Why .NET async await file copy is a lot more CPU consuming than synchronous File.Copy() call?

Question

Why the code below results in:

public static class Program
{
    public static void Main(params string[] args)
    {
        var sourceFileName = @"C:\Users\ehoua\Desktop\Stuff\800MFile.exe";
        var destinationFileName = sourceFileName + ".bak";

        FileCopyAsync(sourceFileName, destinationFileName);

        // The line below is actually faster and a lot less CPU-consuming
        // File.Copy(sourceFileName, destinationFileName, true);

        Console.ReadKey();
    }

    public static async void FileCopyAsync(string sourceFileName, string destinationFileName, int bufferSize = 0x1000, CancellationToken cancellationToken = default(CancellationToken))
    {
        using (var sourceFile = File.OpenRead(sourceFileName))
        {
            using (var destinationFile = File.OpenWrite(destinationFileName))
            {
                Console.WriteLine($"Copying {sourceFileName} to {destinationFileName}...");
                await sourceFile.CopyToAsync(destinationFile, bufferSize, cancellationToken);
                Console.WriteLine("Done");
            }
        }
    }
}

While File.Copy(): https://msdn.microsoft.com/en-us/library/system.io.file.copy(v=vs.110).aspx is a lot less cpu-consuming:

So is there still a real interest using async / await for file copy purposes?

I thought saving a thread for copying might worth it but the File.Copy windows function seems to win the battle hands down in terms of CPU %. Some would argue it's because of the real DMA support but still, am I doing anything to ruin the performances? Or is there anything that can be done to improve the CPU usage with my async method?

Answer 1

These are pretty absurd perf numbers. You are simply not measuring what you think you are. This should not take more than a minor blip, a simple memory-to-memory copy for cached file data. Like File.Copy() did. Operates at ~35 gigabytes/second on a machine with decent DDR3 RAM so can't take more than a few dozen milliseconds. Even if the file is not cached or the machine doesn't have enough RAM then you still can't get this kind of CPU load, your code would be blocked waiting for the disk.

What you are actually seeing is the perf of your installed anti-malware product. It always gets its underwear in a bundle when it sees programs manipulating executable files.

Simple to verify, disable it or make an exclusion and try again.