netdev - Re: splice() performance for TCP socket forwarding

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Date:   Thu, 13 Dec 2018 06:03:25 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Marek Majkowski <marek@...udflare.com>, eric.dumazet@...il.com
Cc:     netdev@...r.kernel.org
Subject: Re: splice() performance for TCP socket forwarding

On 12/13/2018 05:33 AM, Marek Majkowski wrote:
> Ok, 4.19 does seem to kinda fix the SO_RCVLOWAT with splice, but I
> don't fully understand it:
> 
> fcntl(8, F_SETPIPE_SZ, 1048576)         = 1048576 <0.000033>
> setsockopt(4, SOL_SOCKET, SO_RCVLOWAT, [131072], 4) = 0 <0.000014>
> splice(4, NULL, 9, NULL, 1048576, SPLICE_F_MOVE) = 121435 <71.039385>
> splice(8, NULL, 5, NULL, 121435, SPLICE_F_MOVE) = 121435 <0.000118>
> splice(4, NULL, 9, NULL, 1048576, SPLICE_F_MOVE) = 11806 <0.000019>
> splice(8, NULL, 5, NULL, 11806, SPLICE_F_MOVE) = 11806 <0.000018>
>

Good point.

At this moment SO_RCVLOWAT only tries to reduce number of POLLIN events.

But if your splice() system call is performed while there are already
available skbs in the receive queue, splice() wont block and deliver
what is available in the queue.

I guess that we would need to add some logic in recvmsg() and tcp_splice_read()
to truly implement SO_RCVLOWAT : block until at least sk->sk_rcvlowat bytes are
available in receive queue.