Le jeudi 06 janvier 2011 Ã 18:33 +0900, Simon Horman a Ãcrit : > Hi, > > Back in October I reported that I noticed a problem whereby flow control > breaks down when openvswitch is configured to mirror a port[1]. > > I have (finally) looked into this further and the problem appears to relate > to cloning of skbs, as Jesse Gross originally suspected. > > More specifically, in do_execute_actions[2] the first n-1 times that an skb > needs to be transmitted it is cloned first and the final time the original > skb is used. > > In the case that there is only one action, which is the normal case, then > the original skb will be used. But in the case of mirroring the cloning > comes into effect. And in my case the cloned skb seems to go to the (slow) > eth1 interface while the original skb goes to the (fast) dummy0 interface > that I set up to be a mirror. The result is that dummy0 "paces" the flow, > and its a cracking pace at that. > > As an experiment I hacked do_execute_actions() to use the original skb > for the first action instead of the last one. In my case the result was > that eth1 "paces" the flow, and things work reasonably nicely. > > Well, sort of. Things work well for non-GSO skbs but extremely poorly for > GSO skbs where only 3 (yes 3, not 3%) end up at the remote host running > netserv. I'm unsure why, but I digress. > > It seems to me that my hack illustrates the point that the flow ends up > being "paced" by one interface. However I think that what would be > desirable is that the flow is "paced" by the slowest link. Unfortunately > I'm unsure how to achieve that. > Hi Simon ! "pacing" is done because skb is attached to a socket, and a socket has a limited (but configurable) sndbuf. sk->sk_wmem_alloc is the current sum of all truesize skbs in flight. When you enter something that : 1) Get a clone of the skb, queue the clone to device X 2) queue the original skb to device Y Then : Socket sndbuf is not affected at all by device X queue. This is speed on device Y that matters. You want to get servo control on both X and Y You could try to 1) Get a clone of skb Attach it to socket too (so that socket get a feedback of final orphaning for the clone) with skb_set_owner_w() queue the clone to device X Unfortunatly, stacked skb->destructor() makes this possible only for known destructor (aka sock_wfree()) > One idea that I had was to skb_get() the original skb each time it is > cloned - that is easy enough. But unfortunately it seems to me that > approach would require some sort of callback mechanism in kfree_skb() so > that the cloned skbs can kfree_skb() the original skb. > > Ideas would be greatly appreciated. > > [1] http://openvswitch.org/pipermail/dev_openvswitch.org/2010-October/003806.html > [2] http://openvswitch.org/cgi-bin/gitweb.cgi?p=openvswitch;a=blob;f=datapath/actions.c;h=5e16143ca402f7da0ee8fc18ee5eb16c3b7598e6;hb=HEAD > -- -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
