netdev - Re: net/mlx4_en: Pad ethernet packets smaller than 17 bytes

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Message-ID: <20140227154635.GA24226@mtl-eit-vdi-22.mtl.labs.mlnx>
Date:	Thu, 27 Feb 2014 17:46:37 +0200
From:	Amir Vadai <amirv@...lanox.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	David Laight <David.Laight@...LAB.COM>,
	"David S. Miller" <davem@...emloft.net>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	Yevgeny Petrilin <yevgenyp@...lanox.com>,
	Or Gerlitz <ogerlitz@...lanox.com>,
	Eugenia Emantayev <eugenia@...lanox.com>
Subject: Re: net/mlx4_en: Pad ethernet packets smaller than 17 bytes

On 27/02/14 06:44 -0800, Eric Dumazet wrote:
> On Thu, 2014-02-27 at 16:02 +0200, Amir Vadai wrote:
> >  
> > Yes, skb will not be classified as inline-able if nr_frags != 1.
> > See is_inline()

Maybe I wasn't accurate enough. inline-able skb must have:
1. skb->len < inline_thold 
2. nr_frags <= 1

Default value for inline_thold is 104 and it is a module parameter
that can be changed.

> 
> How often this inline stuff ever happens ?
> 
> It looks like the length of the frame needs to be shorter than 44 bytes,
> right ?
It should be shorter than 104 - inline_thold default (and max) value is 104.
I don't understand why you say 44.

> 
> Minimal length for TCP is 54 bytes.
> 
> I wonder why you tried to inline stuff that is probably never
> seen in real workload.
> 

If you mean why did I try to send packets shorter than 17 using inline
- the answer is that it is easy to do it, so why not to?

Amir
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