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

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Date:	Tue, 02 Dec 2014 10:39:49 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Ezequiel Garcia <ezequiel.garcia@...e-electrons.com>
Cc:	Willy Tarreau <w@....eu>, 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

On Tue, 2014-12-02 at 14:12 -0300, Ezequiel Garcia wrote:
> 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?

This was indeed the case, but we added recently xmit_more support, and
it uses BQL information. So you might add BQL anyway, if xmit_more
support is useful for this hardware.



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