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(also parallel evolution), a principle of evolution of groups of organisms, whereby the organisms independently acquire similar structural modifications of features inherited from their common ancestors. For example, in the course of evolution perissodactyls in the northern hemisphere and extinct South American ungulates—litopterns—which originated from a common five-toed ancestor, underwent a parallel reduction in the number of toes from five to one. The trait of saber-like teeth was acquired independently by different groups of predatory mammals. Parallelism is due to natural selection acting in a similar direction on ancestral groups that had originally diverged. It is sometimes defined as the convergence of closely related groups.
in poetics, the distribution in adjacent parts of a text of identical or similar elements of speech that interrelate to create a single poetic image.
The following lines are an illustration of parallelism:
Oh, if no frosts descended on flowers,
Flowers would bloom even in winter;
Oh, if no sorrow befell me,
I would not grieve over anything.
Parallelism of the above type, which juxtaposes an image from nature and one from the life of man, is widespread in folk poetry. Sometimes the use of negation and other literary devices creates greater complexity:
It was not a blade of grass fluttering about in the open
It was myself roaming homeless.
Written literature made use of parallelism at an early stage; to a great extent it forms the basis of the poetic style of the Bible. Three of the most ancient figures of speech of Greek rhetoric are elaborations of parallelism: isocolon, or phrases of equal length; antithesis, or phrases of contrasting meaning; and homeoteleu-thon, or phrases with similar endings.
By analogy with verbal parallelism, other types of parallelism are sometimes referred to: sound parallelism (alliteration and rhyme), rhythmic parallelism (the strophe and antistrophe in Greek lyric poetry), and compositional parallelism (parallel plot lines in a novel).
M. L. GASPAROV
(a + b) *
(c + d) the expressions a, b, c and d can all be calculated in parallel giving a degree of parallelism of (at least) four. Once they have been evaluated then the expressions a + b and c + d can be calculated as two independent parallel processes.
The Bernstein condition states that processes P and Q can be executed in parallel (or in either sequential order) only if:
(i) there is no overlap between the inputs of P and the outputs of Q and vice versa and
(ii) there is no overlap between the outputs of P, the outputs of Q and the inputs of any other task.
If process P outputs value v which process Q reads then P must be executed before Q. If both processes write to some variable then its final value will depend on their execution order so they cannot be executed in parallel if any other process depends on that variable's value.