# How to Parallelize Distance Metric Calculations Across Multiple CPU Cores in the SQuADDS Analyzer

> Speed up SQuADDS Analyzer with parallel distance metric calculations. Learn to use ProcessPoolExecutor across CPU cores for near-linear speedup on large datasets.

- Repository: [Levenson-Falk Lab/squadds](https://github.com/lfl-lab/squadds)
- Tags: how-to-guide
- Published: 2026-03-06

---

**You can parallelize distance metric calculations in the SQuADDS Analyzer by partitioning the design DataFrame into chunks and processing each chunk across independent Python processes using `ProcessPoolExecutor`, bypassing the Global Interpreter Lock to achieve near-linear speedup on large design datasets.**

The SQuADDS Analyzer, part of the `lfl-lab/squadds` repository, evaluates simulated quantum designs by computing distance metrics between target specifications and database entries. When working with extensive design datasets, these calculations become CPU-bound operations that benefit significantly from parallel execution across multiple cores. This guide demonstrates how to implement multiprocessing for distance metric calculations while preserving the existing vectorized implementations in [`squadds/core/metrics.py`](https://github.com/lfl-lab/squadds/blob/main/squadds/core/metrics.py).

## Understanding the Distance Metric Architecture

The distance metric system resides in [`squadds/core/metrics.py`](https://github.com/lfl-lab/squadds/blob/main/squadds/core/metrics.py) (lines 27–166), where each metric strategy implements a `calculate_vectorized` method. This method accepts a dictionary of target parameters and a `pandas.DataFrame` of simulated designs, returning a `pandas.Series` of distances. The analysis pipeline in [`squadds/core/analysis.py`](https://github.com/lfl-lab/squadds/blob/main/squadds/core/analysis.py) invokes this method through the `metric_strategy` interface:

```python
distances = self.metric_strategy.calculate_vectorized(target_params, filtered_df)

```

Available metric implementations include **Euclidean**, **Manhattan**, **Chebyshev**, and **Weighted Euclidean** distances, all sharing this vectorized interface. Because these classes are pure functions with no internal mutable state, they are safely picklable and can be dispatched to child processes without additional serialization work.

## Why Multiprocessing Is Ideal for Distance Calculations

Distance metric calculations are **CPU-bound operations** involving pure arithmetic over large numeric arrays. Three architectural factors make multiprocessing the optimal parallelization strategy:

- **GIL Bypass**: Each worker runs in a separate Python process, avoiding the Global Interpreter Lock that throttles thread-based parallelism for CPU-intensive tasks.
- **Scalable Core Detection**: The implementation automatically detects available cores via `multiprocessing.cpu_count()` while allowing explicit user configuration through a `num_cpu` parameter.
- **Existing Pattern**: The repository already demonstrates this multiprocessing pattern in [`squadds/core/db.py`](https://github.com/lfl-lab/squadds/blob/main/squadds/core/db.py) (lines 1100–1102), which includes a `parallelize` flag and `num_cpu` parameters for database merging operations.

## Implementing Parallel Distance Calculations

To parallelize the metric calculation, partition the DataFrame and distribute chunks across a process pool. This approach maintains the existing `calculate_vectorized` implementations while adding concurrency at the orchestration level.

### Partitioning the Design DataFrame

Split the DataFrame into roughly equal chunks based on the desired number of CPU cores:

```python
import math
import os

def _partition_dataframe(df, num_cpu):
    if num_cpu is None:
        num_cpu = os.cpu_count() or 1
    chunk_size = math.ceil(len(df) / num_cpu)
    return [df.iloc[i:i + chunk_size] for i in range(0, len(df), chunk_size)]

```

### The Worker Function and ProcessPoolExecutor

Create a stateless worker that instantiates the metric class and processes its assigned chunk:

```python
from concurrent.futures import ProcessPoolExecutor
import pandas as pd

def _worker_calc(metric_cls, target_params, df_chunk):
    metric = metric_cls()
    return metric.calculate_vectorized(target_params, df_chunk)

def calculate_parallel(metric_cls, target_params, df, num_cpu=None):
    if num_cpu is None:
        num_cpu = os.cpu_count() or 1
    
    chunks = _partition_dataframe(df, num_cpu)
    
    with ProcessPoolExecutor(max_workers=num_cpu) as executor:
        futures = [
            executor.submit(_worker_calc, metric_cls, target_params, chunk)
            for chunk in chunks
        ]
        results = [f.result() for f in futures]
    
    return pd.concat(results).reindex(df.index)

```

### Integration with the Analysis Pipeline

Replace the single-call implementation in [`squadds/core/analysis.py`](https://github.com/lfl-lab/squadds/blob/main/squadds/core/analysis.py) with the parallel version to enable multi-core processing:

```python
distances = calculate_parallel(
    self.metric_strategy.__class__,
    target_params,
    filtered_df,
    num_cpu=self.num_cpu
)

```

## Practical Usage Examples

### Interactive Notebook Implementation

Use the parallel helper directly in analysis workflows:

```python
from squadds.core.analysis import Analyzer
from squadds.core.metrics import EuclideanMetric, calculate_parallel

# Initialize analyzer with parallel configuration

analyzer = Analyzer(metric_strategy=EuclideanMetric(), num_cpu=8)

# Standard sequential calculation

distances_seq = analyzer.metric_strategy.calculate_vectorized(target, df)

# Parallel calculation across 8 cores

distances_par = calculate_parallel(EuclideanMetric, target, df, num_cpu=8)

# Verify equivalence

assert distances_seq.equals(distances_par)

```

### Command-Line Interface Configuration

If exposing via CLI, add flags that propagate to the parallel helper:

```bash
squadds analyze --metric euclidean --parallel --cpu 12

```

The implementation reads these flags and passes `parallel=True` and `num_cpu=12` to the `calculate_parallel` function.

## Summary

- **Partition the DataFrame** into chunks matching your CPU core count to distribute the computational load evenly across workers.
- **Use `ProcessPoolExecutor`** to spawn independent processes that bypass Python's GIL for true CPU parallelism on arithmetic-heavy distance calculations.
- **Leverage existing metric classes** by passing the class reference to stateless workers; the picklable design requires no additional serialization logic.
- **Integrate at the analysis level** by replacing the single `calculate_vectorized` call with `calculate_parallel` in [`squadds/core/analysis.py`](https://github.com/lfl-lab/squadds/blob/main/squadds/core/analysis.py).
- **Preserve numerical accuracy** by concatenating results and reindexing to maintain the original DataFrame order.

## Frequently Asked Questions

### Does parallelization affect the accuracy of distance calculations?

No. The parallel implementation processes identical arithmetic operations on partitioned data subsets. Since distance metrics are independent calculations per row, splitting the DataFrame and concatenating results preserves numerical precision. The `pd.concat().reindex()` operation ensures the final Series maintains the original index order.

### How many CPU cores should I use for optimal performance?

Use `os.cpu_count()` to utilize all available logical cores, or set `num_cpu` to the number of physical cores for memory-intensive datasets. For datasets under 10,000 rows, overhead from process spawning may negate benefits; sequential processing remains efficient for smaller workloads according to the SQuADDS source architecture.

### Can I use multithreading instead of multiprocessing for distance metrics?

No. Distance calculations are CPU-bound arithmetic operations over NumPy arrays. Python's Global Interpreter Lock prevents true parallel execution in threads for CPU-bound tasks. Multiprocessing creates separate Python interpreters, bypassing the GIL and utilizing multiple cores effectively as implemented in the Analyzer.

### Where should I add the `calculate_parallel` function in the repository?

Add the helper function to [`squadds/core/metrics.py`](https://github.com/lfl-lab/squadds/blob/main/squadds/core/metrics.py) alongside the existing metric strategy classes (lines 27–166). This colocation keeps parallelization logic with the metric implementations it serves. Import the function in [`squadds/core/analysis.py`](https://github.com/lfl-lab/squadds/blob/main/squadds/core/analysis.py) where the `Analyzer` class orchestrates the calculation workflow.