netdev - Re: [PATCH v2] tcp: tsq: restore minimal amount of queueing

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Message-Id: <20131114.162606.1037335356168882336.davem@davemloft.net>
Date:	Thu, 14 Nov 2013 16:26:06 -0500 (EST)
From:	David Miller <davem@...emloft.net>
To:	eric.dumazet@...il.com
Cc:	arno@...isbad.org, sujith@...jith.org, xiyou.wangcong@...il.com,
	netdev@...r.kernel.org, nbd@...nwrt.org
Subject: Re: [PATCH v2] tcp: tsq: restore minimal amount of queueing

From: Eric Dumazet <eric.dumazet@...il.com>
Date: Wed, 13 Nov 2013 06:32:54 -0800

> From: Eric Dumazet <edumazet@...gle.com>
> 
> After commit c9eeec26e32e ("tcp: TSQ can use a dynamic limit"), several
> users reported throughput regressions, notably on mvneta and wifi
> adapters.
> 
> 802.11 AMPDU requires a fair amount of queueing to be effective.
> 
> This patch partially reverts the change done in tcp_write_xmit()
> so that the minimal amount is sysctl_tcp_limit_output_bytes.
> 
> It also remove the use of this sysctl while building skb stored
> in write queue, as TSO autosizing does the right thing anyway.
> 
> Users with well behaving NICS and correct qdisc (like sch_fq),
> can then lower the default sysctl_tcp_limit_output_bytes value from
> 128KB to 8KB.
> 
> This new usage of sysctl_tcp_limit_output_bytes permits each driver
> authors to check how their driver performs when/if the value is set
> to a minimum of 4KB.
> 
> Normally, line rate for a single TCP flow should be possible, 
> but some drivers rely on timers to perform TX completion and
> too long TX completion delays prevent reaching full throughput.
> 
> Fixes: c9eeec26e32e ("tcp: TSQ can use a dynamic limit")
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> Reported-by: Sujith Manoharan <sujith@...jith.org>
> Reported-by: Arnaud Ebalard <arno@...isbad.org>
> Tested-by: Sujith Manoharan <sujith@...jith.org>

Applied and queued up for -stable, thanks Eric.
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