MIME-Version: 1.0
References: <20220104003722.73982-1-ivan@cloudflare.com> <20220103164443.53b7b8d5@hermes.local>
In-Reply-To: <20220103164443.53b7b8d5@hermes.local>
From:   Eric Dumazet <edumazet@google.com>
Date:   Tue, 4 Jan 2022 00:33:03 -0800
Message-ID: <CANn89i+bLN4=mHxQoWg88_MTaFRkn9FAeCy9dn3b9W+x=jowRQ@mail.gmail.com>
Subject: Re: [PATCH net] tcp: note that tcp_rmem[1] has a limited range
To:     Stephen Hemminger <stephen@networkplumber.org>
Cc:     Ivan Babrou <ivan@cloudflare.com>, netdev <netdev@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        kernel-team <kernel-team@cloudflare.com>,
        "David S . Miller" <davem@davemloft.net>,
        Jakub Kicinski <kuba@kernel.org>,
        Jonathan Corbet <corbet@lwn.net>
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

On Mon, Jan 3, 2022 at 4:44 PM Stephen Hemminger
<stephen@networkplumber.org> wrote:
>
> On Mon,  3 Jan 2022 16:37:22 -0800
> Ivan Babrou <ivan@cloudflare.com> wrote:
>
> > The value of rcv_ssthresh is limited to tcp_rwin, which is limited
> > to 64k at the handshake time, since window scaling is not allowed there.
> >
> > Let's add a note to the docs that increasing tcp_rmem[1] does not have
> > any effect on raising the initial value of rcv_ssthresh past 64k.

I guess you have to define what is the initial window.

There seems to be a confusion between rcv_ssthresh and sk_rcvbuf

If you want to document what is rcv_ssthresh and how it relates to sk_rcvbuf,
you probably need more than few lines in Documentation/networking/ip-sysctl.rst

> >
> > Link: https://lkml.org/lkml/2021/12/22/652
> > Signed-off-by: Ivan Babrou <ivan@cloudflare.com>
> > ---
> >  Documentation/networking/ip-sysctl.rst | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/Documentation/networking/ip-sysctl.rst b/Documentation/networking/ip-sysctl.rst
> > index 2572eecc3e86..16528bc92e65 100644
> > --- a/Documentation/networking/ip-sysctl.rst
> > +++ b/Documentation/networking/ip-sysctl.rst
> > @@ -683,7 +683,8 @@ tcp_rmem - vector of 3 INTEGERs: min, default, max
> >       default: initial size of receive buffer used by TCP sockets.
> >       This value overrides net.core.rmem_default used by other protocols.
> >       Default: 131072 bytes.
> > -     This value results in initial window of 65535.
> > +     This value results in initial window of 65535. Increasing this value
> > +     won't raise the initial advertised window above 65535.
> >
> >       max: maximal size of receive buffer allowed for automatically
> >       selected receiver buffers for TCP socket. This value does not override
>
> Why not add error check or warning in write to sysctl?

Please do not. We set this sysctl to 0.5 MB
DRS is known to have quantization artifacts.