Hi Morten,

On Tue, 21 Feb 2017 10:22:57 +0100, Morten Brørup
[off-list ref] wrote:

Hi Olivier,

There has been a lot of debate about the size and location (1st or
2nd cache line) of the mbuf fields.

May I suggest that you add a comment to each field briefly mentioning
why it has a specific size (8 or 16 bits) (e.g. by example use case),
and optionally also why it deserves to be in the first cache line. (A
comment to the mbuf structure can say that fields manipulated by PMDs
on ingress generally belong in the 1st cache line.)

It's not necessary for all fields, just the non-obvious ones (the
ones discussed here on the mailing list). It will make the
information more easily accessible to avoid repeating the same
discussions in the future.

E.g. the port field is 16 bits because a hypervisor can serve more
than 256 virtual machines. And it is in the 1st cache line because a
PMD for a multi-port NIC with a shared ingress queue needs to set it
on ingress.

And the refcnt field has the same size as the port field to support
L3 multicast and L2 port flooding on all ports.

I understand your point about not rediscussing things several times. I
don't think having a comment for fields is really relevant because it
would add many info that are not useful for the user of the structure,
but I think adding something in the API documentation of the rte_mbuf
structure itself makes sense to me.

I'll add something in the next version of the patch.

Thanks,
Olivier