netdev - Re: Packet loss when txqueuelen is zero

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Date:	Sat, 28 Jun 2014 07:40:18 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Stefan Wahren <info@...egoodbye.de>
Cc:	netdev@...r.kernel.org, arnd@...db.de
Subject: Re: Packet loss when txqueuelen is zero

On Sat, 2014-06-28 at 12:59 +0200, Stefan Wahren wrote:
> Hi,
> 
> i'm new to Linux network driver development and currently i want to port 
> the QCA7000 network driver to mainline [1]. I concentrate my tests on tx 
> buffering since my last QCA7000 patch RFC [2]. Now i've found a test 
> scenario which leads to packet loss:
> 
> host A          Powerline       host B
> (QCA7000)       Ethernet        Ethernet
> 192.168.1.3     adaptor         192.168.1.5
> |------------------|---------------|
>       Homeplug         Ethernet
>       10 Mbit          100 Mbit
> 
> 1. Reduce the txqueuelen from 100 (default value) to 0
> 2. Run ping in flood mode on host A to host B
> 
> ping -c 200 -s 10000 -f 192.168.1.5
> 
> 3. ping reports a high packet loss
> 
> Additional information:
> - QCA7000 network driver has a tx ring size of 10 packets
> - the packet loss doesn't appear when txqueuelen is 100
> 
> Here are my questions:
> 
> Is the packet loss a expected result for this scenario?

Sure it is totally expected.

-f is a flood ping, and you remove ability to store packets in the Qdisc
(pifo_fast limit is device txqueuelen).

If you have txqueuelen = 100, then the socket used by ping will more
likely hit its SO_SNDBUF limit and ping will handle this properly (it
detects that a sendmsg() returns -1, errno = ENOBUF

If QCA7000 network driver has a tx ring size of 10 packets, you really
want a qdisc being able to store bursts.

If bufferbloat is your concern, you can switch pfifo_fast to fq_codel or
fq

tc qdisc replace dev eth0 root fq_codel  (or fq to get TCP pacing)




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