异步I / O密集​​型code上比非异步慢，为什么呢？ [英] Async I/O intensive code is running slower than non-async, why?

问题描述

我重构一个应用程序，并尝试添加现有功能的异步版本来提高性能倍ASP.NET MVC应用程序。据我所知，有涉及异步功能的开销，但我预计，有足够的迭代，装载从数据库中的数据的I / O密集​​型的性质将超过补偿开销罚款，我会接受显著的性能提升。

I am refactoring an application and trying to add an asynchronous version of an existing function to improve performance times in an ASP.NET MVC application. I understand that there is an overhead involved with asynchronous functions, but I expected that with enough iterations, the I/O intensive nature of loading the data from the database would more than compensate for the overhead penalty and that I would receive significant performance gains.

通过（使用ADO.NET和Oracle托管客户端）从数据库中检索一堆数据表的 TermusRepository.LoadByTermusId 函数加载数据，填充一个模型，然后返回它。 TermusRepository.LoadByTermusIdAsync 是相似的，除了它异步，与装载了一个稍微不同的方式数据表下载任务时，有多个数据表来检索。

The TermusRepository.LoadByTermusId function loads data by retrieving a bunch of datatables from the database (using ADO.NET and the Oracle Managed Client), populates a model, and returns it. TermusRepository.LoadByTermusIdAsync is similar, except it does so asynchronously, with a slightly different method of loading up datatable download tasks when there's multiple datatables to retrieve.

public async Task<ActionResult> AsyncPerformanceTest()
{
    var vm = new AsyncPerformanceTestViewModel();
    Stopwatch watch = new Stopwatch();
    watch.Start();
    for (int i = 0; i < 60; i++)
    {
        TermusRepository.LoadByTermusId<Termus2011_2012EndYear>("1");
        TermusRepository.LoadByTermusId<Termus2011_2012EndYear>("5");
        TermusRepository.LoadByTermusId<Termus2011_2012EndYear>("6");
        TermusRepository.LoadByTermusId<Termus2011_2012EndYear>("7");
    }
    watch.Stop();
    vm.NonAsyncElapsedTime = watch.Elapsed;
    watch.Reset();
    watch.Start();
    var tasks = new List<Task<Termus2011_2012EndYear>>();
    for (int i = 0; i < 60; i++)
    {
        tasks.Add(TermusRepository.LoadByTermusIdAsync<Termus2011_2012EndYear>("1"));
        tasks.Add(TermusRepository.LoadByTermusIdAsync<Termus2011_2012EndYear>("5"));
        tasks.Add(TermusRepository.LoadByTermusIdAsync<Termus2011_2012EndYear>("6"));
        tasks.Add(TermusRepository.LoadByTermusIdAsync<Termus2011_2012EndYear>("7"));               
    }
    await Task.WhenAll(tasks.ToArray());
    watch.Stop();
    vm.AsyncElapsedTime = watch.Elapsed;            
    return View(vm);
}

public static async Task<T> LoadByTermusIdAsync<T>(string termusId) where T : Appraisal
{
    var AppraisalHeader = new OracleCommand("select tu.termus_id, tu.manager_username, tu.evaluee_name, tu.evaluee_username, tu.termus_complete_date, termus_start_date, tu.termus_status, tu.termus_version, tn.managername from tercons.termus_users tu left outer join tercons.termus_names tn on tu.termus_id=tn.termus_id where tu.termus_id=:termusid");
    AppraisalHeader.BindByName = true;
    AppraisalHeader.Parameters.Add("termusid", termusId);
    var dt = await Database.GetDataTableAsync(AppraisalHeader);
    T Termus = Activator.CreateInstance<T>();
    var row = dt.AsEnumerable().Single();
    Termus.TermusId = row.Field<decimal>("termus_id").ToString();
    Termus.ManagerUsername = row.Field<string>("manager_username");
    Termus.EvalueeUsername = row.Field<string>("evaluee_username");
    Termus.EvalueeName = row.Field<string>("evaluee_name");
    Termus.ManagerName = row.Field<string>("managername");
    Termus.TERMUSCompleteDate = row.Field<DateTime?>("termus_complete_date");
    Termus.TERMUSStartDate = row.Field<DateTime>("termus_start_date");
    Termus.Status = row.Field<string>("termus_status");
    Termus.TERMUSVersion = row.Field<string>("termus_version");
    Termus.QuestionsAndAnswers = new Dictionary<string, string>();

    var RetrieveQuestionIdsCommand = new OracleCommand("select termus_question_id from tercons.termus_questions where termus_version=:termus_version");
    RetrieveQuestionIdsCommand.BindByName = true;
    RetrieveQuestionIdsCommand.Parameters.Add("termus_version", Termus.TERMUSVersion);
    var QuestionIdsDt = await Database.GetDataTableAsync(RetrieveQuestionIdsCommand);
    var QuestionIds = QuestionIdsDt.AsEnumerable().Select(r => r.Field<string>("termus_question_id"));

    //There's about 60 questions/answers, so this should result in 60 calls to the database. It'd be a good spot to combine to a single DB call, but left it this way so I could see if async would speed it up for learning purposes.
    var DownloadAnswersTasks = new List<Task<DataTable>>();
    foreach (var QuestionId in QuestionIds)
    {
        var RetrieveAnswerCommand = new OracleCommand("select termus_response, termus_question_id from tercons.termus_responses where termus_id=:termus_id and termus_question_id=:questionid");
        RetrieveAnswerCommand.BindByName = true;
        RetrieveAnswerCommand.Parameters.Add("termus_id", termusId);
        RetrieveAnswerCommand.Parameters.Add("questionid", QuestionId);
        DownloadAnswersTasks.Add(Database.GetDataTableAsync(RetrieveAnswerCommand));
    }
    while (DownloadAnswersTasks.Count > 0)
    {
        var FinishedDownloadAnswerTask = await Task.WhenAny(DownloadAnswersTasks);
        DownloadAnswersTasks.Remove(FinishedDownloadAnswerTask);
        var AnswerDt = await FinishedDownloadAnswerTask;
        var Answer = AnswerDt.AsEnumerable().Select(r => r.Field<string>("termus_response")).SingleOrDefault();
        var QuestionId = AnswerDt.AsEnumerable().Select(r => r.Field<string>("termus_question_id")).SingleOrDefault();
        if (!String.IsNullOrEmpty(Answer))
        {
            Termus.QuestionsAndAnswers.Add(QuestionId, System.Net.WebUtility.HtmlDecode(Answer));
        }
    }
    return Termus;
}

public static async Task<DataTable> GetDataTableAsync(OracleCommand command)
{
    DataTable dt = new DataTable();
    using (var connection = GetDefaultOracleConnection())
    {
        command.Connection = connection;
        await connection.OpenAsync();
        dt.Load(await command.ExecuteReaderAsync());
    }
    return dt;
}

public static T LoadByTermusId<T>(string TermusId) where T : Appraisal
{
    var RetrieveAppraisalHeaderCommand = new OracleCommand("select tu.termus_id, tu.manager_username, tu.evaluee_name, tu.evaluee_username, tu.termus_complete_date, termus_start_date, tu.termus_status, tu.termus_version, tn.managername from tercons.termus_users tu left outer join tercons.termus_names tn on tu.termus_id=tn.termus_id where tu.termus_id=:termusid");
    RetrieveAppraisalHeaderCommand.BindByName = true;
    RetrieveAppraisalHeaderCommand.Parameters.Add("termusid", TermusId);
    var AppraisalHeaderDt = Database.GetDataTable(RetrieveAppraisalHeaderCommand);
    T Termus = Activator.CreateInstance<T>();
    var AppraisalHeaderRow = AppraisalHeaderDt.AsEnumerable().Single();
    Termus.TermusId = AppraisalHeaderRow.Field<decimal>("termus_id").ToString();
    Termus.ManagerUsername = AppraisalHeaderRow.Field<string>("manager_username");
    Termus.EvalueeUsername = AppraisalHeaderRow.Field<string>("evaluee_username");
    Termus.EvalueeName = AppraisalHeaderRow.Field<string>("evaluee_name");
    Termus.ManagerName = AppraisalHeaderRow.Field<string>("managername");
    Termus.TERMUSCompleteDate = AppraisalHeaderRow.Field<DateTime?>("termus_complete_date");
    Termus.TERMUSStartDate = AppraisalHeaderRow.Field<DateTime>("termus_start_date");
    Termus.Status = AppraisalHeaderRow.Field<string>("termus_status");
    Termus.TERMUSVersion = AppraisalHeaderRow.Field<string>("termus_version");
    Termus.QuestionsAndAnswers = new Dictionary<string, string>();

    var RetrieveQuestionIdsCommand = new OracleCommand("select termus_question_id from tercons.termus_questions where termus_version=:termus_version");
    RetrieveQuestionIdsCommand.BindByName = true;
    RetrieveQuestionIdsCommand.Parameters.Add("termus_version", Termus.TERMUSVersion);
    var QuestionIdsDt = Database.GetDataTable(RetrieveQuestionIdsCommand);
    var QuestionIds = QuestionIdsDt.AsEnumerable().Select(r => r.Field<string>("termus_question_id"));
    //There's about 60 questions/answers, so this should result in 60 calls to the database. It'd be a good spot to combine to a single DB call, but left it this way so I could see if async would speed it up for learning purposes.
    foreach (var QuestionId in QuestionIds)
    {
        var RetrieveAnswersCommand = new OracleCommand("select termus_response from tercons.termus_responses where termus_id=:termus_id and termus_question_id=:questionid");
        RetrieveAnswersCommand.BindByName = true;
        RetrieveAnswersCommand.Parameters.Add("termus_id", TermusId);
        RetrieveAnswersCommand.Parameters.Add("questionid", QuestionId);
        var AnswersDt = Database.GetDataTable(RetrieveAnswersCommand);
        var Answer = AnswersDt.AsEnumerable().Select(r => r.Field<string>("termus_response")).SingleOrDefault();
        if (!String.IsNullOrEmpty(Answer))
        {
            Termus.QuestionsAndAnswers.Add(QuestionId, System.Net.WebUtility.HtmlDecode(Answer));
        }
    }
    return Termus;
}

public static DataTable GetDataTable(OracleCommand command)
{
    DataTable dt = new DataTable();
    using (var connection = GetDefaultOracleConnection())
    {
        command.Connection = connection;
        connection.Open();
        dt.Load(command.ExecuteReader());
    }
    return dt;
}

public static OracleConnection GetDefaultOracleConnection()
{
    return new OracleConnection(ConfigurationManager.ConnectionStrings[connectionstringname].ConnectionString);
}

结果60次迭代：

Results for 60 iterations are:

Non Async 18.4375460 seconds

Async     19.8092854 seconds

本试验的结果是一致的。无论我多少次迭代经过的for循环 AsyncPerformanceTest（）的操作方法，异步运行的东西约1秒比非异步慢。 （我运行测试多次连续占抖动热身。）我在做什么错误是造成异步比非异步慢？我误解基本的东西有关编写异步code？

The results of this test are consistent. No matter how many iterations I go through of the for loop in AsyncPerformanceTest() action method, the async stuff runs about 1 second slower than the non-async. (I run the test multiple times in a row to account for the JITter warming up.) What am I doing wrong that's causing the async to be slower than the non-async? Am I misunderstanding something fundamental about writing asynchronous code?

推荐答案

异步版本总是会比同步版本慢时，有没有并发。它是所有做同样的工作，非异步版本，但开销少量添加到管理不同步。

The asynchronous version will always be slower than the synchronous version when there is no concurrency. It's doing all of the same work as the non-async version, but with a small amount of overhead added to manage the asynchrony.

异步是有利的，在性能方面，通过允许提高可用性。每个人的要求会慢一些，但如果你在同一时间1000个请求，异步执行将能更迅速地（在某些情况下至少）处理它们。

Asynchrony is advantageous, with respect to performance, by allowing improved availability. Each individual request will be slower, but if you make 1000 requests at the same time, the asynchronous implementation will be able to handle them all more quickly (at least in certain circumstances).

这是因为异步解决方案允许被分配给处理回到游泳池和处理其他请求，请求的线程，而同步解决方案强制线程坐在那里，什么也不做，而它等待异步操作去完成。有开销的方式，允许线程被释放，做其他工作，构建方案，但优点是线程去做其他工作的能力。在程序中没有为线程去做其它任何工作，所以它最终是净损失。

This happens because the asynchronous solution allows the thread that was allocated to handle the request to go back to the pool and handle other requests, whereas the synchronous solution forces the thread to sit there and do nothing while it waits for the asynchronous operation to complete. There is overhead in structuring the program in a way that allows the thread to be freed up to do other work, but the advantage is the ability of that thread to go do other work. In your program there is no other work for the thread to go do, so it ends up being a net loss.

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