Java NIO socket IO degenerating read chunk size

I'm writing a non-blocking client that reads data from a network socket using selector SelectionKey.OP_READ and selector.select() calls. The reading part is handled like this:

if (selectionKey.isReadable()) {
    int len = inChannel.read(buf);
    System.out.println(len);
    . . .
}

The problem I see is that the read size (len) is gradually degenerating as the download progresses:

Does anyone know why this is and how to improve it? Of course, it works as-is, but I need to reduce cpu overhead, and thus it is best to process large chunks instead of small ones. Adding a small sleep (20ms) helps, but will obviously limit scalability, as I need to handle thousands of streams simultaneously.

Solution

Going to answer my own question...

Never mind; I wasn't using the buffer correctly. Forgot the compact() call after reading from buffer.

if (selectionKey.isReadable()) {
    int len = inChannel.read(buf);
    System.out.println(len);
    . . .
    buf.flip();
    // ... read from buf ...
    buf.compact();
}

Apparently the buffers are not circular; even though a buffer is empty, the position and limit must be reset before next use.