Date: Tue, 2 May 2023 06:55:20 -0700 (PDT) From: "Rodney W. Grimes" <freebsd-rwg@gndrsh.dnsmgr.net> To: "Rodney W. Grimes" <freebsd-rwg@gndrsh.dnsmgr.net> Cc: Chen Shuo <chenshuo@chenshuo.com>, freebsd-net@FreeBSD.org, freebsd-transport@FreeBSD.org Subject: Re: Cwnd grows slowly during slow-start due to LRO of the receiver side. Message-ID: <202305021355.342DtKWj021076@gndrsh.dnsmgr.net> In-Reply-To: <202305021346.342Dkt32021016@gndrsh.dnsmgr.net>
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Second attempt, first one failed due to not being a member of the list :-(. > Adding freebsd-transport@freebsd.org to get that specific groups > eyes on this issue. > > Rod > > > As per newreno_ack_received() in sys/netinet/cc/cc_newreno.c, > > FreeBSD TCP sender strictly follows RFC 5681 with RFC 3465 extension > > That is, during slow-start, when receiving an ACK of 'bytes_acked' > > > > cwnd += min(bytes_acked, abc_l_var * SMSS); // abc_l_var = 2 dflt > > > > As discussed in sec3.2 of RFC 3465, L=2*SMSS bytes exactly balances > > the negative impact of the delayed ACK algorithm. RFC 5681 also > > requires that a receiver SHOULD generate an ACK for at least every > > second full-sized segment, so bytes_acked per ACK is at most 2 * SMSS. > > If both sender and receiver follow it. cwnd should grow exponentially > > during slow-slow: > > > > cwnd *= 2 (per RTT) > > > > However, LRO and TSO are widely used today, so receiver may generate > > much less ACKs than it used to do. As I observed, Both FreeBSD and > > Linux generates at most one ACK per segment assembled by LRO/GRO. > > The worst case is one ACK per 45 MSS, as 45 * 1448 = 65160 < 65535. > > > > Sending 1MB over a link of 100ms delay from FreeBSD 13.2: > > > > 0.000 IP sender > sink: Flags [S], seq 205083268, win 65535, options > > [mss 1460,nop,wscale 10,sackOK,TS val 495212525 ecr 0], length 0 > > 0.100 IP sink > sender: Flags [S.], seq 708257395, ack 205083269, win > > 65160, options [mss 1460,sackOK,TS val 563185696 ecr > > 495212525,nop,wscale 7], length 0 > > 0.100 IP sender > sink: Flags [.], ack 1, win 65, options [nop,nop,TS > > val 495212626 ecr 563185696], length 0 > > // TSopt omitted below for brevity. > > > > // cwnd = 10 * MSS, sent 10 * MSS > > 0.101 IP sender > sink: Flags [.], seq 1:14481, ack 1, win 65, length 14480 > > > > // got one ACK for 10 * MSS, cwnd += 2 * MSS, sent 12 * MSS > > 0.201 IP sink > sender: Flags [.], ack 14481, win 427, length 0 > > 0.201 IP sender > sink: Flags [.], seq 14481:31857, ack 1, win 65, length 17376 > > > > // got ACK of 12*MSS above, cwnd += 2 * MSS, sent 14 * MSS > > 0.301 IP sink > sender: Flags [.], ack 31857, win 411, length 0 > > 0.301 IP sender > sink: Flags [.], seq 31857:52129, ack 1, win 65, length 20272 > > > > // got ACK of 14*MSS above, cwnd += 2 * MSS, sent 16 * MSS > > 0.402 IP sink > sender: Flags [.], ack 52129, win 395, length 0 > > 0.402 IP sender > sink: Flags [P.], seq 52129:73629, ack 1, win 65, > > length 21500 > > 0.402 IP sender > sink: Flags [.], seq 73629:75077, ack 1, win 65, length 1448 > > > > As a consequence, instead of growing exponentially, cwnd grows > > more-or-less quadratically during slow-start, unless abc_l_var is > > set to a sufficiently large value. > > > > NewReno took more than 20 seconds to ramp up throughput to 100Mbps > > over an emulated 100ms delay link. While Linux took ~2 seconds. > > I can provide the pcap file if anyone is interested. > > > > Switching to CUBIC won't help, because it uses the logic in NewReno > > ack_received() for slow start. > > > > Is this a well-known issue and abc_l_var is the only cure for it? > > https://calomel.org/freebsd_network_tuning.html > > > > Thank you! > > > > Best, > > Shuo Chen > > > > > > -- > Rod Grimes rgrimes@freebsd.org > > -- Rod Grimes rgrimes@freebsd.org
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