Re: Add PGM protocol support to the IP stack

Christoph Lameter
2010-03-22T19:32:26Z

On Mon, 22 Mar 2010, Andi Kleen wrote:

> > Multiple processes would communicate via shm segments. Maybe defer to the
> > future but its an important operation mode as the systems grow bigger and bigger.
> > SHM segment would have to contain some sort of ring buffer that the
> > receivers could tap into. But that mode has not really been thought
> > through.
>
> AF_UNIX is not SHM today.
>
> The only point is to avoid one copy? (user1 -> kernel -> user2 to user1 -> user2)
> Not sure if that is really worth it. Don't you need another copy to the reliability
> buffer anyways?

Not sure either. Access of multiple processes to one reliability buffer
would be best. Some sort of multiended pipe I guess.

> But in principle AF_INET over localhost should not be that less efficient
> than AF_UNIX, so you can probably drop it for now (unless you need special AF_UNIX
> features like credentials)

Well lets skip it for now and see if there are performance implications in
the future.

> > > That's unusual to have such a option (except the MTU). What is it good for?
> >
> > No idea why it was implemented. It can be used to use send() for portions
> > of a message. Triggers the send() only when all bytes have been provided.
> > Probably necessary if one wants to have very long (megabytes) messages.
>
> Those could be a problem in kernel memory consumption. One would need
> to be very careful to have a good memory management scheme for the socket
> in place.

Lets not support it then unless someone can make a convincing case.

> > Reliable multicast protocols have a defined time period / "reliabilty
> > buffer" so that they can resend a message that was missed for a time
> > period. It is customary to either specify a time period or define the size
> > of the "reliability buffer".
>
> One problem is memory management then. What happens when a process opens 100 of those
> sockets and fills them all?

Pushes out the app? Same as the user space apps now. Some sort of
upper limit is needed I guess.

> I guess you would still need a suitable global limit like TCP has.

Yes.