IE：之前/：在td的100％高度问题之后(IE :before/:after 100% height issues on td)

 我发现这个问题非常有趣，特别是因为我在Ado.Net和EF 6上使用async 。我希望有人给出这个问题的解释，但并没有发生。 所以我试图重现这个问题在我身边。 我希望你们中有人会发现这个有趣的。 
 
 第一个好消息：我转载它:)和差异是巨大的。 有一个因子8 ... 
 
 
 首先，我怀疑与CommandBehavior有关的事情，因为我读了一篇有关与Ado async 的有趣的文章 ，他说： 
 
 “由于非顺序访问模式必须存储整行的数据，如果从服务器读取大列（如varbinary（MAX），varchar（MAX），nvarchar（MAX））或XML ）“。 
 
 我怀疑ToList()调用是CommandBehavior.SequentialAccess和异步的CommandBehavior.Default是CommandBehavior.Default （非连续的，这可能会导致问题）。 所以我下载了EF6的来源，并将破解点放在任何地方（当然是使用CommandBehavior地方）。 
 
 结果： 没有 。 所有的调用都是使用CommandBehavior.Default ....所以我试图进入EF代码来了解会发生什么...和.. ooouch ...我从来没有看到这样一个委托代码，一切似乎都懒惰执行... 
 
 所以我试图做一些分析来了解会发生什么 
 
 我想我有东西 
 
 这里是创建我的基准测试表的模型，其中有3500行，每个varbinary(MAX) 256 Kb的随机数据。 （EF 6.1 - CodeFirst - CodePlex ）： 
 
public class TestContext : DbContext
{
    public TestContext()
        : base(@"Server=(localdb)\\v11.0;Integrated Security=true;Initial Catalog=BENCH") // Local instance
    {
    }
    public DbSet<TestItem> Items { get; set; }
}

public class TestItem
{
    public int ID { get; set; }
    public string Name { get; set; }
    public byte[] BinaryData { get; set; }
}
 
 这里是我用来创建测试数据的代码，以及基准的EF。 
 
using (TestContext db = new TestContext())
{
    if (!db.Items.Any())
    {
        foreach (int i in Enumerable.Range(0, 3500)) // Fill 3500 lines
        {
            byte[] dummyData = new byte[1 << 18];  // with 256 Kbyte
            new Random().NextBytes(dummyData);
            db.Items.Add(new TestItem() { Name = i.ToString(), BinaryData = dummyData });
        }
        await db.SaveChangesAsync();
    }
}

using (TestContext db = new TestContext())  // EF Warm Up
{
    var warmItUp = db.Items.FirstOrDefault();
    warmItUp = await db.Items.FirstOrDefaultAsync();
}

Stopwatch watch = new Stopwatch();
using (TestContext db = new TestContext())
{
    watch.Start();
    var testRegular = db.Items.ToList();
    watch.Stop();
    Console.WriteLine("non async : " + watch.ElapsedMilliseconds);
}

using (TestContext db = new TestContext())
{
    watch.Restart();
    var testAsync = await db.Items.ToListAsync();
    watch.Stop();
    Console.WriteLine("async : " + watch.ElapsedMilliseconds);
}

using (var connection = new SqlConnection(CS))
{
    await connection.OpenAsync();
    using (var cmd = new SqlCommand("SELECT ID, Name, BinaryData FROM dbo.TestItems", connection))
    {
        watch.Restart();
        List<TestItem> itemsWithAdo = new List<TestItem>();
        var reader = await cmd.ExecuteReaderAsync(CommandBehavior.SequentialAccess);
        while (await reader.ReadAsync())
        {
            var item = new TestItem();
            item.ID = (int)reader[0];
            item.Name = (String)reader[1];
            item.BinaryData = (byte[])reader[2];
            itemsWithAdo.Add(item);
        }
        watch.Stop();
        Console.WriteLine("ExecuteReaderAsync SequentialAccess : " + watch.ElapsedMilliseconds);
    }
}

using (var connection = new SqlConnection(CS))
{
    await connection.OpenAsync();
    using (var cmd = new SqlCommand("SELECT ID, Name, BinaryData FROM dbo.TestItems", connection))
    {
        watch.Restart();
        List<TestItem> itemsWithAdo = new List<TestItem>();
        var reader = await cmd.ExecuteReaderAsync(CommandBehavior.Default);
        while (await reader.ReadAsync())
        {
            var item = new TestItem();
            item.ID = (int)reader[0];
            item.Name = (String)reader[1];
            item.BinaryData = (byte[])reader[2];
            itemsWithAdo.Add(item);
        }
        watch.Stop();
        Console.WriteLine("ExecuteReaderAsync Default : " + watch.ElapsedMilliseconds);
    }
}

using (var connection = new SqlConnection(CS))
{
    await connection.OpenAsync();
    using (var cmd = new SqlCommand("SELECT ID, Name, BinaryData FROM dbo.TestItems", connection))
    {
        watch.Restart();
        List<TestItem> itemsWithAdo = new List<TestItem>();
        var reader = cmd.ExecuteReader(CommandBehavior.SequentialAccess);
        while (reader.Read())
        {
            var item = new TestItem();
            item.ID = (int)reader[0];
            item.Name = (String)reader[1];
            item.BinaryData = (byte[])reader[2];
            itemsWithAdo.Add(item);
        }
        watch.Stop();
        Console.WriteLine("ExecuteReader SequentialAccess : " + watch.ElapsedMilliseconds);
    }
}

using (var connection = new SqlConnection(CS))
{
    await connection.OpenAsync();
    using (var cmd = new SqlCommand("SELECT ID, Name, BinaryData FROM dbo.TestItems", connection))
    {
        watch.Restart();
        List<TestItem> itemsWithAdo = new List<TestItem>();
        var reader = cmd.ExecuteReader(CommandBehavior.Default);
        while (reader.Read())
        {
            var item = new TestItem();
            item.ID = (int)reader[0];
            item.Name = (String)reader[1];
            item.BinaryData = (byte[])reader[2];
            itemsWithAdo.Add(item);
        }
        watch.Stop();
        Console.WriteLine("ExecuteReader Default : " + watch.ElapsedMilliseconds);
    }
}
 
 对于常规EF调用（ .ToList() ），分析似乎是“正常”，易于阅读： 
 
  
 
 在这里，我们发现与秒表相比，我们拥有8.4秒的时间（分析速度减慢了perfs）。 我们还在呼叫路径上找到HitCount = 3500，女巫在测试中与3500线一致。 在TDS解析器端，事情开始变得更糟，因为我们在TryReadByteArray()方法中读取了118 353个调用，这是缓冲循环发生的。 （平均33.8调用256kb的每个byte[] 
 
 对于async情况，它真的是不同的....首先， .ToListAsync()调用安排在ThreadPool上，然后等待。 这里没什么惊喜 但是，现在，这里是ThreadPool中的async地狱： 
 
  
 
 首先，在第一种情况下，我们沿着完整的通话路径只有3500次点击，这里我们有118 371.此外，你必须想象所有的同步电话，我没有放在屏幕上... 
 
 第二，在第一种情况下，我们对TryReadByteArray()方法进行了“只有118 353”的调用，这里我们有2 050 210个调用！ 它是17倍以上（在大型1Mb阵列的测试中，它是160倍） 
 
 此外还有： 
 
 
 
 创建了120 000个Task实例 
 
 
 727 519 Interlocked电话 
 
 
 290 569 Monitor来电 
 
 
 98 283 ExecutionContext实例，具有264 481捕获 
 
 
 208 733 SpinLock电话 
 
 
 我的猜测是缓冲是以异步方式（而不是一个好的）进行的，并行任务试图从TDS中读取数据。 创建太多任务只是为了解析二进制数据。 
 
 作为一个初步的结论，我们可以说Async很棒，但是EF6很好，但EF6在当前实现中异步的用法在性能方面增加了一个主要的开销，即线程方面和CPU方面（12％的CPU使用率ToList()情况和20％的ToListAsync情况下，工作时间为8到10倍...我运行在旧的i7 920上）。 
 
 在做一些测试时，我再次考虑这篇文章 ，我注意到我想念的事情： 
 
 “对于.Net 4.5中的新异步方法，它们的行为与同步方法完全相同，除了一个显着的异常：非顺序模式下的ReadAsync。 
 
 什么 ？！！！ 
 
 所以我扩展了我的基准，包括Ado.Net在常规/异步调用，并与CommandBehavior.SequentialAccess / CommandBehavior.Default ，这是一个惊喜！ ： 
 
 
 我们与Ado.Net有完全相同的行为！ 捂脸...... 
 
 我的最终结论是 ：在EF 6实现中有一个错误。 当对包含binary(max)列的表执行异步调用时，它应将CommandBehavior切换为SequentialAccess 。 在Ado.Net方面，创建太多任务的问题会减缓进程。 EF问题是它不应该使用Ado.Net。 
 
 现在你知道，而不是使用EF6异步方法，你最好以非常异步的方式调用EF，然后使用TaskCompletionSource<T>以异步方式返回结果。 
 
 注1：我修改了我的帖子，因为一个可耻的错误....我做了我的第一次测试网络，而不是在本地，有限的带宽扭曲的结果。 以下是更新后的结果。 
 
 注2：我没有将测试扩展到其他用例（例如： nvarchar(max) ），但有很多机会发生相同的行为。 
 
 注3：对于ToList()情况，通常情况下，是12％的CPU（CPU的1/8 = 1逻辑内核）。 对于ToListAsync()情况，不寻常的是最多20％，就好像Scheduler不能使用所有的Treads。 这可能是由于太多的任务创建，或者可能是TDS解析器的瓶颈，我不知道... 

I found this question very interesting, especially since I'm using async everywhere with Ado.Net and EF 6. I was hoping someone to give an explanation for this question, but it doesn't happened. So I tried to reproduce this problem on my side. I hope some of you will find this interesting.
 
First good news : I reproduced it :) And the difference is enormous. With a factor 8 ...
 
 
First I was suspecting something dealing with CommandBehavior, since I read an interesting article about async with Ado, saying this : 
 
"Since non-sequential access mode has to store the data for the entire row, it can cause issues if you are reading a large column from the server (such as varbinary(MAX), varchar(MAX), nvarchar(MAX) or XML)."
 
I was suspecting ToList() calls to be CommandBehavior.SequentialAccess and async ones to be CommandBehavior.Default (non-sequential, which can cause issues). So I downloaded EF6's sources, and put breakpoints everywhere (where CommandBehavior where used, of course). 
 
Result : nothing. All the calls are made with CommandBehavior.Default .... So I tried to step into EF code to understand what happens... and.. ooouch... I never see such a delegating code, everything seems lazy executed...
 
So I tried to do some profiling to understand what happens...
 
And I think I have something...
 
Here's the model to create the table I benchmarked, with 3500 lines inside of it, and 256 Kb random data in each varbinary(MAX). (EF 6.1 - CodeFirst - CodePlex) :
 
public class TestContext : DbContext
{
    public TestContext()
        : base(@"Server=(localdb)\\v11.0;Integrated Security=true;Initial Catalog=BENCH") // Local instance
    {
    }
    public DbSet<TestItem> Items { get; set; }
}

public class TestItem
{
    public int ID { get; set; }
    public string Name { get; set; }
    public byte[] BinaryData { get; set; }
}
 
And here's the code I used to create the test data, and benchmark EF.
 
using (TestContext db = new TestContext())
{
    if (!db.Items.Any())
    {
        foreach (int i in Enumerable.Range(0, 3500)) // Fill 3500 lines
        {
            byte[] dummyData = new byte[1 << 18];  // with 256 Kbyte
            new Random().NextBytes(dummyData);
            db.Items.Add(new TestItem() { Name = i.ToString(), BinaryData = dummyData });
        }
        await db.SaveChangesAsync();
    }
}

using (TestContext db = new TestContext())  // EF Warm Up
{
    var warmItUp = db.Items.FirstOrDefault();
    warmItUp = await db.Items.FirstOrDefaultAsync();
}

Stopwatch watch = new Stopwatch();
using (TestContext db = new TestContext())
{
    watch.Start();
    var testRegular = db.Items.ToList();
    watch.Stop();
    Console.WriteLine("non async : " + watch.ElapsedMilliseconds);
}

using (TestContext db = new TestContext())
{
    watch.Restart();
    var testAsync = await db.Items.ToListAsync();
    watch.Stop();
    Console.WriteLine("async : " + watch.ElapsedMilliseconds);
}

using (var connection = new SqlConnection(CS))
{
    await connection.OpenAsync();
    using (var cmd = new SqlCommand("SELECT ID, Name, BinaryData FROM dbo.TestItems", connection))
    {
        watch.Restart();
        List<TestItem> itemsWithAdo = new List<TestItem>();
        var reader = await cmd.ExecuteReaderAsync(CommandBehavior.SequentialAccess);
        while (await reader.ReadAsync())
        {
            var item = new TestItem();
            item.ID = (int)reader[0];
            item.Name = (String)reader[1];
            item.BinaryData = (byte[])reader[2];
            itemsWithAdo.Add(item);
        }
        watch.Stop();
        Console.WriteLine("ExecuteReaderAsync SequentialAccess : " + watch.ElapsedMilliseconds);
    }
}

using (var connection = new SqlConnection(CS))
{
    await connection.OpenAsync();
    using (var cmd = new SqlCommand("SELECT ID, Name, BinaryData FROM dbo.TestItems", connection))
    {
        watch.Restart();
        List<TestItem> itemsWithAdo = new List<TestItem>();
        var reader = await cmd.ExecuteReaderAsync(CommandBehavior.Default);
        while (await reader.ReadAsync())
        {
            var item = new TestItem();
            item.ID = (int)reader[0];
            item.Name = (String)reader[1];
            item.BinaryData = (byte[])reader[2];
            itemsWithAdo.Add(item);
        }
        watch.Stop();
        Console.WriteLine("ExecuteReaderAsync Default : " + watch.ElapsedMilliseconds);
    }
}

using (var connection = new SqlConnection(CS))
{
    await connection.OpenAsync();
    using (var cmd = new SqlCommand("SELECT ID, Name, BinaryData FROM dbo.TestItems", connection))
    {
        watch.Restart();
        List<TestItem> itemsWithAdo = new List<TestItem>();
        var reader = cmd.ExecuteReader(CommandBehavior.SequentialAccess);
        while (reader.Read())
        {
            var item = new TestItem();
            item.ID = (int)reader[0];
            item.Name = (String)reader[1];
            item.BinaryData = (byte[])reader[2];
            itemsWithAdo.Add(item);
        }
        watch.Stop();
        Console.WriteLine("ExecuteReader SequentialAccess : " + watch.ElapsedMilliseconds);
    }
}

using (var connection = new SqlConnection(CS))
{
    await connection.OpenAsync();
    using (var cmd = new SqlCommand("SELECT ID, Name, BinaryData FROM dbo.TestItems", connection))
    {
        watch.Restart();
        List<TestItem> itemsWithAdo = new List<TestItem>();
        var reader = cmd.ExecuteReader(CommandBehavior.Default);
        while (reader.Read())
        {
            var item = new TestItem();
            item.ID = (int)reader[0];
            item.Name = (String)reader[1];
            item.BinaryData = (byte[])reader[2];
            itemsWithAdo.Add(item);
        }
        watch.Stop();
        Console.WriteLine("ExecuteReader Default : " + watch.ElapsedMilliseconds);
    }
}
 
For the regular EF call (.ToList()), the profiling seems "normal" and is easy to read :
 
 
Here we find the 8.4 seconds we have with the Stopwatch (profiling slow downs the perfs). We also find HitCount = 3500 along the call path, which is consistent with the 3500 lines in the test. On the TDS parser side, things start to became worse since we read 118 353 calls on TryReadByteArray() method, which is were the buffering loop occurs. (an average 33.8 calls for each byte[] of 256kb)
 
For the async case, it's really really different.... First, the .ToListAsync() call is scheduled on the ThreadPool, and then awaited. Nothing amazing here. But, now, here's the async hell on the ThreadPool :
 
 
First, in the first case we were having just 3500 hit counts along the full call path, here we have 118 371. Moreover, you have to imagine all the synchronization calls I didn't put on the screenshoot...
 
Second, in the first case, we were having "just 118 353" calls to the TryReadByteArray() method, here we have 2 050 210 calls ! It's 17 times more... (on a test with large 1Mb array, it's 160 times more)
 
Moreover there are :
 
 
 
120 000 Task instances created
 
 
727 519 Interlocked calls
 
 
290 569 Monitor calls 
 
 
98 283 ExecutionContext instances, with 264 481 Captures
 
 
208 733 SpinLock calls
 
 
My guess is the buffering is made in an async way (and not a good one), with parallel Tasks trying to read data from the TDS. Too many Task are created just to parse the binary data. 
 
As a preliminary conclusion, we can say Async is great, EF6 is great, but EF6's usages of async in it's current implementation adds a major overhead, on the performance side, the Threading side, and the CPU side (12% CPU usage in the ToList() case and 20% in the ToListAsync case for a 8 to 10 times longer work... I run it on an old i7 920). 
 
While doings some tests, I was thinking about this article again and I notice something I miss :
 
"For the new asynchronous methods in .Net 4.5, their behavior is exactly the same as with the synchronous methods, except for one notable exception: ReadAsync in non-sequential mode."
 
What ?!!!
 
So I extend my benchmarks to include Ado.Net in regular / async call, and with CommandBehavior.SequentialAccess / CommandBehavior.Default, and here's a big surprise ! :
 
 
We have the exact same behavior with Ado.Net !!! Facepalm...
 
My definitive conclusion is : there's a bug in EF 6 implementation. It should toggle the CommandBehavior to SequentialAccess when an async call is made over a table containing a binary(max) column. The problem of creating too many Task, slowing down the process, is on the Ado.Net side. The EF problem is that it doesn't use Ado.Net as it should. 
 
Now you know instead of using the EF6 async methods, you would better have to call EF in a regular non-async way, and then use a TaskCompletionSource<T> to return the result in an async way.
 
Note 1 : I edited my post because of a shameful error.... I've done my first test over the network, not locally, and the limited bandwidth have distorted the results. Here are the updated results.
 
Note 2 : I didn't extends my test to other uses cases (ex : nvarchar(max) with a lot of data), but there are chances the same behavior happens.
 
Note 3 : Something usual for the ToList() case, is the 12% CPU (1/8 of my CPU = 1 logical core). Something unusual is the maximum 20% for the ToListAsync() case, as if the Scheduler could not use all the Treads. It's probably due to the too many Task created, or maybe a bottleneck in TDS parser, I don't know...