mirror of
https://github.com/triqs/dft_tools
synced 2024-11-01 19:53:45 +01:00
edd1ff4529
A first general restructuration of the doc according to the pattern [tour|tutorial|reference]. In the reference part, objects are documented per topic. In each topic, [definition|c++|python|hdf5] (not yet implemented)
64 lines
2.2 KiB
ReStructuredText
64 lines
2.2 KiB
ReStructuredText
.. highlight:: c
|
|
|
|
.. _arr_view_memory:
|
|
|
|
View memory management
|
|
------------------------
|
|
|
|
View classes contains a reference counting system to the memory block they view
|
|
(like a std::shared_ptr, but more sophisticated to properly interface to Python numpy).
|
|
|
|
This guarantees that memory will not be dellocated before the destruction of the view.
|
|
|
|
The memory block will be dellocated when its array and all array_view
|
|
pointing to it or to a portion of it will be destroyed, and only at that moment.
|
|
|
|
Example::
|
|
|
|
array<int,2> *p = new array<int,2> (2,3); // create an array p
|
|
array_view<int,2> B = *p; // making a view
|
|
delete p; // p is gone...
|
|
B(0,0) = 314; cout<<B<<endl; // B (and the data) is still alive
|
|
|
|
|
|
.. warning::
|
|
|
|
TRIQS arrays, and in particular views are NOT thread-safe, contrary to std::shared_ptr.
|
|
This is a deliberate choice for optimisation.
|
|
|
|
|
|
Weak views [Advanced]
|
|
^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
Unfortunately this cannot be the end of the story, at least on current C++11 compilers.
|
|
It turns out that, in very performance sensitive loops, increasing this tiny
|
|
reference counter can break a lot of optimisations in almost all compilers, including the most
|
|
recent ones (gcc 4.8, clang 3.3).
|
|
|
|
It was therefore decided to introduce `weak views`.
|
|
|
|
* Weak views are completely similar to "normal" views, except that they do `not` increase the memory
|
|
reference counter.
|
|
|
|
* Normal views are be implicitely constructed from weak view (the reverse is also true),
|
|
As soon as you store the weak view into a normal view (e.g. array_view<T,N>)
|
|
the reference counting is again incremented, and the memory guarantee holds.
|
|
|
|
* Explicit construction of weak views is intentionally not documented here.
|
|
It is designed to be (almost) an implementation detail.
|
|
|
|
* The () operator returns a weak view `on a fully compliant C++11 compiler` (in particular, `rvalue reference for *this` must be implemented)
|
|
allowing therefore for better performance on such compiler, in some loops.
|
|
Older supported compiler will therefore generate code with lower performances.
|
|
|
|
* It is however necessary for the advanced user to know about this implementation detail,
|
|
because in some convolved cases, the memory guarantee may not hold.
|
|
Example ::
|
|
|
|
TO BE WRITTEN
|
|
|
|
|
|
|
|
|
|
|