netdev - Re: [PATCH] net: mvneta: fix Tx interrupt delay

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Date:	Tue, 02 Dec 2014 14:12:10 -0300
From:	Ezequiel Garcia <ezequiel.garcia@...e-electrons.com>
To:	Eric Dumazet <eric.dumazet@...il.com>, Willy Tarreau <w@....eu>
CC:	netdev@...r.kernel.org,
	Maggie Mae Roxas <maggie.mae.roxas@...il.com>,
	Thomas Petazzoni <thomas.petazzoni@...e-electrons.com>,
	Gregory CLEMENT <gregory.clement@...e-electrons.com>
Subject: Re: [PATCH] net: mvneta: fix Tx interrupt delay

Eric,

On 12/02/2014 09:18 AM, Eric Dumazet wrote:
[..]
> 
> I am surprised TCP even worked correctly with this problem.
> 
> I highly suggest BQL for this driver, now this issue is fixed.
> 

Implementing BQL for the mvneta driver was something I wanted to do a
while ago, but you explained that these kind drivers (i.e. those with
software TSO) didn't need BQL, because the latency that resulted from
the ring was too small.

Quoting (http://www.spinics.net/lists/netdev/msg284439.html):
""
Note that a full size TSO packet (44 or 45 MSS) requires about 88 or 90
descriptors.

So I do not think BQL is really needed, because a 512 slots TX ring wont
add a big latency : About 5 ms max.

BQL is generally nice for NIC supporting hardware TSO, where a 64KB TSO
packet consumes 3 or 4 descriptors.

Also note that TCP Small Queues should limit TX ring occupancy of a
single bulk flow anyway.
""

Maybe I misunderstood something?
-- 
Ezequiel García, Free Electrons
Embedded Linux, Kernel and Android Engineering
http://free-electrons.com
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