On Tue, Apr 16, 2013 at 9:02 AM, Roland Dreier <[email protected]> wrote: > On Mon, Apr 15, 2013 at 2:10 PM, Wendy Cheng <[email protected]> wrote: >> Before closing this thread ..... I see there is a clear warning in >> IPOIB code that enabling connected mode will cause multicast packet to >> drop. I assume this is due to message ordering requirement and the >> multicast packet could fall behind the large packet fragment that ends >> up causing multicast logic to time out. If my assumption is not >> correct, please do kindly correct me. > > No, it's because multicasts can't be sent over the RC connections, so > they need to be sent via UD. And the datagram size is limited by the > underlying IB MTU, which is going to be either 4K or 2K most likely. > However if you enable CM and then set your IPoIB MTU to something like > 64K, the IP stack will happily create multicast messages bigger than > the datagram MTU, and the IPoIB driver really can't do anything with > those except throw them away. >
Great thanks (for the clarification) ! Now, assuming CM is only turned on with devices (e.g. MLX card) that are capable of doing segmentation off-load, i.e. skb_is_gso() is true, will this limitation (losing multicast packets) still exist ? I see multicast and normal data packets are directed to different QPs, aren't they ? I'm trying to understand the ramifications of running IPOIB-CM in a production environment, as it does perform (much) better. Thanks, Wendy -- To unsubscribe from this list: send the line "unsubscribe linux-rdma" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html
