Re: [PATCH net-next] tcp: avoid frag allocation for small frames

[Eric Dumazet <eric.dumazet@gmail.com>]

Le lundi 28 novembre 2011 à 17:04 -0800, Vijay Subramanian a écrit :
> On 22 October 2011 05:23, Eric Dumazet <eric.dumazet@gmail.com> wrote:
> > tcp_sendmsg() uses select_size() helper to choose skb head size when a
> > new skb must be allocated.
> >
> > If GSO is enabled for the socket, current strategy is to force all
> > payload data to be outside of headroom, in PAGE fragments.
> >
> > This strategy is not welcome for small packets, wasting memory.
> >
> > Experiments show that best results are obtained when using 2048 bytes
> > for skb head (This includes the skb overhead and various headers)
> >
> > This patch provides better len/truesize ratios for packets sent to
> > loopback device, and reduce memory needs for in-flight loopback packets,
> > particularly on arches with big pages.
> >
> > If a sender sends many 1-byte packets to an unresponsive application,
> > receiver rmem_alloc will grow faster and will stop queuing these packets
> > sooner, or will collapse its receive queue to free excess memory.
> >
> > netperf -t TCP_RR results are improved by ~4 %, and many workloads are
> > improved as well (tbench, mysql...)
> >
> > Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
> > ---
> >  net/ipv4/tcp.c |    9 ++++++---
> >  1 file changed, 6 insertions(+), 3 deletions(-)
> >
> > diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
> > index 704adad..cd45b44 100644
> > --- a/net/ipv4/tcp.c
> > +++ b/net/ipv4/tcp.c
> > @@ -897,9 +897,12 @@ static inline int select_size(const struct sock *sk, int sg)
> >        int tmp = tp->mss_cache;
> >
> >        if (sg) {
> > -               if (sk_can_gso(sk))
> > -                       tmp = 0;
> > -               else {
> > +               if (sk_can_gso(sk)) {
> > +                       /* Small frames wont use a full page:
> > +                        * Payload will immediately follow tcp header.
> > +                        */
> > +                       tmp = SKB_WITH_OVERHEAD(2048 - MAX_TCP_HEADER);
> > +               } else {
> >                        int pgbreak = SKB_MAX_HEAD(MAX_TCP_HEADER);
> >
> >                        if (tmp >= pgbreak &&
> >
> >
> 
> 
> 
> This patch from Eric fixing select_size in tcp.c was queued to be
> applied but does not seem to be in net-next tree. Was this somehow
> overlooked or have I missed something?
> 

Good catch Vijay, thanks for noticing !

David, do you want me to respin this ?