Max iteration being hit on QV circuits targetting ECR-based systems

[nonhermitian]

Environment

• Qiskit Terra version: latest
• Python version: 3.9
• Operating system: ubuntu

What is happening?

It seems transpiling QV circuits for ecr based backends raises transpilation warnings for various seed values. It is not hard to find a seed where this issue occurs.

How can we reproduce the issue?

from qiskit import *
from qiskit_ibm_provider import IBMProvider
from qiskit.circuit.library import QuantumVolume

provider = IBMProvider()

qc = QuantumVolume(7, seed=12345)
qc.measure_all()

backend = provider.get_backend('ibm_sherbrooke')
trans_qc = transpile(qc, backend, optimization_level=3)

gives

TranspilerError: 'Maximum iteration reached. max_iteration=1000'

Switching the backend to ibm_kyiv that has the same topology, but uses cx gates rather than ecr works fine.

What should happen?

It should work fine.

Any suggestions?

No response

[jakelishman]

Thanks - we're fairly sure this is because an optimisation loop tends to get stuck oscillating between two possible solutions. #9612 should hopefully solve this.

[1ucian0]

confirmed that #9612 fixes this issue.