ios - Swift 并发操作慢 2 倍

Question

我有一个大的 JSON 数组需要保存到 Realm，问题是这个操作持续大约 45 秒而且太长了。我尝试像这样为 JSON 数组中的每个元素同时运行保存操作:

for element in jsonArray { // jsonArray has about 25 elements
  DispatchQueue.global(qos: .userInitiated).async {
    let realm = try! Realm()
    let savedObject = realm.objects(MyObject.self).filter("name == '\(element.name)'")

    for subElement in element { // element is an array that has around 1000 elements
      let myModel = MyModel(initWith: subElement) // MyModel initialization is a simple light weight process that copies values from one model to another
      savedObject.models.append(myModel)
    }

  }
}

当我尝试运行相同的代码但使用 DispatchQueue.main.async 时，即使它不是并发的，它的完成速度也快了大约 2 倍。我还尝试以服务质量 .userInteractive 运行上面的代码，但速度相同。

当我运行这段代码时，CPU 利用率约为 30%，内存约为 45 MB。是否有可能加快此操作或我到达了死胡同？

Answer 1

1. 整个循环应该在 DispatchQueue.global(qos: .userInitiated).async block 内。

2. 如 the Realm website 中所述:

   Realm write operations are synchronous and blocking, not asynchronous. If thread A starts a write operation, then thread B starts a write operation on the same Realm before thread A is finished, thread A must finish and commit its transaction before thread B’s write operation takes place. Write operations always refresh automatically on beginWrite(), so no race condition is created by overlapping writes.

   这意味着尝试在多线程中编写不会获得任何好处。