Re: kcomd - was Re: OpenMOSIX process migration mechanism

Ian,

On Sun, 2006-08-13 at 11:31 +0000, "Ian Latter"  wrote:
> Thanks for the response - I've not been clear, its my fault
> for not being explicit.  In both questions, I'm working from
> a user-space perception;
> 
> 
>   On "node awareness"
> 
>   Assuming that nodes have been discovered some
>   other way - such as omautod - then does my question
>   hold true?  That is, does kcomd provide a mechanism
>   to allow oM to exclude nodes that have "disappeared",
>   regardless of the autodiscovery daemon's perception?
> 
>   Alternatively, consider a static map.  Will oM drop its
>   in-kernel host entries for nodes that do not 
>   acknowledge kcomd messages?

Yes for both. kcomd and 'omautod' have independent node lists.  Nodes
only show up in kcomd's list if there is an active connection.  If a
connection has not yet been made (via a process migration) or a
connection is lost or severed, that node does not show up in the list.


> 
> 
>   On node "message passing"
> 
>   As there is now a concept of a node-to-node 
>   communication  (as opposed to a PID-to-PID-on-other-
>   node communication),  I would like to know if there is
>   a comms-channel that is now available for me to use
>   from user space on node "a" to user space on node
>   "b".  

No.  kcomd does not allow for a userspace to userspace connection.
migration is transparent to userspace so the process has no concept of
nodes.


> 
>   My current technique is via a socket, with a TCP
>   channel via an IPv4 network transport.
> 
>   Currently (production oM) there are native Linux
>   techniques available for message passing - such as
>   named pipes - but there is nothing specific to oM to
>   forward a message between two oM nodes.  
>   Ultimately, I'm not expecting a message passing 
>   protocol/facility to exist, as oM aims to be transparent,
>   but if one is avaliable, I'd like to use it.



<snip>

> > as far as I understand kcomd doesn't have a discovery functionality
-
> > this should be done in userspace. kcomd is used by oM to communicate
> > between migrating/migrated processes. This is al running in-kernel
and
> > will only provide the most neccesary functions. By using this
mechanism
> > (see it as communication layer) oM should become more stable.

Correct.

> > 
> > Node-to-node message passing is quite a general term... But I think
the
> > answer is more or less 'it depends, but kcomd sends and receives
> > messages'. If a node-to-node-message and a kcomd-message are the 
> same is
> > not clear to me.

They are not the same in that kcomd-messages are specific to kcomd
daemons running on distinct nodes and the processes themselves are
unaware and unable to use them from userspace.

Hope this helps,
Matt




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