WebSocket MessageInputStream.read() spends a lot of time in ByteBuffer.compact()

[lachlan-roberts]

Jetty version
10.0.2

Description
Originally reported by @dcitron in issue #6127.

I did notice that org.eclipse.jetty.websocket.core.internal.messages.MessageInputStream.read() is taking a larger percentage of time in Jetty 10 than Jetty 9. In Jetty 9 it kicked around in java.nio.Buffer.hasRemaining() a bit, but in Jetty 10 it's taking quite a bit of time in the following call stack:

MessageInputStream.read() 
    -> org.eclipse.jetty.util.BufferUtil.append() 
         -> org.eclipse.jetty.util.BufferUtil.flipToFill() 
              -> java.nio.HeapByteBuffer.compact()

In a simple test that I ran, processing 1,000 messages, it actually seemed to spend 24% of its time in HeapByteBuffer.compact() so maybe that's something you might want to look into further, or maybe it's just a memory-vs-CPU tradeoff.

[lachlan-roberts]

This is fixed by #6170.