Re: tbench wrt. loopback TSO

[David Miller <davem@davemloft.net>]

From: David Miller <davem@davemloft.net>
Date: Mon, 27 Oct 2008 11:39:04 -0700 (PDT)

> 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.