netdev - Re: TSQ accounting skb->truesize degrades throughput for large packets

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Message-ID: <1378574494.26319.14.camel@edumazet-glaptop>
Date:	Sat, 07 Sep 2013 10:21:34 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Zoltan Kiss <zoltan.kiss@...rix.com>
Cc:	Wei Liu <wei.liu2@...rix.com>,
	Jonathan Davies <Jonathan.Davies@...citrix.com>,
	Ian Campbell <ian.campbell@...rix.com>, netdev@...r.kernel.org,
	xen-devel@...ts.xenproject.org
Subject: Re: TSQ accounting skb->truesize degrades throughput for large
 packets

On Fri, 2013-09-06 at 10:00 -0700, Eric Dumazet wrote:
> On Fri, 2013-09-06 at 17:36 +0100, Zoltan Kiss wrote:
> 
> > So I guess it would be good to revisit the default value of this 
> > setting.
> 
> If ixgbe requires 3 TSO packets in TX ring to get line rate, you also
> can tweak dev->gso_max_size from 65535 to 64000.

Another idea would be to no longer use tcp_limit_output_bytes but

max(sk_pacing_rate / 1000, 2*MSS)

This means that number of packets in FQ would be limited to the
equivalent of 1ms, so TCP could have faster response to packet losses : 

Retransmitted packets would not have to wait for prior packets being
drained from FQ

For a 8Gbps flow, 1Gbyte/s, sk_pacing_rate would be 2Gbyte, this would
translate to ~2 Mbytes in Qdisc/TX ring.

sk_pacing_rate was introduced in linux-3.12, but could be backported
easily.



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