Re: [RFC] Patch to improve TSO limitation formula in general

On 09/06/14 00:09, Rick Macklem wrote:
> Hans Petter Selesky wrote:
>> On 09/05/14 23:19, Eric Joyner wrote:
>>> There are some concerns if we use this with devices that ixl
>>> supports:
>>>
>>> - The maximum fragment size is 16KB-1, which isn't a power of 2.
>>>
>>
>> Hi Eric,
>>
>> Multiplying by powers of two are more fast, than non-powers of two.
>> So
>> in this case you would have to use 8KB as a maximum.
>>
> Well, I'm no architecture expert, but I really doubt the CPU delay of a
> non-power of 2 multiply/divide is significant related to doing smaller
> TSO segments. Long ago (as in 1970s) I did work on machines where shifts
> for power of 2 multiply/divide was preferable, but these days I doubt it
> is going to matter??
>
>>> - You can't get the maximum TSO size for ixl devices by multiplying
>>> the
>>> maximum number of fragments by the maximum size.
>>> Instead the number of fragments is AFAIK unlimited, but a segment
>>> can only
>>> span 8 mbufs (including the [up to 3] mbufs containing the header),
>>> and the
>>> maximum TSO size is 256KB.

Hi,

Maybe that can be a separate parameter?

I see that your patch assumes that a segment can be any-length. That is 
not always the case. Remember there are JUMBO mbufs too.

With my patch, the maximum segment size is a separate parameter. The 
total number of TSO bytes is then not so useful.

--HPS