sabre algorithm improvements in transpilation at level 3 not in effect #9090
Description
Environment
- Qiskit Terra version: 0.22.2 (vs. 0.21.1)
- Python version: 3.8.2
- Operating system: Windows
What is happening?
transpilation at optimization_level=3 does not (significantly) perform better in terra 0.22.2 than 0.21.1 despite improvements in sabre algorithm.
How can we reproduce the issue?
both in terra 0.22.2 and 0.21.1 run:
qc.x(5)
qc.h(range(6))
qc.cx(range(5),5)
qc.h(range(5))
qc.measure(range(5), range(5))
qc.draw('mpl')
# Transpile the circuit 'qc' 1000 times
trans_circ_list = transpile([qc]*1000, backend, optimization_level=3)
# get the number of cnot gates
cx_counts = np.asarray([circ.count_ops().get('cx', 0) for circ in trans_circ_list])
# create violin plot
import seaborn as sns
ax = sns.violinplot(data=[cx_counts])
What should happen?
Acc to level 3 preset , there should be 20 swap_trials performed for each iteration of transpile. So, the performance should be improved by these 20 iterations. Instead, there is no visible difference, but doing 20 iterations of transpile manually in qiskit 0.21.1 performs much better (will take about 60mins on a 4core-CPU):
global_circ_list=[]
for i in range(1000):
# Transpile the circuit 'qc' 20 times
trans_circ_list_loop = transpile([qc]*20, backend, optimization_level=3)
# get the number of cnot gates
cx_counts_loop = np.asarray([circ.count_ops().get('cx', 0) for circ in trans_circ_list_loop])
#print('Number of cnots:', cx_counts_loop)
# select the circuit with the lowest number
best_idx_loop = np.argmin(cx_counts_loop)
best_trans_qc_loop = trans_circ_list_loop[best_idx_loop]
global_circ_list.append(best_trans_qc_loop)
cx_counts_global = np.asarray([circ.count_ops().get('cx', 0) for circ in global_circ_list])
combining all in the plot shows that manually pre-transpiling in the older qiskit version performs much better
Any suggestions?
there might be a minimization step missing, or a mixup of max_iterations and swap_trials in
https://github.com/Qiskit/qiskit-terra/blob/90b158c7e02432db957762e58c4c2ed75d89a1ca/qiskit/transpiler/preset_passmanagers/level3.py#L140