Netmap: head vs cur vs tail?

[Luigi Rizzo]

On Fri, Oct 17, 2014 at 9:55 AM, Matthew P. Grosvenor <
matthew.grosvenor at cl.cam.ac.uk> wrote:

> Hi all,
> I’m trying to understand how to use the netmap framework, specifically how
> the head, tail and current “pointers” interact with each other.
>
> Looking in man NETMAP(4) (
> http://www.freebsd.org/cgi/man.cgi?query=netmap&sektion=4) under data
> structures, struct netmap_ring   it says: "  contains the index of he
> current read or write slot (cur), “. In the example code, the following
> pattern is used:
>

​the default netmap manpage at the above URL is the old one,
please use the one
for 10-stable or 11-current

http://www.freebsd.org/cgi/man.cgi
​?​
query=netmap&manpath=FreeBSD+10.0-stable

​which​
 explains in more detail the role
of the three pointers (with some ascii graphics too).

Feel free to ask for more details if the page is not clear

cheers
luigi


>         i = ring->cur;
>         ...
>         ring->cur = NETMAP_RING_NEXT(ring, i);
>
> However, in the example that ships with the netmap source (
> https://code.google.com/p/netmap/source/browse/examples/bridge.c#72 &
> https://code.google.com/p/netmap/source/browse/examples/pkt-gen.c#660)
> the following pattern is used:
>
>         j = rxring->cur;
>         while(…){
>                 j = nm_ring_next(rxring, j);
>         …
>         }
>         rxring->head = rxring->cur = j;
>
> So the obvious question is, what is the relationship between head and
> current? Do I believe the man page (and man page example) that head is not
> necessary, or do I believe the example code that head is necessary and
> should be set to the same value as current? And if so, what is the point of
> head? And why is it updated outside of the loop in both of the examples?
>
> At a high level, I’m looking for a better understanding of what head, tail
> and current mean and how they affect the processing of rings.
>
>


> Cheers,
> Matt
>
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>



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 Prof. Luigi RIZZO, rizzo at iet.unipi.it  . Dip. di Ing. dell'Informazione
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