On 18/02/2017 00:33, Lawrence Stewart wrote:
On 18/02/2017 09:37, Steven Hartland wrote:
[snip]
So the questions:
1. Has anyone looked at this issue before or is working on it?
2. Do people think we could add support for RTT estimates and enable
receive scaling without major tcp stack changes?
We already have a non-TS-enabled RTT estimator in place c.f. the
relevant code in tcp_input.c:
if ((to.to_flags & TOF_TS) != 0 &&
to.to_tsecr) {
uint32_t t;
t = tcp_ts_getticks() - to.to_tsecr;
if (!tp->t_rttlow || tp->t_rttlow > t)
tp->t_rttlow = t;
tcp_xmit_timer(tp,
TCP_TS_TO_TICKS(t) + 1);
} else if (tp->t_rtttime &&
SEQ_GT(th->th_ack, tp->t_rtseq)) {
if (!tp->t_rttlow ||
tp->t_rttlow > ticks - tp->t_rtttime)
tp->t_rttlow = ticks - tp->t_rtttime;
tcp_xmit_timer(tp,
ticks - tp->t_rtttime);
}
The autoscaling implementation just needs to have its insistence on
using timestamps stuffed into a cannon and shot in the direction of the
sun. This is the relevant block of rcvbuf code from tcp_input.c:
if (V_tcp_do_autorcvbuf &&
(to.to_flags & TOF_TS) &&
to.to_tsecr &&
(so->so_rcv.sb_flags & SB_AUTOSIZE)) {
if (TSTMP_GT(to.to_tsecr, tp->rfbuf_ts) &&
to.to_tsecr - tp->rfbuf_ts < hz) {
if (tp->rfbuf_cnt >
(so->so_rcv.sb_hiwat / 8 * 7) &&
so->so_rcv.sb_hiwat <
V_tcp_autorcvbuf_max) {
newsize =
min(so->so_rcv.sb_hiwat +
V_tcp_autorcvbuf_inc,
V_tcp_autorcvbuf_max);
}
/* Start over with next RTT. */
tp->rfbuf_ts = 0;
tp->rfbuf_cnt = 0;
} else
tp->rfbuf_cnt += tlen; /* add up */
}
It's pretty trivial to fix by anyone so inclined.
Thanks for the pointers Lawrence, I'm not very familiar with network
stack but I've had a stab at this, after reading through the current
code in the blocks you pointed out, along side some experimentation ;-)
The results seem pretty encouraging, with S3 downloads with a ~17ms
latency on a 1Gbps jumping from ~3MB/s to ~80MB/s.
I'd appreciate feedback on the review which can be found here:
https://reviews.freebsd.org/D9668
Its a bit rough ATM in order to facilitate monitoring the behaviour with
dtrace.
My initial attempt did try to replicate the tests done to facilitate the
calls to tcp_xmit_timer, but they couldn't be reused due to the changes
the tcp_xmit_timer calls themselves trigger, so I based it off the
already calculated and stored smoothed RTT instead.
Regards
Steve
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