byte[] and efficiently passing by reference

问题

So this is in relationship to dealing with the Large Object Heap and trying to minimize the number of times I instantiate a byte[]. Basically, I'm having OutOfMemoryExceptions and I feel like it's because we're instantiating too many byte array's. The program works fine when we process a couple of files, but it needs to scale, and it currently can't.

In a nutshell, I've got a loop that pulls documents from a database. Currently, it's pulling one document at a time and then processing the document. Documents can range from less than a meg to 400+ megs. (hence why i'm processing one at a time). The following is pseudo-code and before I've optimized.

So the steps I'm doing are:

1. Make a call to the database to find the largest file size (and then multiplying it by 1.1)

   var maxDataSize = new BiztalkBinariesData().GetMaxFileSize();
   maxDataSize = (maxDataSize != null && maxDataSize > 0)
       ? (long)(maxDataSize * 1.1)
       : 0;
   var FileToProcess = new byte[maxDataSize];
   

2. Then I make another database call pulling all of the documents (without data) from the database and place these into an IEnumerable.

   UnprocessedDocuments =
       claimDocumentData.Select(StatusCodes.CurrentStatus.WaitingToBeProcessed);
   foreach (var currentDocument in UnprocessDocuments)
   {
        // all of the following code goes here
   }
   

3. Then I populate my byte[] array from an external source:

   FileToProcess = new BiztalkBinariesData()
       .Get(currentDocument.SubmissionSetId, currentDocument.FullFileName);
   

4. Here is the question. It would be much cleaner to pass the currentDocument (IClaimDocument) to other methods to process. So if I set the data part of the currentDocument to the pre-formatted array, will this use the existing reference? Or does this create a new array in the Large Object Heap?

   currentDocument.Data = FileToProcess;
   

5. At the end of the loop, I would then clear FileToProcess

   Array.Clear(FileToProcess, 0, FileToProcess.length);
   

Was that clear? If not, I'll try to clean it up.

回答1:

Step 1:

var FileToProcess = new byte[maxDataSize];

Step 3:

FileToProcess = new BiztalkBinariesData()
    .Get(currentDocument.SubmissionSetId, currentDocument.FullFileName);

Your step 1 is completely unnecessary, since you re-assign the array in step 3 - you are creating a new array, you do not populate the existing array - So essentially step 1 is just creating more work for the GC, which if you do it in quick order (and if it is not optimized away by the compiler, which is entirely possible) might explain some of the memory pressure you are seeing.

回答2:

Arrays are reference types and as such you will be passing a copy of the reference, not a copy of the array itself. That would only be true with value types.

This simple snippet illustrates how arrays behave as reference types:

public void Test()
{    
    var intArray = new[] {1, 2, 3, 4};
    EditArray(intArray);
    Console.WriteLine(intArray[0].ToString()); //output will be 0
}

public void EditArray(int[] intArray)
{
    intArray[0] = 0;
}

回答3:

It will use the existing reference, don't worry. The array's contents aren't copied.

回答4:

Your problem could be in the implementation and usage of the "BiztalkBinariesData" class.

I'm not sure how it is implemented but I do see you're declaring a new instance each time

new BiztalkBinariesData()

Something to think about..

回答5:

I'm having OutOfMemoryExceptions and I feel like it's because we're instantiating too many byte array's

No, it is because yo allocate LARGE arrays. Limit them to like 48kb or 64kb and "combine" them with a custom container. 64kb means you can take the higher 2 bytes of the index to determine the array to use. The container contains an arrays of arrays. Handling very large objects leads to fragemntation and the inability to allocate one large array later down.

来源：https://stackoverflow.com/questions/9082420/byte-and-efficiently-passing-by-reference