Question

我正在尝试保存压力图的流数据。基本上，我的压力矩阵定义为：

double[,] pressureMatrix = new double[e.Data.GetLength(0), e.Data.GetLength(1)];

基本上，我每10毫秒就会收到一个pressureMatrix，我想将所有信息保存在JSON文件中以便以后重现。

首先，我要做的就是用所有用于记录的设置来写我所谓的标题：

recordedData.softwareVersion = Assembly.GetExecutingAssembly().GetName().Version.Major.ToString() + "." + Assembly.GetExecutingAssembly().GetName().Version.Minor.ToString();
recordedData.calibrationConfiguration = calibrationConfiguration;
recordedData.representationConfiguration = representationSettings;
recordedData.pressureData = new List<PressureMap>();

var json = JsonConvert.SerializeObject(csvRecordedData, Formatting.None);

File.WriteAllText(this.filePath, json);

然后，每次获得新的压力图时，我都会创建一个新线程以添加新的PressureMatrix并重新编写文件：

var newPressureMatrix = new PressureMap(datos, DateTime.Now);
recordedData.pressureData.Add(newPressureMatrix);
var json = JsonConvert.SerializeObject(recordedData, Formatting.None);
File.WriteAllText(this.filePath, json);

大约20-30分钟后，我收到OutOfMemory异常，因为系统无法保存 recordedData 变量，因为其中的List<PressureMatrix>太大。

我该如何处理以保存数据？我想保存24-48小时的信息。

Answer 1

您的基本问题是，您将所有压力图样本保存在内存中，而不是分别编写每个样本，然后将其垃圾回收。更糟糕的是，您在两个不同的地方这样做：

您需要先将整个样本列表序列化为JSON字符串json，然后再将其写入文件。

相反，如Performance Tips: Optimize Memory Usage中所述，在这种情况下，应直接对文件进行序列化和反序列化。有关如何执行此操作的说明，请参见this answer至 Can Json.NET serialize / deserialize to / from a stream? 以及Serialize JSON to a file。
recordedData.pressureData = new List<PressureMap>();会累加所有压力图样本，然后在每次创建样本时都写出所有这些。

一个更好的解决方案是将每个样本编写一次而忘记，但是每个样本都必须嵌套在JSON中的某些容器对象内的要求使得如何做到这一点变得不明显。

那么，如何应对问题2？

首先，让我们如下修改数据模型，将标头数据划分为单独的类：

public class PressureMap
{
    public double[,] PressureMatrix { get; set; }
}

public class CalibrationConfiguration 
{
    // Data model not included in question
}

public class RepresentationConfiguration 
{
    // Data model not included in question
}

public class RecordedDataHeader
{
    public string SoftwareVersion { get; set; }
    public CalibrationConfiguration CalibrationConfiguration { get; set; }
    public RepresentationConfiguration RepresentationConfiguration { get; set; }
}

public class RecordedData
{
    // Ensure the header is serialized first.
    [JsonProperty(Order = 1)]
    public RecordedDataHeader RecordedDataHeader { get; set; }
    // Ensure the pressure data is serialized last.
    [JsonProperty(Order = 2)]
    public IEnumerable<PressureMap> PressureData { get; set; }
}

选项＃1 是producer-comsumer pattern的版本。它涉及两个线程：一个用于生成PressureData样本，另一个用于序列化RecordedData。第一个线程将生成样本，并将其添加到传递到第二个线程的BlockingCollection<PressureMap>集合中。然后，第二个线程将序列化BlockingCollection<PressureMap>.GetConsumingEnumerable() 作为RecordedData.PressureData的值。

以下代码为执行此操作提供了框架：

var sampleCount = 400;    // Or whatever stopping criterion you prefer
var sampleInterval = 10;  // in ms

using (var pressureData = new BlockingCollection<PressureMap>())
{
    // Adapted from
    // https://docs.microsoft.com/en-us/dotnet/standard/collections/thread-safe/blockingcollection-overview
    // https://docs.microsoft.com/en-us/dotnet/api/system.collections.concurrent.blockingcollection-1?view=netframework-4.7.2

    // Spin up a Task to sample the pressure maps
    using (Task t1 = Task.Factory.StartNew(() =>
    {
        for (int i = 0; i < sampleCount; i++)
        {
            var data = GetPressureMap(i);
            Console.WriteLine("Generated sample {0}", i);
            pressureData.Add(data);
            System.Threading.Thread.Sleep(sampleInterval);
        }
        pressureData.CompleteAdding();
    }))
    {
        // Spin up a Task to consume the BlockingCollection
        using (Task t2 = Task.Factory.StartNew(() =>
        {
            var recordedDataHeader = new RecordedDataHeader
            {
                SoftwareVersion = softwareVersion,
                CalibrationConfiguration = calibrationConfiguration,
                RepresentationConfiguration = representationConfiguration,
            };

            var settings = new JsonSerializerSettings
            {
                ContractResolver = new CamelCasePropertyNamesContractResolver(),
            };

            using (var stream = new FileStream(this.filePath, FileMode.Create))
            using (var textWriter = new StreamWriter(stream))
            using (var jsonWriter = new JsonTextWriter(textWriter))
            {
                int j = 0;

                var query = pressureData
                    .GetConsumingEnumerable()
                    .Select(p => 
                            { 
                                // Flush the writer periodically in case the process terminates abnormally
                                jsonWriter.Flush();
                                Console.WriteLine("Serializing item {0}", j++);
                                return p;
                            });

                var recordedData = new RecordedData
                {
                    RecordedDataHeader = recordedDataHeader,
                    // Since PressureData is declared as IEnumerable<PressureMap>, evaluation will be lazy.
                    PressureData = query,
                };                          

                Console.WriteLine("Beginning serialization of {0} to {1}:", recordedData, this.filePath);
                JsonSerializer.CreateDefault(settings).Serialize(textWriter, recordedData);
                Console.WriteLine("Finished serialization of {0} to {1}.", recordedData, this.filePath);
            }
        }))
        {
            Task.WaitAll(t1, t2);
        }
    }
}

注意：

此解决方案使用以下事实：序列化IEnumerable<T>时，Json.NET将不具体化为列表。取而代之的是，它将充分利用惰性评估，并简单地枚举它，编写然后忘记遇到的每个项目。
第一个线程对PressureData进行采样，并将它们添加到阻塞集合中。
第二个线程将阻塞集合包装在IEnumerable<PressureData>中，然后将其序列化为RecordedData.PressureData。

序列化期间，序列化程序将通过IEnumerable<PressureData>枚举枚举，将每个流传输到JSON文件，然后继续进行下一个-有效地阻塞直到一个可用。
您将需要进行一些实验，以确保序列化线程可以“跟上”采样线程，这可能是通过在构造过程中设置BoundedCapacity来实现的。如果没有，您可能需要采用其他策略。
PressureMap GetPressureMap(int count)应该是您的某种方法（问题中未显示），该方法可以返回当前压力图样本。
在此技术中，JSON文件在采样会话期间保持打开状态。如果采样异常终止，则文件可能会被截断。我尝试通过定期刷新编写器来缓解此问题。
虽然数据序列化将不再需要无限制的内存量，但稍后反序列化RecordedData会将PressureData数组反序列化为具体的List<PressureMap>。这可能会在下游处理期间导致内存问题。

演示小提琴＃1 here。

选项＃2 是将JSON文件切换为Newline Delimited JSON文件。这样的文件由用换行符分隔的JSON对象序列组成。在您的情况下，您将使第一个对象包含RecordedDataHeader信息，随后的对象为PressureMap类型：

var sampleCount = 100; // Or whatever
var sampleInterval = 10;

var recordedDataHeader = new RecordedDataHeader
{
    SoftwareVersion = softwareVersion,
    CalibrationConfiguration = calibrationConfiguration,
    RepresentationConfiguration = representationConfiguration,
};

var settings = new JsonSerializerSettings
{
    ContractResolver = new CamelCasePropertyNamesContractResolver(),
};

// Write the header
Console.WriteLine("Beginning serialization of sample data to {0}.", this.filePath);

using (var stream = new FileStream(this.filePath, FileMode.Create))
{
    JsonExtensions.ToNewlineDelimitedJson(stream, new[] { recordedDataHeader });
}

// Write each sample incrementally

for (int i = 0; i < sampleCount; i++)
{
    Thread.Sleep(sampleInterval);
    Console.WriteLine("Performing sample {0} of {1}", i, sampleCount);
    var map = GetPressureMap(i);

    using (var stream = new FileStream(this.filePath, FileMode.Append))
    {
        JsonExtensions.ToNewlineDelimitedJson(stream, new[] { map });
    }
}

Console.WriteLine("Finished serialization of sample data to {0}.", this.filePath);

使用扩展方法：

public static partial class JsonExtensions
{
    // Adapted from the answer to
    // https://stackoverflow.com/questions/44787652/serialize-as-ndjson-using-json-net
    // by dbc https://stackoverflow.com/users/3744182/dbc
    public static void ToNewlineDelimitedJson<T>(Stream stream, IEnumerable<T> items)
    {
        // Let caller dispose the underlying stream 
        using (var textWriter = new StreamWriter(stream, new UTF8Encoding(false, true), 1024, true))
        {
            ToNewlineDelimitedJson(textWriter, items);
        }
    }

    public static void ToNewlineDelimitedJson<T>(TextWriter textWriter, IEnumerable<T> items)
    {
        var serializer = JsonSerializer.CreateDefault();

        foreach (var item in items)
        {
            // Formatting.None is the default; I set it here for clarity.
            using (var writer = new JsonTextWriter(textWriter) { Formatting = Formatting.None, CloseOutput = false })
            {
                serializer.Serialize(writer, item);
            }
            // http://specs.okfnlabs.org/ndjson/
            // Each JSON text MUST conform to the [RFC7159] standard and MUST be written to the stream followed by the newline character \n (0x0A). 
            // The newline charater MAY be preceeded by a carriage return \r (0x0D). The JSON texts MUST NOT contain newlines or carriage returns.
            textWriter.Write("\n");
        }
    }

    // Adapted from the answer to 
    // https://stackoverflow.com/questions/29729063/line-delimited-json-serializing-and-de-serializing
    // by Yuval Itzchakov https://stackoverflow.com/users/1870803/yuval-itzchakov
    public static IEnumerable<TBase> FromNewlineDelimitedJson<TBase, THeader, TRow>(TextReader reader)
        where THeader : TBase
        where TRow : TBase
    {
        bool first = true;

        using (var jsonReader = new JsonTextReader(reader) { CloseInput = false, SupportMultipleContent = true })
        {
            var serializer = JsonSerializer.CreateDefault();

            while (jsonReader.Read())
            {
                if (jsonReader.TokenType == JsonToken.Comment)
                    continue;
                if (first)
                {
                    yield return serializer.Deserialize<THeader>(jsonReader);
                    first = false;
                }
                else
                {
                    yield return serializer.Deserialize<TRow>(jsonReader);
                }
            }
        }
    }
}

稍后，您可以按以下方式处理换行符分隔的JSON文件：

using (var stream = File.OpenRead(filePath))
using (var textReader = new StreamReader(stream))
{
    foreach (var obj in JsonExtensions.FromNewlineDelimitedJson<object, RecordedDataHeader, PressureMap>(textReader))
    {
        if (obj is RecordedDataHeader)
        {
            var header = (RecordedDataHeader)obj;
            // Process the header
            Console.WriteLine(JsonConvert.SerializeObject(header));
        }
        else
        {
            var row = (PressureMap)obj;
            // Process the row.
            Console.WriteLine(JsonConvert.SerializeObject(row));
        }
    }
}

注意：

这种方法看起来更简单，因为样本是逐步添加到文件末尾的，而不是插入到整个JSON容器中。
使用这种方法，可以使用有限的内存来完成序列化和下游处理。
示例文件在采样期间不会保持打开状态，因此被截断的可能性较小。
下游应用程序可能没有内置工具来处理换行符分隔的JSON。
此策略可以更简单地与您当前的线程代码集成。

演示小提琴＃2 here。

C＃-OutOfMemoryException将列表保存在JSON文件中

1 个答案: