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Eric Dumazet says: ==================== tcp: switch to Early Departure Time model In the early days, pacing has been implemented in sch_fq (FQ) in a generic way : - SO_MAX_PACING_RATE could be used by any sockets. - TCP would vary effective pacing rate based on CWND*MSS/SRTT - FQ would ensure delays between packets based on current sk->sk_pacing_rate, but with some quantum based artifacts. (inflating RPC tail latencies) - BBR then tweaked the pacing rate in its various phases (PROBE, DRAIN, ...) This worked reasonably well, but had the side effect that TCP RTT samples would be inflated by the sojourn time of the packets in FQ. Also note that when FQ is not used and TCP wants pacing, the internal pacing fallback has very different behavior, since TCP emits packets at the time they should be sent (with unreasonable assumptions about scheduling costs) Van Jacobson gave a talk at Netdev 0x12 in Montreal, about letting TCP (or applications for UDP messages) decide of the Earliest Departure Time, instead of letting packet schedulers derive it from pacing rate. https://www.netdevconf.org/0x12/session.html?evolving-from-afap-teaching-nics-about-time https://www.files.netdevconf.org/d/46def75c2ef345809bbe/files/?p=/Evolving%20from%20AFAP%20%E2%80%93%20Teaching%20NICs%20about%20time.pdf Recent additions in linux provided SO_TXTIME and a new ETF qdisc supporting the new skb->tstamp role This patch series converts TCP and FQ to the same model. This might in the future allow us to relax tight TSQ limits (if FQ is present in the output path), and thus lower number of callbacks to tcp_write_xmit(), thanks to batching. This will be followed by FQ change allowing SO_TXTIME support so that QUIC servers can let the pacing being done in FQ (or offloaded if network device permits) For example, a TCP flow rated at 24Mbps now shows a more meaningful RTT Before : ESTAB 0 211408 10.246.7.151:41558 10.246.7.152:33723 cubic wscale:8,8 rto:203 rtt:2.195/0.084 mss:1448 rcvmss:536 advmss:1448 cwnd:20 ssthresh:20 bytes_acked:36897937 segs_out:25488 segs_in:12454 data_segs_out:25486 send 105.5Mbps lastsnd:1 lastrcv:12851 lastack:1 pacing_rate 24.0Mbps/24.0Mbps delivery_rate 22.9Mbps busy:12851ms unacked:4 rcv_space:29200 notsent:205616 minrtt:0.026 After : ESTAB 0 192584 10.246.7.151:61612 10.246.7.152:34375 cubic wscale:8,8 rto:201 rtt:0.165/0.129 mss:1448 rcvmss:536 advmss:1448 cwnd:20 ssthresh:20 bytes_acked:170755401 segs_out:117931 segs_in:57651 data_segs_out:117929 send 1404.1Mbps lastsnd:1 lastrcv:56915 lastack:1 pacing_rate 24.0Mbps/24.0Mbps delivery_rate 24.2Mbps busy:56915ms unacked:4 rcv_space:29200 notsent:186792 minrtt:0.054 A nice side effect of this patch series is a reduction of max/p99 latencies of RPC workloads, since the FQ quantum no longer adds artifact. ==================== Signed-off-by: David S. Miller <davem@davemloft.net> |
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| README | ||
README
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
See Documentation/00-INDEX for a list of what is contained in each file.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.