Use OnceLock instead of OnceCell #13410
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OnceLock is a thread-safe version of OnceCell that enables us to use PackedInstruction from a threaded environment. There is some overhead associated with this, primarily in memory as the OnceLock is a larger type than a OnceCell. But the tradeoff is worth it to start leverage multithreading for circuits. Fixes Qiskit#13219
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With Qiskit#13410 removing the non-threadsafe structure from our circuit representation we're now able to read and iterate over a DAGCircuit from multiple threads. This commit is the first small piece doing this, it moves the analysis portion of the BarrierBeforeFinalMeasurements pass to execure in parallel. The pass checks every node to ensure all it's decendents are either a measure or a barrier before reaching the end of the circuit. This commit iterates over all the nodes and does the check in parallel.
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kevinhartman
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After 8bc4844 merged fixing the asv tests I did a quick asv run which yielded basically no change: This is mostly expected because we only use the py gate cache if when we're interacting with Python which will always end up being the bottleneck. The exception I guess is |
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* Use OnceLock instead of OnceCell OnceLock is a thread-safe version of OnceCell that enables us to use PackedInstruction from a threaded environment. There is some overhead associated with this, primarily in memory as the OnceLock is a larger type than a OnceCell. But the tradeoff is worth it to start leverage multithreading for circuits. Fixes #13219 * Update twirling too * Perform BarrierBeforeFinalMeasurements analysis in paralle With #13410 removing the non-threadsafe structure from our circuit representation we're now able to read and iterate over a DAGCircuit from multiple threads. This commit is the first small piece doing this, it moves the analysis portion of the BarrierBeforeFinalMeasurements pass to execure in parallel. The pass checks every node to ensure all it's decendents are either a measure or a barrier before reaching the end of the circuit. This commit iterates over all the nodes and does the check in parallel. * Remove allocation for node scan * Refactor pass to optimize search and set parallel threshold This commit updates the logic in the pass to simplify the search algorithm and improve it's overall efficiency. Previously the pass would search the entire dag for all barrier and measurements and then did a BFS from each found node to check that all descendants are either barriers or measurements. Then with the set of nodes matching that condition a full topological sort of the dag was run, then the topologically ordered nodes were filtered for the matching set. That sorted set is then used for filtering This commit refactors this to do a reverse search from the output nodes which reduces the complexity of the algorithm. This new algorithm is also conducive for parallel execution because it does a search starting from each qubit's output node. Doing a test with a quantum volume circuit from 10 to 1000 qubits which scales linearly in depth and number of qubits a crossover point between the parallel and serial implementations was found around 150 qubits. * Update crates/circuit/src/dag_circuit.rs Co-authored-by: Raynel Sanchez <[email protected]> * Rework logic to check using StandardInstruction * Add comments explaining the search function * Update crates/circuit/src/dag_circuit.rs Co-authored-by: Raynel Sanchez <[email protected]> --------- Co-authored-by: Raynel Sanchez <[email protected]>
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Summary
OnceLock is a thread-safe version of OnceCell that enables us to use PackedInstruction from a threaded environment. There is some overhead associated with this, primarily in memory as the OnceLock is a larger type than a OnceCell. But the tradeoff is worth it to start leverage multithreading for circuits.
Details and comments
Fixes #13219