On Mon, Mar 20, 2006 at 02:37:04AM -0800, David S. Miller wrote: > From: "Michael S. Tsirkin" <[EMAIL PROTECTED]> > Date: Mon, 20 Mar 2006 12:22:34 +0200 > > > Quoting r. David S. Miller <[EMAIL PROTECTED]>: > > > The path an SKB can take is opaque and unknown until the very last > > > moment it is actually given to the device transmit function. > > > > Why, I was proposing looking at dst cache. If that's NULL, well, > > we won't stretch ACKs. Worst case we apply the wrong optimization. > > Right? > > Where you receive a packet from isn't very useful for determining > even the full patch on which that packet itself flowed. > > More importantly, packets also do not necessarily go back out over the > same path on which packets are received for a connection. This is > actually quite common. > > Maybe packets for this connection come in via IPoIB but go out via > gigabit ethernet and another route altogether. > > > What I'd like to clarify, however: rfc2581 explicitly states that in > > some cases it might be OK to generate ACKs less frequently than > > every second full-sized segment. Given Matt's measurements, TCP on > > top of IP over InfiniBand on Linux seems to hit one of these cases. > > Do you agree to that? > > I disagree with Linux changing it's behavior. It would be great to > turn off congestion control completely over local gigabit networks, > but that isn't determinable in any way, so we don't do that. > > The IPoIB situation is no different, you can set all the bits you want > in incoming packets, the barrier to doing this remains the same. > > It hurts performance if any packet drop occurs because it will require > an extra round trip for recovery to begin to be triggered at the > sender. > > The network is a black box, routes to and from a destination are > arbitrary, and so is packet rewriting and reflection, so being able to > say "this all occurs on IPoIB" is simply infeasible. > > I don't know how else to say this, we simply cannot special case IPoIB > or any other topology type.
David is right. If you care about performance, you are already using SDP or verbs layer for the transport anyway. If I am going to be doing IPoIB, it's because eventually I expect the packet might get off the IB network and onto some other network and go halfway across the country. - To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html
