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Large Send Offload and Network Performance | Peer Wisdom March 18, 2015

Posted by sandyclaus in Microsoft Windows.
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Large Send Offload and Network Performance

April 3, 2013 by crutledge · Leave a comment

One issue that I continually see reported by customers is slow network performance.  Although there are literally a ton of issues that can effect how fast data moves to and from a server, there is one fix I’ve found that will resolve this 99% of time — disable Large Send Offload on the Ethernet adapter.

So what is Large Send Offload (also known as Large Segmetation Offload, and LSO for short)?  It’s a feature on modern Ethernet adapters that allows the TCP\IP network stack to build a large TCP message of up to 64KB in length before sending to the Ethernet adapter.  Then the hardware on the Ethernet adapter — what I’ll call the LSO engine — segments it into smaller data packets (known as “frames” in Ethernet terminology) that can be sent over the wire. This is up to 1500 bytes for standard Ethernet frames and up to 9000 bytes for jumbo Ethernet frames.  In return, this frees up the server CPU from having to handle segmenting large TCP messages into smaller packets that will fit inside the supported frame size.  Which means better overall server performance.  Sounds like a good deal.  What could possibly go wrong?

Quite a lot, as it turns out.  In order for this to work, the other network devices — the Ethernet switches through which all traffic flows — all have to agree on the frame size.  The server cannot send frames that are larger than the Maximum Transmission Unit (MTU) supported by the switches.  And this is where everything can, and often does, fall apart.

via Large Send Offload and Network Performance | Peer Wisdom.

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