bc6d41af   
   From: rick.jones2@hp.com   
      
   Miguel Sanders  wrote:   
   > I was wondering if there is an additional performance gain in   
   > enabled TCP segmentation offload in a jumbo frame network. What's   
   > your opinion on that?   
      
   I used to call TSO "Poor Man's Jumbo Frames" since it offered   
   send-side benefits similar to that of Jumbo Frames but not receive   
   side.   
      
   TSO is sending side only.  It does not reduce the burden on the   
   reciever.  If the receiver does not support LRO (Large Receive   
   Offload) then enabling JumboFrames will help the receiver   
   considerably during bulk transfers.   
      
   Otherwise, adding JumboFrames to TSO (or TSO to JumboFrames) does have   
   a benefit, but it is rapidly approaching diminishing returns.  One   
   benefit to JumboFrames (not sure if it also happens with LRO) is a   
   reduction in the number of ACK segments per KB transferred.  In the   
   presence of CKO (a pre-req for TSO) and TSO and perhaps copy   
   avoidance, ACK processing starts to dominate in bulk transfer.  The   
   TSO sender can send up to 64K at a time down the stack, so has one   
   "send" as far as CPU util is concerned, but will receive upwards of 22   
   ACKs from the reciever with the typical 1500 byte MTU and   
   corresponding MSS.   
      
   If one happens to be sending large UDP datagrams, TSO does nothing for   
   that, but JumboFrames can reduce the IP fragmentation and so help.   
   With UDP though there is no MSS exchange so one must be certain that   
   both ends can do JumboFrames.  With TCP's MSS exchange, if you enable   
   JumboFrames at one end, but not the other, you get your backside   
   covered and traffic will still flow, based on the smaller MTU.   
      
   > Also, what are the caveats of TCP segmentation offload?   
      
   It adds a little bit of overhead to the sending path to decide when to   
   do a large send and by how much.  It may also make traffic a bit more   
   bursty.   
      
   It does virtually nothing for small sends, particularly small   
   request/response exchanges.   
      
   rick jones   
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