On Wed, Dec 23, 2020 at 11:59:13AM +0000, Stuart Henderson wrote:
> On 2020/12/17 20:50, Jan Klemkow wrote:
> > ping
> >
> > On Fri, Nov 06, 2020 at 01:10:52AM +0100, Jan Klemkow wrote:
> > > bluhm and I make some network performance measurements and kernel
> > > profiling.
>
> I've been running this on my workstation since you sent it out - lots
> of long-running ssh connections, hourly reposync, daily rsync of base
> snapshots.
>
> I don't know enough about TCP stack behaviour to really give a meaningful
> OK, but certainly not seeing any problems with it.
Thanks, Stuart. Has someone else tested this diff? Or, are there some
opinions or objections about it? Even bike-shedding is welcome :-)
Thanks,
Jan
> > > Setup: Linux (iperf) -10gbit-> OpenBSD (relayd) -10gbit-> Linux (iperf)
> > >
> > > We figured out, that the kernel uses a huge amount of processing time
> > > for sending ACKs to the sender on the receiving interface. After
> > > receiving a data segment, we send our two ACK. The first one in
> > > tcp_input() direct after receiving. The second ACK is send out, after
> > > the userland or the sosplice task read some data out of the socket
> > > buffer.
> > >
> > > The fist ACK in tcp_input() is called after receiving every other data
> > > segment like it is discribed in RFC1122:
> > >
> > > 4.2.3.2 When to Send an ACK Segment
> > > A TCP SHOULD implement a delayed ACK, but an ACK should
> > > not be excessively delayed; in particular, the delay
> > > MUST be less than 0.5 seconds, and in a stream of
> > > full-sized segments there SHOULD be an ACK for at least
> > > every second segment.
> > >
> > > This advice is based on the paper "Congestion Avoidance and Control":
> > >
> > > 4 THE GATEWAY SIDE OF CONGESTION CONTROL
> > > The 8 KBps senders were talking to 4.3+BSD receivers
> > > which would delay an ack for atmost one packet (because
> > > of an ack’s clock’ role, the authors believe that the
> > > minimum ack frequency should be every other packet).
> > >
> > > Sending the first ACK (on every other packet) coasts us too much
> > > processing time. Thus, we run into a full socket buffer earlier. The
> > > first ACK just acknowledges the received data, but does not update the
> > > window. The second ACK, caused by the socket buffer reader, also
> > > acknowledges the data and also updates the window. So, the second ACK,
> > > is much more worth for a fast packet processing than the fist one.
> > >
> > > The performance improvement is between 33% with splicing and 20% without
> > > splice:
> > >
> > > splicing relaying
> > >
> > > current 3.1 GBit/s 2.6 GBit/s
> > > w/o first ack 4.1 GBit/s 3.1 GBit/s
> > >
> > > As far as I understand the implementation of other operating systems:
> > > Linux has implement a custom TCP_QUICKACK socket option, to turn this
> > > kind of feature on and off. FreeBSD and NetBSD sill depend on it, when
> > > using the New Reno implementation.
> > >
> > > The following diff turns off the direct ACK on every other segment. We
> > > are running this diff in production on our own machines at genua and on
> > > our products for several month, now. We don't noticed any problems,
> > > even with interactive network sessions (ssh) nor with bulk traffic.
> > >
> > > Another solution could be a sysctl(3) or an additional socket option,
> > > similar to Linux, to control this behavior per socket or system wide.
> > > Also, a counter to ACK every 3rd, 4th... data segment could beat the
> > > problem.
> > >
> > > bye,
> > > Jan
> > >
> > > Index: netinet/tcp_input.c
> > > ===================================================================
> > > RCS file: /cvs/src/sys/netinet/tcp_input.c,v
> > > retrieving revision 1.365
> > > diff -u -p -r1.365 tcp_input.c
> > > --- netinet/tcp_input.c 19 Jun 2020 22:47:22 -0000 1.365
> > > +++ netinet/tcp_input.c 5 Nov 2020 23:00:34 -0000
> > > @@ -165,8 +165,8 @@ do { \
> > > #endif
> > >
> > > /*
> > > - * Macro to compute ACK transmission behavior. Delay the ACK unless
> > > - * we have already delayed an ACK (must send an ACK every two segments).
> > > + * Macro to compute ACK transmission behavior. Delay the ACK until
> > > + * a read from the socket buffer or the delayed ACK timer causes one.
> > > * We also ACK immediately if we received a PUSH and the ACK-on-PUSH
> > > * option is enabled or when the packet is coming from a loopback
> > > * interface.
> > > @@ -176,8 +176,7 @@ do { \
> > > struct ifnet *ifp = NULL; \
> > > if (m && (m->m_flags & M_PKTHDR)) \
> > > ifp = if_get(m->m_pkthdr.ph_ifidx); \
> > > - if (TCP_TIMER_ISARMED(tp, TCPT_DELACK) || \
> > > - (tcp_ack_on_push && (tiflags) & TH_PUSH) || \
> > > + if ((tcp_ack_on_push && (tiflags) & TH_PUSH) || \
> > > (ifp && (ifp->if_flags & IFF_LOOPBACK))) \
> > > tp->t_flags |= TF_ACKNOW; \
> > > else \
> > >
> >
>
