Skip to content
New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Add topic about "Lazy Initiliazation" pattern to Design patterns #790

Merged
merged 15 commits into from
Mar 9, 2022
1 change: 1 addition & 0 deletions doc/main/tbb_userguide/design_patterns/Design_Patterns.rst
Original file line number Diff line number Diff line change
Expand Up @@ -51,6 +51,7 @@ expressions into equivalent C++03 code.
../../tbb_userguide/design_patterns/Divide_and_Conquer
../../tbb_userguide/design_patterns/GUI_Thread
../../tbb_userguide/design_patterns/Non-Preemptive_Priorities
../../tbb_userguide/design_patterns/Lazy_Initialization
../../tbb_userguide/design_patterns/Local_Serializer
../../tbb_userguide/design_patterns/Fenced_Data_Transfer
../../tbb_userguide/design_patterns/Reference_Counting
Expand Down
Loading
Sorry, something went wrong. Reload?
Sorry, we cannot display this file.
Sorry, this file is invalid so it cannot be displayed.
Loading
Sorry, something went wrong. Reload?
Sorry, we cannot display this file.
Sorry, this file is invalid so it cannot be displayed.
109 changes: 109 additions & 0 deletions doc/main/tbb_userguide/design_patterns/Lazy_Initialization.rst
Original file line number Diff line number Diff line change
@@ -0,0 +1,109 @@
.. _Lazy_Initialization:

Lazy Initialization
==================


.. container:: section


.. rubric:: Problem
:class: sectiontitle

Delay the creation of an object, potentially expensive, until it is accessed.
In parallel programming, initialization must also be guarded against race conditions.


.. container:: section


.. rubric:: Context
:class: sectiontitle

The cost of operations that take place during the initialization
of the object may be considerably high. In that case, the object
should be initialized only when needed. Lazy initialization is
the common tactic that allows implementing such an approach.


.. container:: section


.. rubric:: Solution
:class: sectiontitle

Using ``oneapi::tbb::collaborative_call_once`` with ``oneapi::tbb::collaborative_once_flag``
helps to implement thread-safe lazy initialization for a user object.


In addition, ``collaborative_call_once`` allows other thread blocked on
the same ``collaborative_once_flag`` to join other |short_name|
parallel constructions called within the initializing function.


.. container:: section


.. rubric:: Example
:class: sectiontitle

This example illustrates the implementation of lazy initialization
for the calculation of the Fibonacci numbers. Here is a graphical
representation of the Fibonacci recursion tree for N=4.


|image0|


As seen in the diagram, some elements are recalculated more than once. These operations are redundant,
so the "lazy initialized" Fibonacci numbers are relevant here.


An implementation without the use of lazy initialization would have *O(2^N)* time complexity due to
the full recursion tree traversal and recalculation of values. Since all the nodes are traversed once,
the tree becomes a list, making the time complexity *O(N)*.


|image1|


Here you can see the code for the implementation. Already calculated values are stored in a buffer
paired with ``collaborative_once_flag`` and will not be recalculated when ``collaborative_call_once``
is invoked when initialization has already been done.


::


using FibBuffer = std::vector<std::pair<oneapi::tbb::collaborative_once_flag, std::uint64_t>>;

std::uint64_t LazyFibHelper(int n, FibBuffer& buffer) {
// Base case
if (n <= 1) {
return n;
}
// Calculate nth value only once and store it in the buffer.
// Other threads won't be blocked on already taken collaborative_once_flag
// but join parallelism inside functor
oneapi::tbb::collaborative_call_once(buffer[n].first, [&]() {
std::uint64_t a, b;
oneapi::tbb::parallel_invoke([&] { a = LazyFibHelper(n - 2, buffer); },
[&] { b = LazyFibHelper(n - 1, buffer); });
buffer[n].second = a + b;
});

return buffer[n].second;
}

std::uint64_t Fib(int n) {
FibBuffer buffer(n+1);
return LazyFibHelper(n, buffer);
}


.. |image0| image:: Images/image008a.jpg
Copy link
Contributor

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

can you remake screenshots so it would have a space above and beneath the figures?

Copy link
Contributor Author

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Updated images.

:width: 744px
:height: 367px
.. |image1| image:: Images/image009a.jpg
:width: 744px
:height: 367px