netdev - Re: [RFC] TCP_NOTSENT

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Message-ID: <1487345896.1311.80.camel@edumazet-glaptop3.roam.corp.google.com>
Date:   Fri, 17 Feb 2017 07:38:16 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Josh Hunt <johunt@...mai.com>
Cc:     edumazet@...gle.com, netdev@...r.kernel.org, jbaron@...mai.com
Subject: Re: [RFC] TCP_NOTSENT_LOWAT behavior

On Thu, 2017-02-16 at 23:07 -0800, Eric Dumazet wrote:

> This adds a cost to fast path. tcp_sendmsg() is insane.
> 
> We have one skb granularity (64KB) already for SO_SNDBUF, regardless of
> TCP_NOTSENT_LOWAT being used or not.

Note that this granularity is wanted for performance, otherwise our per
task 32KB page (current->task_frag) might be split in very small chunks
over multiple sockets on applications dealing with many TCP sockets.

Having nice initial 64KB TSO packets with 2 or 3 frags allows for better
sack coalescing

Ref :
https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=3613b3dbd1ade9a6a626dae1f608c57638eb5e8a

Thanks.