Lazy Initialization
Problem
Delay the creation of an object, potentially expensive, until it is accessed.
In parallel programming, initialization must also be guarded against race conditions.
Context
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.
Solution
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 oneTBB
parallel constructions called within the initializing function.Example
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.
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)
.
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);
}