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

[Sujith Manoharan <sujith@msujith.org>]

Eric Dumazet wrote:
> From: Eric Dumazet <edumazet@google.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