netdev - Re: TCP performance regression

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Date:	Mon, 11 Nov 2013 09:38:35 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Felix Fietkau <nbd@...nwrt.org>
Cc:	Sujith Manoharan <sujith@...jith.org>, netdev@...r.kernel.org,
	Dave Taht <dave.taht@...il.com>
Subject: Re: TCP performance regression

On Mon, 2013-11-11 at 17:38 +0100, Felix Fietkau wrote:
> On 2013-11-11 17:13, Sujith Manoharan wrote:
> > Eric Dumazet wrote:
> >> We have many choices.
> >> 
> >> 1) Add back a minimum of ~128 K of outstanding bytes per TCP session,
> >>    so that buggy drivers can sustain 'line rate'.
> >> 
> >>    Note that with 100 concurrent TCP streams, total amount of bytes
> >>    queued on the NIC is 12 MB.
> >>    And pfifo_fast qdisc will drop packets anyway.
> >> 
> >>    Thats what we call 'BufferBloat'
> >> 
> >> 2) Try lower values like 64K. Still bufferbloat.
> >> 
> >> 3) Fix buggy drivers, using a proper logic, or shorter timers (mvneta
> >> case for example)
> >> 
> >> 4) Add a new netdev attribute, so that well behaving NIC drivers do not
> >> have to artificially force TCP stack to queue too many bytes in
> >> Qdisc/NIC queues.
> > 
> > I think the quirks of 802.11 aggregation should be taken into account.
> > I am adding Felix to this thread, who would have more to say on latency/bufferbloat
> > with wireless drivers.
> I don't think this issue is about something as simple as timer handling
> for tx completion (or even broken/buggy drivers).
> 
> There's simply no way to make 802.11 aggregation work well and have
> similar tx completion latency characteristics as Ethernet devices.
> 
> 802.11 aggregation reduces the per-packet airtime overhead by combining
> multiple packets into one transmission (saving a lot of time getting a
> tx opportunity, transmitting the PHY header, etc.), which makes the
> 'line rate' heavily depend on the amount of buffering.

How long a TX packet is put on hold hoping a following packet will
come ?



> Aggregating multiple packets into one transmission also causes extra
> packet loss, which is compensated by retransmission and reordering, thus
> introducing additional latency.
> 
> I don't think that TSQ can do a decent job of mitigating bufferbloat on
> 802.11n devices without a significant performance hit, so adding a new
> netdev attribute might be a good idea.

The netdev attribute would work, but might not work well if using a
tunnel...



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