问题
I want to read big binary file containing millions of records and I want to get some reports for the records. I use BinaryReader to read (which I think has the best performance in readers) and convert read bytes to data model. Due to the count of records, passing model to the report layer is another issue: I prefer to use IEnumerable to have LINQ functionality and features when developing the reports.
Here is sample data class:
Public Class MyData
Public A1 As UInt64
Public A2 As UInt64
Public A3 As Byte
Public A4 As UInt16
Public A5 As UInt64
End Class
I used this sub to create the file:
Sub CreateSampleFile()
Using streamWriter As New FileStream(fileName, FileMode.Create, FileAccess.Write, FileShare.Write)
For i As Integer = 1 To 1000
For j As Integer = 1 To 1000
For k = 1 To 30
Dim item As New MyData With {.A1 = i, .A2 = j, .A3 = k, .A4 = j, .A5 = i * j}
Dim bytes() As Byte = BitConverter.GetBytes(item.A1).Concat(BitConverter.GetBytes(item.A2)).Concat({item.A3}).Concat(BitConverter.GetBytes(item.A4)).Concat(BitConverter.GetBytes(item.A5)).ToArray
streamWriter.Write(bytes, 0, bytes.Length)
Next
Next
Next
End Using
End Sub
And here is my reader class:
Imports System.IO
Public Class FileReader
Public Const BUFFER_LENGTH As Long = 4096 * 256 * 27
Public Const MY_DATA_LENGTH As Long = 27
Private _buffer(BUFFER_LENGTH - 1) As Byte
Private _streamWriter As FileStream
Public Event OnByteRead(sender As FileReader, bytes() As Byte, index As Long)
Public Sub StartReadBinary(fileName As String)
Dim currentBufferReadCount As Long = 0
Using fileStream As New FileStream(fileName, FileMode.Open, FileAccess.Read, FileShare.Read)
Using streamReader As New BinaryReader(fileStream)
currentBufferReadCount = streamReader.Read(Me._buffer, 0, Me._buffer.Length)
While currentBufferReadCount > 0
For i As Integer = 0 To currentBufferReadCount - 1 Step MY_DATA_LENGTH
RaiseEvent OnByteRead(Me, Me._buffer, i)
Next
currentBufferReadCount = streamReader.Read(Me._buffer, 0, Me._buffer.Length)
End While
End Using
End Using
End Sub
Public Iterator Function GetAll(fileName As String) As IEnumerable(Of MyData)
Dim currentBufferReadCount As Long = 0
Using fileStream As New FileStream(fileName, FileMode.Open, FileAccess.Read, FileShare.Read)
Using streamReader As New BinaryReader(fileStream)
currentBufferReadCount = streamReader.Read(Me._buffer, 0, Me._buffer.Length)
While currentBufferReadCount > 0
For i As Integer = 0 To currentBufferReadCount - 1 Step MY_DATA_LENGTH
Yield GetInstance(_buffer, i)
Next
currentBufferReadCount = streamReader.Read(Me._buffer, 0, Me._buffer.Length)
End While
End Using
End Using
End Function
Public Function GetInstance(bytes() As Byte, index As Long) As MyData
Return New MyData With {.A1 = BitConverter.ToUInt64(bytes, index), .A2 = BitConverter.ToUInt64(bytes, index + 8), .A3 = bytes(index + 16), .A4 = BitConverter.ToUInt16(bytes, index + 17), .A5 = BitConverter.ToUInt64(bytes, index + 19)}
End Function
End Class
I was thinking about the IEnumerable performance, so I tried to use both GetAll method as IEnumerable and raising event for each record which is read from file. Here is the test module:
Imports System.IO
Module Module1
Private fileName As String = "MyData.dat"
Private readerJustTraverse As New FileReader
Private WithEvents readerWithoutInstance As New FileReader
Private WithEvents readerWithInstance As New FileReader
Private readerIEnumerable As New FileReader
Sub Main()
Dim s As New Stopwatch
s.Start()
readerJustTraverse.StartReadBinary(fileName)
s.Stop()
Console.WriteLine("Read bytes: {0}", s.ElapsedMilliseconds)
s.Restart()
readerWithoutInstance.StartReadBinary(fileName)
s.Stop()
Console.WriteLine("Read bytes, raise event: {0}", s.ElapsedMilliseconds)
s.Restart()
readerWithInstance.StartReadBinary(fileName)
s.Stop()
Console.WriteLine("Read bytes, raise event, get instance: {0}", s.ElapsedMilliseconds)
s.Restart()
For Each item In readerIenumerable.GetAll(fileName)
Next
Console.WriteLine("Read bytes, get instance, return yield: {0}", s.ElapsedMilliseconds)
s.Stop()
Console.ReadLine()
End Sub
Private Sub readerWithInstance_OnByteRead(sender As FileReader, bytes() As Byte, index As Long) Handles readerWithInstance.OnByteRead
Dim item As MyData = sender.GetInstance(bytes, index)
End Sub
Private Sub readerWithoutInstance_OnByteRead(sender As FileReader, bytes() As Byte, index As Long) Handles readerWithoutInstance.OnByteRead
'do nothing
End Sub
End Module
The thing which I'm wondering is elapsed time for each process, here is the test result (tested on ASUS Ultrabook - Zenbook Core i7):
Read bytes: 384 (without touching the read bytes!)
Read bytes, raise event: 583
Read bytes, raise event, get instance: 3923
Read bytes, get instance, return yield: 4917
It shows that reading file as byte is incredibly fast, and converting bytes to the model is slow. Also raising event instead of getting IEnumerable result, is 25% faster.
Is iterating in IEnumerable is really has this performance cost or I missed something?
回答1:
Yes, using Iterator Functions carries a performance penalty.
I compiled your code and I got the same results as you did. I looked at the generated IL code. The state machine created from the GetAll method does contain a lot of stuff but most of the instructions are nop's or simple operations.
The results with/without using the iterator functions differ, as you say, by 25%. That's not too much. When you are using StartReadBinary, there is simply one big cycle which calls the OnByteRead method (via the event) three billion times. However, when you create the objects in a foreach cycle, what you must do for each object is call the GetCurrent() method and MoveNext() of the generated enumerator, the latter of which is not trivial (most of the code from GetAll was moved there) and uses quite an amount of compiler-generated variables.
Using "Yield" generally slows down your program because the compiler has to create complicated IL code to represent the state machine.
来源：https://stackoverflow.com/questions/19786246/performance-comparison-of-ienumerable-and-raising-event-for-each-item-in-source