netdev - Re: [PATCH net-next] tcp: reduce POLLOUT events caused by TCP_NOTSENT

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Message-Id: <20181204.212145.97568551688753578.davem@davemloft.net>
Date:   Tue, 04 Dec 2018 21:21:45 -0800 (PST)
From:   David Miller <davem@...emloft.net>
To:     edumazet@...gle.com
Cc:     netdev@...r.kernel.org, eric.dumazet@...il.com, soheil@...gle.com
Subject: Re: [PATCH net-next] tcp: reduce POLLOUT events caused by
 TCP_NOTSENT_LOWAT

From: Eric Dumazet <edumazet@...gle.com>
Date: Tue,  4 Dec 2018 07:58:17 -0800

> TCP_NOTSENT_LOWAT socket option or sysctl was added in linux-3.12
> as a step to enable bigger tcp sndbuf limits.
> 
> It works reasonably well, but the following happens :
> 
> Once the limit is reached, TCP stack generates
> an [E]POLLOUT event for every incoming ACK packet.
> 
> This causes a high number of context switches.
> 
> This patch implements the strategy David Miller added
> in sock_def_write_space() :
> 
>  - If TCP socket has a notsent_lowat constraint of X bytes,
>    allow sendmsg() to fill up to X bytes, but send [E]POLLOUT
>    only if number of notsent bytes is below X/2
> 
> This considerably reduces TCP_NOTSENT_LOWAT overhead,
> while allowing to keep the pipe full.
 ...
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> Acked-by: Soheil Hassas Yeganeh <soheil@...gle.com>

Applied, thanks Eric.