netdev - Re: Throughput regression with `tcp: refine TSO autosizing`

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Date:	Fri, 06 Feb 2015 05:53:05 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Michal Kazior <michal.kazior@...to.com>
Cc:	Neal Cardwell <ncardwell@...gle.com>,
	linux-wireless <linux-wireless@...r.kernel.org>,
	Network Development <netdev@...r.kernel.org>,
	eyalpe@....mellanox.co.il
Subject: Re: Throughput regression with `tcp: refine TSO autosizing`

On Fri, 2015-02-06 at 05:40 -0800, Eric Dumazet wrote:

> tcp_wfree() could maintain in tp->tx_completion_delay_ms an EWMA
> of TX completion delay. But this would require yet another expensive
> call to ktime_get() if HZ < 1000.
> 
> Then tcp_write_xmit() could use it to adjust :
> 
>    limit = max(2 * skb->truesize, sk->sk_pacing_rate >> 9);
> 
> to
> 
>    amount = (2 + tp->tx_completion_delay_ms) * sk->sk_pacing_rate 
> 
>    limit = max(2 * skb->truesize, amount / 1000);
> 
> I'll cook a patch.

Hmm... doing this in all protocols would be too expensive,
and we do not want to include time spent in qdiscs.

wifi could eventually do that, providing in skb->tx_completion_delay_us
the time spent in wifi driver.

This way, we would have no penalty for network devices doing normal skb
orphaning (loopback interface, ethernet, ...)


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