d2000361e4
The current MPTCP-level RTT estimator has several issues. On high speed
links, the MPTCP-level receive buffer auto-tuning happens with a
frequency well above the TCP-level's one. That in turn can cause
excessive/unneeded receive buffer increase.
On such links, the initial rtt_us value is considerably higher than the
actual delay, and the current mptcp_rcv_space_adjust() updates
msk->rcvq_space.rtt_us with a period equal to the such field previous
value. If the initial rtt_us is 40ms, its first update will happen after
40ms, even if the subflows see actual RTT orders of magnitude lower.
Additionally:
- setting the msk RTT to the maximum among all the subflows RTTs makes
DRS constantly overshooting the rcvbuf size when a subflow has
considerable higher latency than the other(s).
- during unidirectional bulk transfers with multiple active subflows,
the TCP-level RTT estimator occasionally sees considerably higher
value than the real link delay, i.e. when the packet scheduler reacts
to an incoming ACK on given subflow pushing data on a different
subflow.
- currently inactive but still open subflows (i.e. switched to backup
mode) are always considered when computing the msk-level RTT.
Address the all the issues above with a more accurate RTT estimation
strategy: the MPTCP-level RTT is set to the minimum of all the subflows
actually feeding data into the MPTCP receive buffer, using a small
sliding window.
While at it, also use EWMA to compute the msk-level scaling_ratio, to
that MPTCP can avoid traversing the subflow list is
mptcp_rcv_space_adjust().
Use some care to avoid updating msk and ssk level fields too often.
Fixes:
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|---|---|---|
| .. | ||
| events | ||
| misc | ||
| stages | ||
| bpf_probe.h | ||
| define_custom_trace.h | ||
| define_trace.h | ||
| perf.h | ||
| syscall.h | ||
| trace_custom_events.h | ||
| trace_events.h | ||