netdev - Re: tbench wrt. loopback TSO

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Message-ID: <4911EA97.8080101@hp.com>
Date:	Wed, 05 Nov 2008 10:48:55 -0800
From:	Rick Jones <rick.jones2@...com>
To:	Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>
CC:	Evgeniy Polyakov <zbr@...emap.net>,
	David Miller <davem@...emloft.net>,
	Netdev <netdev@...r.kernel.org>, efault@....de, mingo@...e.hu,
	a.p.zijlstra@...llo.nl, Herbert Xu <herbert@...dor.apana.org.au>
Subject: Re: tbench wrt. loopback TSO

Ilpo Järvinen wrote:
> On Wed, 5 Nov 2008, Evgeniy Polyakov wrote:
> 
> 
>>On Wed, Nov 05, 2008 at 02:25:57PM +0200, Ilpo Järvinen (ilpo.jarvinen@...sinki.fi) wrote:
>>
>>>The problem is that we'd need to _resegment with the next skb_ since the 
>>>mss boundary and skb boundary would basically constantly be running 
>>>out-of-sync. That won't get done currently by anything.
>>
>>Btw, what's that wrong if there will be sub-mss frame per tso frame?
> 
> 
> I personally don't consider that to be a big deal... I suppose some see
> it as bad thing because of the slightly larger header vs data ratio...
> Which is significant only if you can saturate the link (or have unbounded 
> bandwidth such as with lo), so slower links are more affected than high 
> speed ones...

Can't say that I tend to "like" subMSS segments out there in a bulk 
transfer but some pseudorandom thoughts:

And the worst that would be would be one full MSS and a single byte, 
getting us an average of (MSS+1)/2 (roughly).  It only gets better from 
there (2MSS+1)/3, (3MSS+1)/4 etc etc.

Ignoring the TSO case for a moment, if there is congestion and receiver 
window available and a user makes a > MSS send that isn't an integral 
multiple of the MSS, we don't delay the last subMSS segment do we?

rick jones
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