netdev - Re: tbench wrt. loopback TSO

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Date:	Wed, 5 Nov 2008 14:49:00 +0300
From:	Evgeniy Polyakov <zbr@...emap.net>
To:	David Miller <davem@...emloft.net>
Cc:	ilpo.jarvinen@...sinki.fi, netdev@...r.kernel.org, efault@....de,
	mingo@...e.hu, a.p.zijlstra@...llo.nl, herbert@...dor.apana.org.au
Subject: Re: tbench wrt. loopback TSO

On Wed, Nov 05, 2008 at 03:42:24AM -0800, David Miller (davem@...emloft.net) wrote:
> > One idea immediately occurs to me.  Since we're effectively limited
> > to a 64K TSO frame, and the MSS is some value smaller than that, we
> > can probably get away with a reciprocol divide.  Even using a 16-bit
> > inverse value would suffice, so we wouldn't need to use u64's like
> > some other pieces of code do.  A u32 would be enough.
> 
> I couldn't get anywhere with this idea.
> 
> The problem is that 16-bits provides not enough precision for accurate
> divisions by multiplication.
> 
> For example, for a divisor of 1500 and a shift of 16 neither 43 nor 44
> provides an accurate calculation.
> 
> So we'd need to use a u64 and a shift of 32, and on 32-bit cpus that
> could be expensive, but perhaps not as expensive as the divide.

But what if we just remove that trimming at all? This may result in a
not fully filled tso frame, but who cares if we copy data from userspace
anyway, will add another potentially small copy at sending time?

-- 
	Evgeniy Polyakov
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