From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ed1-x530.google.com (mail-ed1-x530.google.com [IPv6:2a00:1450:4864:20::530]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by lists.bufferbloat.net (Postfix) with ESMTPS id 2C1033CB42 for ; Tue, 26 Oct 2021 01:32:45 -0400 (EDT) Received: by mail-ed1-x530.google.com with SMTP id r4so8587652edi.5 for ; Mon, 25 Oct 2021 22:32:45 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=broadcom.com; s=google; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=zCTwOPC56dpu+BV1YyiAmCoPyY87GZkCo947IKDaePw=; b=UbYB0wSNhzGIDEQ3pSJEJsSLKS7oVHnrx3YeipfoQd54iWcteL/goZQXxXvNb12vFu MZVqPU9f4vtin2VM0iu8fZjgxWxZl2h589fF0iKM4de40VyjuAwEcHdriJYWTNPlMjOa 9s31NWwj23KI8lN1EySY0H444UhS/H1TSIs34= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=zCTwOPC56dpu+BV1YyiAmCoPyY87GZkCo947IKDaePw=; b=CaFeXat633UXTncCnn4Qzj6rdoVO9poO3+ycrt/HVD7d5AXRQ5nr0jpNynIJ15UEDJ qjVW+UBI0O2iBvuYCGabVAE2Drnj9te96yEfFLwaYdt7Z/TYMgf1oTcG2BM6WNv5fSwn JvSo9qlY62XzSDaNF8X4CENfW3wq9ACYjc7gSB3PYp6Ey4UL+C45Umi8pg5B6rn9WcQ4 r0+P4vKYmcool18QsTRYXAyU8h5ySaQfR93/pB1HFa+Y8fD6M11Gs7HvD4sgOU/apoTO 3r7h/5zPBz2xDhJmzJc4/4pOhoiDqQlC8/YVLaRSx5RwN0iDs10lb4j+A28cxPTKrsQI PfPg== X-Gm-Message-State: AOAM533kSQzDQ+J/ruvQqGNxruAeABsr+N5kNPPysTZRypnkW4HlfJ/F V1q78DkQKt07wz+A/DNi/lNoHlPK1eaTCqeF7eu98eFMLzcv62tVT4hg/czQot+BzXiOGKuL9Lr GK4YzueamojJRO9WsVN0qONU36M3o2SxLHIqUA6N7 X-Google-Smtp-Source: ABdhPJyTPmkDDQuTMXf/GNb/YD1ZiX02IhisrOUfyydL8+Zj3gKbftgtgiIZ2Kp6qLC+wIn8aXXtska+l5dLtVoRfUc= X-Received: by 2002:aa7:d5c2:: with SMTP id d2mr2095622eds.56.1635226363935; Mon, 25 Oct 2021 22:32:43 -0700 (PDT) MIME-Version: 1.0 References: <1625188609.32718319@apps.rackspace.com> <989de0c1-e06c-cda9-ebe6-1f33df8a4c24@candelatech.com> <1625773080.94974089@apps.rackspace.com> <1625859083.09751240@apps.rackspace.com> <257851.1632110422@turing-police> <1632680642.869711321@apps.rackspace.com> In-Reply-To: From: Bob McMahon Date: Mon, 25 Oct 2021 22:32:33 -0700 Message-ID: Subject: Re: [Make-wifi-fast] TCP_NOTSENT_LOWAT applied to e2e TCP msg latency To: Stuart Cheshire Cc: "David P. Reed" , Cake List , =?UTF-8?Q?Valdis_Kl=C4=93tnieks?= , Make-Wifi-fast , starlink@lists.bufferbloat.net, codel , Matt Mathis , cerowrt-devel , bloat , Steve Crocker , Vint Cerf , Neal Cardwell Content-Type: multipart/alternative; boundary="0000000000008d845105cf3ace31" X-List-Received-Date: Tue, 26 Oct 2021 05:32:45 -0000 --0000000000008d845105cf3ace31 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable Thanks Stuart this is helpful. I'm measuring the time just before the first write() (of potentially a burst of writes to achieve a burst size) per a socket fd's select event occurring when TCP_NOT_SENT_LOWAT being set to a small value, then sampling the RTT and CWND and providing histograms for all three, all on that event. I'm not sure the correctness of RTT and CWND at this sample point. This is a controlled test over 802.11ax and OFDMA where the TCP acks per the WiFi clients are being scheduled by the AP using 802.11ax trigger frames so the AP is affecting the end/end BDP per scheduling the transmits and the acks. The AP can grow the BDP or shrink it based on these scheduling decisions. From there we're trying to maximize network power (throughput/delay) for elephant flows and just latency for mouse flows. (We also plan some RF frequency stuff to per OFDMA) Anyway, the AP based scheduling along with aggregation and OFDMA makes WiFi scheduling optimums non-obvious - at least to me - and I'm trying to provide insights into how an AP is affecting end/end performance. The more direct approach for e2e TCP latency and network power has been to measure first write() to final read() and compute the e2e delay. This requires clock sync on the ends. (We're using ptp4l with GPS OCXO atomic references for that but this is typically only available in some labs.) Bob On Mon, Oct 25, 2021 at 8:11 PM Stuart Cheshire wrote: > On 21 Oct 2021, at 17:51, Bob McMahon via Make-wifi-fast < > make-wifi-fast@lists.bufferbloat.net> wrote: > > > Hi All, > > > > Sorry for the spam. I'm trying to support a meaningful TCP message > latency w/iperf 2 from the sender side w/o requiring e2e clock > synchronization. I thought I'd try to use the TCP_NOTSENT_LOWAT event to > help with this. It seems that this event goes off when the bytes are in > flight vs have reached the destination network stack. If that's the case, > then iperf 2 client (sender) may be able to produce the message latency b= y > adding the drain time (write start to TCP_NOTSENT_LOWAT) and the sampled > RTT. > > > > Does this seem reasonable? > > I=E2=80=99m not 100% sure what you=E2=80=99re asking, but I will try to h= elp. > > When you set TCP_NOTSENT_LOWAT, the TCP implementation won=E2=80=99t repo= rt your > endpoint as writable (e.g., via kqueue or epoll) until less than that > threshold of data remains unsent. It won=E2=80=99t stop you writing more = bytes if > you want to, up to the socket send buffer size, but it won=E2=80=99t *ask= * you for > more data until the TCP_NOTSENT_LOWAT threshold is reached. In other word= s, > the TCP implementation attempts to keep BDP bytes in flight + > TCP_NOTSENT_LOWAT bytes buffered and ready to go. The BDP of bytes in > flight is necessary to fill the network pipe and get good throughput. The > TCP_NOTSENT_LOWAT of bytes buffered and ready to go is provided to give t= he > source software some advance notice that the TCP implementation will soon > be looking for more bytes to send, so that the buffer doesn=E2=80=99t run= dry, > thereby lowering throughput. (The old SO_SNDBUF option conflates both > =E2=80=9Cbytes in flight=E2=80=9D and =E2=80=9Cbytes buffered and ready t= o go=E2=80=9D into the same > number.) > > If you wait for the TCP_NOTSENT_LOWAT notification, write a chunk of n > bytes of data, and then wait for the next TCP_NOTSENT_LOWAT notification, > that will tell you roughly how long it took n bytes to depart the machine= . > You won=E2=80=99t know why, though. The bytes could depart the machine in= response > for acks indicating that the same number of bytes have been accepted at t= he > receiver. But the bytes can also depart the machine because CWND is > growing. Of course, both of those things are usually happening at the sam= e > time. > > How to use TCP_NOTSENT_LOWAT is explained in this video: > > > > Later in the same video is a two-minute demo (time offset 42:00 to time > offset 44:00) showing a =E2=80=9Cbefore and after=E2=80=9D demo illustrat= ing the dramatic > difference this makes for screen sharing responsiveness. > > > > Stuart Cheshire --=20 This electronic communication and the information and any files transmitted= =20 with it, or attached to it, are confidential and are intended solely for=20 the use of the individual or entity to whom it is addressed and may contain= =20 information that is confidential, legally privileged, protected by privacy= =20 laws, or otherwise restricted from disclosure to anyone else. If you are=20 not the intended recipient or the person responsible for delivering the=20 e-mail to the intended recipient, you are hereby notified that any use,=20 copying, distributing, dissemination, forwarding, printing, or copying of= =20 this e-mail is strictly prohibited. If you received this e-mail in error,= =20 please return the e-mail to the sender, delete it from your computer, and= =20 destroy any printed copy of it. --0000000000008d845105cf3ace31 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable
Thanks Stuart this is helpful. I'm measuring=C2=A0the = time just before the first write() (of potentially a burst of writes to ach= ieve a burst size) per a socket fd's select event occurring when TCP_NO= T_SENT_LOWAT being set to a small value, then sampling the RTT and CWND and= providing histograms for all three, all on that event. I'm not sure th= e correctness of RTT and CWND at this sample point. This is a controlled te= st over 802.11ax and OFDMA where the TCP acks per the WiFi clients are bein= g scheduled by the AP using 802.11ax trigger frames so the AP is affecting = the end/end BDP per scheduling the transmits and the acks. The AP can grow = the BDP or shrink it based on these scheduling decisions.=C2=A0 From there = we're trying to maximize network power (throughput/delay) for elephant = flows and just latency for mouse flows. (We also plan some RF frequency stu= ff to per OFDMA) Anyway, the AP based scheduling along with aggregation=C2= =A0and OFDMA makes WiFi scheduling optimums non-obvious - at least to me - = and I'm trying to provide insights into how an AP is affecting end/end = performance.

The more direct approach for e2e TCP latency and networ= k power has been to measure first write() to final read() and compute the e= 2e delay. This requires clock sync on the ends. (We're using ptp4l with= GPS OCXO atomic=C2=A0references=C2=A0for that but this is typically only a= vailable in some labs.)=C2=A0

Bob
=C2=A0

On Mon, Oct 25= , 2021 at 8:11 PM Stuart Cheshire <cheshire@apple.com> wrote:
On 21 Oct 2021, at 17:51, Bob McMahon via Make-wifi-fast= <make-wifi-fast@lists.bufferbloat.net> wrote:

> Hi All,
>
> Sorry for the spam. I'm trying to support a meaningful TCP message= latency w/iperf 2 from the sender side w/o requiring e2e clock synchroniza= tion. I thought I'd try to use the TCP_NOTSENT_LOWAT event to help with= this. It seems that this event goes off when the bytes are in flight vs ha= ve reached the destination network stack. If that's the case, then iper= f 2 client (sender) may be able to produce the message latency by adding th= e drain time (write start to TCP_NOTSENT_LOWAT) and the sampled RTT.
>
> Does this seem reasonable?

I=E2=80=99m not 100% sure what you=E2=80=99re asking, but I will try to hel= p.

When you set TCP_NOTSENT_LOWAT, the TCP implementation won=E2=80=99t report= your endpoint as writable (e.g., via kqueue or epoll) until less than that= threshold of data remains unsent. It won=E2=80=99t stop you writing more b= ytes if you want to, up to the socket send buffer size, but it won=E2=80=99= t *ask* you for more data until the TCP_NOTSENT_LOWAT threshold is reached.= In other words, the TCP implementation attempts to keep BDP bytes in fligh= t + TCP_NOTSENT_LOWAT bytes buffered and ready to go. The BDP of bytes in f= light is necessary to fill the network pipe and get good throughput. The TC= P_NOTSENT_LOWAT of bytes buffered and ready to go is provided to give the s= ource software some advance notice that the TCP implementation will soon be= looking for more bytes to send, so that the buffer doesn=E2=80=99t run dry= , thereby lowering throughput. (The old SO_SNDBUF option conflates both =E2= =80=9Cbytes in flight=E2=80=9D and =E2=80=9Cbytes buffered and ready to go= =E2=80=9D into the same number.)

If you wait for the TCP_NOTSENT_LOWAT notification, write a chunk of n byte= s of data, and then wait for the next TCP_NOTSENT_LOWAT notification, that = will tell you roughly how long it took n bytes to depart the machine. You w= on=E2=80=99t know why, though. The bytes could depart the machine in respon= se for acks indicating that the same number of bytes have been accepted at = the receiver. But the bytes can also depart the machine because CWND is gro= wing. Of course, both of those things are usually happening at the same tim= e.

How to use TCP_NOTSENT_LOWAT is explained in this video:

<https://developer.apple.com/v= ideos/play/wwdc2015/719/?time=3D2199>

Later in the same video is a two-minute demo (time offset 42:00 to time off= set 44:00) showing a =E2=80=9Cbefore and after=E2=80=9D demo illustrating t= he dramatic difference this makes for screen sharing responsiveness.

<https://developer.apple.com/v= ideos/play/wwdc2015/719/?time=3D2520>

Stuart Cheshire

This ele= ctronic communication and the information and any files transmitted with it= , or attached to it, are confidential and are intended solely for the use o= f the individual or entity to whom it is addressed and may contain informat= ion that is confidential, legally privileged, protected by privacy laws, or= otherwise restricted from disclosure to anyone else. If you are not the in= tended recipient or the person responsible for delivering the e-mail to the= intended recipient, you are hereby notified that any use, copying, distrib= uting, dissemination, forwarding, printing, or copying of this e-mail is st= rictly prohibited. If you received this e-mail in error, please return the = e-mail to the sender, delete it from your computer, and destroy any printed= copy of it. --0000000000008d845105cf3ace31--