Quoting r. David Miller <[EMAIL PROTECTED]>: > Subject: Re: Dropping NETIF_F_SG since no checksum feature. > > From: "Michael S. Tsirkin" <[EMAIL PROTECTED]> > Date: Thu, 12 Oct 2006 21:12:06 +0200 > > > Quoting r. David Miller <[EMAIL PROTECTED]>: > > > Subject: Re: Dropping NETIF_F_SG since no checksum feature. > > > > > > Numbers? > > > > I created two subnets on top of the same pair infiniband HCAs: > > I was asking for SG vs. non-SG numbers so I could see proof > that it really does help like you say it will. >
Dave, thanks for the clarification. Please note that ib0 is a non-SG device with MTU 2K, sorry that I forgot to mention that. so, to summarize my previous mail: interface flags mtu bandwidth ib0 linear(0) 2044 286.45 ibc0 _F_SG 65484 782.55 If I will set both ib0 and ibc0 to 64K MTU, then benchmark-mode with the same MTU SG is somewhat slower than non-SG (I tested this at some point, by some 10%, don't have the numbers at the moment - do you want to see them?). I did not claim it is faster to do SG with same MTU and it is I think clear why linear should be faster for copy *with the same MTU*. But do you really think that we will be able to allocate even a single 64K linear skb after the machine has been active for a while? My assumption is that if I want to reliably get MTU > PAGE_SIZE I must support SG. Is it the wrong one? If this assumption is correct, then below is my line of thinking: - with infiniband we provably get a 2.5x speedup with MTU of 64K vs to 2K. - to get packets of that size reliably we must declare S/G support - infiniband verbs do not support IP checksumming - per network algorithmics, it is better to piggyback checksum calculation on copying if copying takes place For this reason, I would like to define the meaning of S/G set when checksum bits are all clear as "we support S/G but not checksum, please checksum for us if you copy data anyway". Alternatively, add a new NETIF_F_??_CSUM bit to mean this capability. Does this make sense? Thanks, -- MST - 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
