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

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Message-ID: <21122.18481.785917.621199@gargle.gargle.HOWL>
Date:	Tue, 12 Nov 2013 20:54:33 +0530
From:	Sujith Manoharan <sujith@...jith.org>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Arnaud Ebalard <arno@...isbad.org>,
	David Miller <davem@...emloft.net>,
	Cong Wang <xiyou.wangcong@...il.com>, netdev@...r.kernel.org,
	Felix Fietkau <nbd@...nwrt.org>
Subject: Re: [PATCH] tcp: tsq: restore minimal amount of queueing

Eric Dumazet wrote:
> 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.
> 
> The new usage of sysctl_tcp_limit_output_bytes permits each driver
> author to check how 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 delays prevent reaching full throughput.

I tested the patch with ath9k and performance with a 2-stream card is normal
again, about 195 Mbps in open air.

Thanks for the fix !

Also, I think this needs to be marked as a stable candidate, since 3.12 needs
this fix.

Sujith
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