From: Eric Dumazet <eric.duma...@gmail.com> Date: Sat, 27 Aug 2016 07:37:54 -0700
> From: Eric Dumazet <eduma...@google.com> > > When TCP operates in lossy environments (between 1 and 10 % packet > losses), many SACK blocks can be exchanged, and I noticed we could > drop them on busy senders, if these SACK blocks have to be queued > into the socket backlog. > > While the main cause is the poor performance of RACK/SACK processing, > we can try to avoid these drops of valuable information that can lead to > spurious timeouts and retransmits. > > Cause of the drops is the skb->truesize overestimation caused by : > > - drivers allocating ~2048 (or more) bytes as a fragment to hold an > Ethernet frame. > > - various pskb_may_pull() calls bringing the headers into skb->head > might have pulled all the frame content, but skb->truesize could > not be lowered, as the stack has no idea of each fragment truesize. > > The backlog drops are also more visible on bidirectional flows, since > their sk_rmem_alloc can be quite big. > > Let's add some room for the backlog, as only the socket owner > can selectively take action to lower memory needs, like collapsing > receive queues or partial ofo pruning. > > Signed-off-by: Eric Dumazet <eduma...@google.com> Really nice change, thanks Eric.
