From: "Michael Chan" <[EMAIL PROTECTED]> Date: Tue, 15 May 2007 15:05:28 -0700
> On Tue, 2007-05-15 at 14:08 -0700, Roland Dreier wrote: > > > > Well, IPoIB doesn't do netif_wake_queue() until half the device's TX > > > > queue is free, so we should get batching. However, I'm not sure that > > > > I can count on a fudge factor ensuring that there's enough space to > > > > handle everything skb_gso_segment() gives me -- is there any reliable > > > > way to get an upper bound on how many segments a given gso skb will > > > > use when it's segmented? > > > > > > Take a look at tg3.c. I use (gso_segs * 3) as the upper bound. > > > > Thanks for the pointer... I noticed that code, but could you tell me > > where the "* 3" comes from? > > > For each gso_seg, there will be a header and the payload may span 2 > pages for 1500-byte packets. We always assume 1500-byte packets because > the buggy chips do not support jumbo frames. Correct. I think there may be a case where you could see up to 4 segments. If the user corks the TCP socket, does a sendmsg() (which puts the data in the per-socket page) then does a sendfile() you'll see something like: skb->data IP, TCP, ethernet headers, etc. page0 sendmsg() data page1 sendfile page2 sendfile Ie. this can happen if the sendfile() part starts near the end of a page, so it would get split even for a 1500 MTU frame. Even more complex variants are possible if the user does tiny sendfile() requests to different pages within the file. So in fact it can span up to N pages. But there is an upper limit defined by the original GSO frame, and that is controlled by MAX_SKB_FRAGS, so at most you would see MAX_SKB_FRAGS plus some small constant. - 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
