Re: [PATCH v2 net-next] net: introduce SO_RCVBUFAUTO to let the rcv_buf tune automatically

[Eric Dumazet]

On Tue, Feb 15, 2022 at 9:03 PM <kerneljasonxing@xxxxxxxxx> wrote:
>
> From: Jason Xing <xingwanli@xxxxxxxxxxxx>
>
> Normally, user doesn't care the logic behind the kernel if they're
> trying to set receive buffer via setsockopt. However, once the new
> value of the receive buffer is set even though it's not smaller than
> the initial value which is sysctl_tcp_rmem[1] implemented in
> tcp_rcv_space_adjust(),, the server's wscale will shrink and then
> lead to the bad bandwidth as intended.

Quite confusing changelog, honestly.

Users of SO_RCVBUF specifically told the kernel : I want to use _this_
buffer size, I do not want the kernel to decide for me.

Also, I think your changelog does not really explain that _if_ you set
SO_RCVBUF to a small value before
connect() or in general the 3WHS, the chosen wscale will be small, and
this won't allow future 10x increase
of the effective RWIN.


>
> For now, introducing a new socket option to let the receive buffer
> grow automatically no matter what the new value is can solve
> the bad bandwidth issue meanwhile it's not breaking the application
> with SO_RCVBUF option set.
>
> Here are some numbers:
> $ sysctl -a | grep rmem
> net.core.rmem_default = 212992
> net.core.rmem_max = 40880000
> net.ipv4.tcp_rmem = 4096        425984  40880000
>
> Case 1
> on the server side
>     # iperf -s -p 5201
> on the client side
>     # iperf -c [client ip] -p 5201
> It turns out that the bandwidth is 9.34 Gbits/sec while the wscale of
> server side is 10. It's good.
>
> Case 2
> on the server side
>     #iperf -s -p 5201 -w 425984
> on the client side
>     # iperf -c [client ip] -p 5201
> It turns out that the bandwidth is reduced to 2.73 Gbits/sec while the
> wcale is 2, even though the receive buffer is not changed at all at the
> very beginning.
>
> After this patch is applied, the bandwidth of case 2 is recovered to
> 9.34 Gbits/sec as expected at the cost of consuming more memory per
> socket.

How does your patch allow wscale to increase after flow is established ?

I would remove from the changelog these experimental numbers that look
quite wrong,
maybe copy/pasted from your prior version.

Instead I would describe why an application might want to clear the
'receive buffer size is locked' socket attribute.

>
> Signed-off-by: Jason Xing <xingwanli@xxxxxxxxxxxx>
> --
> v2: suggested by Eric
> - introduce new socket option instead of breaking the logic in SO_RCVBUF
> - Adjust the title and description of this patch
> link: https://lore.kernel.org/lkml/CANn89iL8vOUOH9bZaiA-cKcms+PotuKCxv7LpVx3RF0dDDSnmg@xxxxxxxxxxxxxx/
> ---
>

I think adding another parallel SO_RCVBUF option is not good. It is
adding confusion (and net/core/filter.c has been unchanged)

Also we want CRIU to work correctly.

So if you have a SO_XXXX setsockopt() call, you also need to provide
getsockopt() implementation.

I would suggest an option to clear or set SOCK_RCVBUF_LOCK,  and
getsockopt() would return if the bit is currently set or not.

Something clearly describing the intent, like SO_RCVBUF_LOCK maybe.