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Improve efficiency of CouplingMap.make_symmetric #9571

Merged
merged 2 commits into from
Feb 10, 2023
Merged

Improve efficiency of CouplingMap.make_symmetric #9571

merged 2 commits into from
Feb 10, 2023

Commits on Feb 10, 2023

  1. Improve efficiency of CouplingMap.make_symmetric 

    Right now the CouplingMap.make_symmetric has a quadratic overhead. It
was iterating over a list of each edgess' endpoints in the graph
and for each edge it was iterating over the same list again to check
whether the reverse edge is present or not. The overhead for this for
large coupling maps can be quite large, for example with a 10497 qubit
heavy hex coupling map the time it took to run this method as part of
SabreLayout's initialization was 10x slower than actually running the
pass (a equally sized Bernstein-Vazirani circuit). Instead of doing an
O(n) contains check inside the loop this commit updates it to use a
set which will be an O(1) lookup. This should address the performance
issue with this method. In the future we should leverage the native
rustworkx method to do this, which is being added in:
Qiskit/rustworkx#814
    @mtreinish
    mtreinish committed Feb 10, 2023
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  2. Further reduce the overhead by avoiding CouplingMap.add_edge() 

    The CouplingMap add_edge() method has similar overhead to check whether
nodes need to be added. However, in the case of make_symmetric we don't
need this checking because both endpoints are already in the graph. This
commit further improves the performance of the method by just adding the
reverse edge directly to the graph via the rustworkx api instead.
    @mtreinish
    mtreinish committed Feb 10, 2023
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