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Remove hard time limit from vf2 passes in preset passmanagers #8021

Merged
merged 3 commits into from
May 4, 2022
Merged

Remove hard time limit from vf2 passes in preset passmanagers #8021

merged 3 commits into from
May 4, 2022

Commits on May 4, 2022

  1. Remove hard time limit from vf2 passes in preset passmanagers 

    This commit removes the hard time limit from the preset passmanager. We
were previously setting two limits on the pass a call_limit parameter,
which is used to set the limit of internal state visits the vf2
implementation in retworkx will attempt before giving up, and a time
limit which sets a hard wall time limit that is checked after each
mapping to ensure we don't spend more than a specific amount of time on
trying to find an optimal mapping. As subgraph isomorphism is
NP-complete we could spend a very large amount of time trying to find
the next isomorphic mapping and both these limits were set to avoid a
situation where we spend hours in optimization level 1 trying to find a
better layout. However, as we've seen in Qiskit#8017 setting a hard time limit
limits the level of reproducibility on the output of the transpiler. It
means that setting a fixed seed is no longer sufficient to exactly
reproduce the output as it's also partially a function of the
performance of the local system we're running on. If the local
environment is slower we might get different results because we've hit
the hard time limit.

To address this, this commit removes the hard time limit so that we only
rely on the call limit parameter on the preset pass manager. While this
parameter is a bit more opaque in how it should be used because it
requires an understanding of how the VF2 algorithm works (and is
implemented in retworkx) it should be sufficient to avoid the runaway
execution problem but also means the limits are determinitisically
applied. This should improve the reproducability of the transpiler because
the pass will always try exactly up until a given point regardless of
how fast the local environment is.

For people manually instatiating the pass they can still rely on the
hard time limit parameter if they want it, but for the preset pass
managers and the transpile() function we no longer set this.

Fixes Qiskit#8017
    @mtreinish
    mtreinish committed May 4, 2022
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  2. Add release note

    @mtreinish
    mtreinish committed May 4, 2022
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  3. Fix tense

    @jakelishman
    jakelishman authored May 4, 2022
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