Marshalling of byte[] arrays is causing excessive GC load (and gen2 collections)

My app was serving a large file (~2 GB) through Dokan and I was noticing a gen2 garbage collection every second. This is obviously not good for performance - I was seeing 30% of CPU spent in GC. After investigating, it turns out the allocations were coming from marshaling of the byte[] array in ReadFile(). A managed array was being allocated for every ReadFile() call and thrown away after the call completes.

Today’s code looks like this:

public delegate NtStatus ReadFileDelegate(
    [MarshalAs(UnmanagedType.LPWStr)] string rawFileName,
    [MarshalAs(UnmanagedType.LPArray, SizeParamIndex = 2), Out] byte[] rawBuffer,
    uint rawBufferLength,
...
);

By modifying it to not marshal at all and just pass the IntPtr I was able to completely eliminate the GC load. The implementor of IDokanOperations can then use Marshal.Copy or unsafe code to fill the output.

This is obviously a breaking interface change, but maybe it’s worth adding a new interface, something like IDokanOperationsUnsafe, for those that want the perf?

[hans-olav]

See PR #206 for the implementation of this feature.

[hans-olav]

#206 merged to master, closing issue.