> However, as far as I understand things, the shared_ptr actually points
> to a control structure containing (at least) the following:
> - a counter of strong references (by shared_ptr),
> - a counter of weak references (by weak_ptr),
> - a pointer to the real object in memory.

A quick test shows that on my machine a std::shared_ptr needs 16 bytes.
That's not too bad: you need at least one pointer to the actual
object (8 bytes), and the reference counter (4 bytes).  I haven't
dug in the code deep enough to see what the remaining 4 bytes are.

> Unfortunately, this control structure must be allocated somehow, so
> there is an additional "new" (again, there might be a way around this
> that I don't know of).

True.  But this is why std::make_shared was invented.  It produces
a shared_ptr with only single allocation.  AFAIK that only c++0x, but
dune-common provides a compatibility layer in dune/common/shared_ptr.hh

>Moreover, dereferencing a shared_ptr either
> dereferences two pointers (the one to the control structure and the
> pointer to the real object), or the shared_pointer needs to contain both
> pointers.
>
Not true.  Since you hold the shared_ptr by value you have only on pointer
to dereference.  Copy'n'paste from the stl:

       operator->() const noexcept
       {
	_GLIBCXX_DEBUG_ASSERT(_M_ptr != 0);
	return _M_ptr;
       }

and _M_ptr is a plain pointer.

> In summary: The shared_ptr seems unnecessarily expensive to me if I just
> want a simple reference counting pointer (being prepared to store the
> reference count on the object itself).
>
> Puh, a long answer. Maybe someone more into the stl can enlighten me on
> where I'm right and where I'm wrong. Until then, I prefer coding it
> myself ;-).
>
Conclusio: you may want to give the stl another chance, and save yourself
some coding time & headaches.  :-)

cheers,
Oliver


> Best,
>
> Martin
>
> On 10/12/2012 05:11 PM, Oliver Sander wrote:
>> Hi Martin,
>>
>>> as the name says, the user data field is for your data. GeometryGrid
>>> uses it for the reference count.
>>
>> just out of curiosity: what's wrong with the stl way of reference
>> counting?
>>
>> best,
>> Oliver
>>
>>> If you specify an empty class, no
>>> memory is used (a trick I learned from gcc's std::vector). If you don't
>>> need it, just leave it at the default (empty).
>>>
>>> Best,
>>>
>>> Martin
>>>
>>> On 10/12/2012 04:50 PM, Oliver Sander wrote:
>>>> Hi Martin,
>>>> thanks! I'm looking forward to making UGGrid use that instead of
>>>> BasicGeometry.
>>>> Can you give a short explanation what the userData is good for?
>>>> best,
>>>> Oliver
>>>>
>>>> Am 12.10.2012 16:03, schrieb mnolte at dune-project.org:
>>>>> Author: mnolte
>>>>> Date: 2012-10-12 16:03:14 +0200 (Fri, 12 Oct 2012)
>>>>> New Revision: 1072
>>>>>
>>>>> Modified:
>>>>> recent_changes.wml
>>>>> Log:
>>>>> announce the new MultiLinearGeometry (see FS#1182)
>>>>>
>>>>>
>>>>> Modified: recent_changes.wml
>>>>> ===================================================================
>>>>> --- recent_changes.wml 2012-10-09 16:18:26 UTC (rev 1071)
>>>>> +++ recent_changes.wml 2012-10-12 14:03:14 UTC (rev 1072)
>>>>> @@ -54,6 +54,11 @@
>>>>> dimension and codimension. Since such hypercubes have a lot of
>>>>> structure, the
>>>>> implementation
>>>>> is very fast.
>>>>> </li>
>>>>> +<li>A reimplementation of the generic geometries,
>>>>> named<kbd>MultiLinearGeometry</kbd>
>>>>> + has been added. It comes without a complicated TMP and abandons the
>>>>> use of
>>>>> dynamic
>>>>> + polymorphism in the case of multiple geometry types. Despite these
>>>>> simplifications,
>>>>> + it shows slightly better performance.
>>>>> +</li>
>>>>> </ul>
>>>>>
>>>>> <h3>Deprecated and removed features</h3>
>>>>> @@ -91,6 +96,9 @@
>>>>> Jacobians to be FieldMatrices anymore. The precise interface
>>>>> requirements for
>>>>> these matrices
>>>>> is still being worked out.
>>>>> </li>
>>>>> +<li><kbd>GeometryGrid</kbd> now uses the
>>>>> new<kbd>MultiLinearGeometry<kbd>
>>>>> instead of the
>>>>> + generic geometry implementaton.
>>>>> +</li>
>>>>> </ul>
>>>>>
>>>>> <h3>Deprecated and removed features</h3>
>>>>>
>>>>>
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