dotnet-parallel

Development

.NET 병렬 처리 패턴 (Parallel, PLINQ, ConcurrentCollections)

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.NET 병렬 처리

CPU 바운드 작업의 병렬 처리를 위한 API 및 패턴 가이드입니다.

1. 핵심 API

API	용도
`Parallel.For`, `Parallel.ForEach`	CPU 바운드 병렬 처리
`PLINQ (.AsParallel())`	LINQ 쿼리 병렬화
`Partitioner<T>`	대용량 데이터 파티셔닝
`ConcurrentDictionary<K,V>`	Thread-safe 딕셔너리

2. Parallel 클래스

2.1 Parallel.ForEach

public sealed class ImageProcessor
{
    public void ProcessImages(IEnumerable<string> imagePaths)
    {
        var options = new ParallelOptions
        {
            MaxDegreeOfParallelism = Environment.ProcessorCount
        };

        Parallel.ForEach(imagePaths, options, path =>
        {
            ProcessImage(path);
        });
    }
}

2.2 조기 중단

Parallel.For(0, data.Length, (i, state) =>
{
    if (data[i] == target)
    {
        state.Break();
    }
});

3. PLINQ

var results = data
    .AsParallel()
    .WithDegreeOfParallelism(Environment.ProcessorCount)
    .Where(d => d.IsValid)
    .Select(d => Transform(d))
    .ToList();

// 순서 유지 필요 시
var results = data
    .AsParallel()
    .AsOrdered()
    .Select(d => Process(d))
    .ToList();

4. Thread-Safe 컬렉션

컬렉션	용도
`ConcurrentDictionary<K,V>`	Thread-safe 딕셔너리
`ConcurrentQueue<T>`	Thread-safe FIFO 큐
`ConcurrentBag<T>`	Thread-safe 순서 없는 컬렉션

// 병렬 처리 중 결과 수집
var results = new ConcurrentBag<Result>();

Parallel.ForEach(data, item =>
{
    var result = Process(item);
    results.Add(result);
});

5. ThreadLocal

스레드별 독립 변수로 경합 방지

private readonly ThreadLocal<StringBuilder> _localBuilder =
    new(() => new StringBuilder());

public void ProcessInParallel()
{
    Parallel.For(0, 1000, i =>
    {
        var sb = _localBuilder.Value!;
        sb.Clear();
        sb.Append(i);
    });
}

6. 주의사항

⚠️ 병렬 처리 vs 비동기

CPU 바운드: Parallel 사용
I/O 바운드: async-await 사용

// ❌ I/O 작업에 Parallel 사용
Parallel.ForEach(urls, url => httpClient.GetAsync(url).Result);

// ✅ I/O 작업에 async-await 사용
await Task.WhenAll(urls.Select(url => httpClient.GetAsync(url)));

⚠️ 공유 상태 동기화

// ❌ 일반 컬렉션에 병렬 쓰기
var list = new List<int>();
Parallel.For(0, 1000, i => list.Add(i)); // Race condition!

// ✅ Thread-safe 컬렉션 사용
var bag = new ConcurrentBag<int>();
Parallel.For(0, 1000, i => bag.Add(i));