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

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Message-ID: <CAA93jw4H4MBMgP-gpF6R35RZhYnpZ78r5RQ=6GT2TtGfuECadQ@mail.gmail.com>
Date:	Tue, 2 Dec 2014 09:31:20 -0800
From:	Dave Taht <dave.taht@...il.com>
To:	Ezequiel Garcia <ezequiel.garcia@...e-electrons.com>
Cc:	Eric Dumazet <eric.dumazet@...il.com>, Willy Tarreau <w@....eu>,
	"netdev@...r.kernel.org" <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, Dec 2, 2014 at 9:12 AM, Ezequiel Garcia
<ezequiel.garcia@...e-electrons.com> 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.

at a gigabit.

Wildly variable with pause frames. And: 50ms at 100mbit. 500ms at 10mbit.

Secondly, why accept any latency in the driver when you can get it well below
2ms in most circumstances with a couple lines of BQL code?

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

BQL is nice at any speed, and darn useful at lower speeds,
as well.

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

We only do one upload or one download at a time on our systems,
followed by a ping measurement... never do more than one flow
at the same time, right?


> ""
>
> Maybe I misunderstood something?

Try it, test it, you'll like it.

> --
> Ezequiel García, Free Electrons
> Embedded Linux, Kernel and Android Engineering
> http://free-electrons.com
> --
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-- 
Dave Täht

http://www.bufferbloat.net/projects/bloat/wiki/Upcoming_Talks
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