Subject: Re: Add PGM protocol support to the IP stack

From: Andi Kleen <[email-masked]>
Date: Mon, 22 Mar 2010 19:53:10 +0100

On Mon, Mar 22, 2010 at 01:07:37PM -0500, Christoph Lameter wrote:
> > >         B. PGM over UDP
> > >
> > >                 fd = socket(AF_INET, SOCK_RDM, IPPROTO_UDP)
> > >
> > >         C. PGM over SHM (?)
> > >
> > >                 fd = socket(AF_UNIX, SOCK_RDM, 0)
> >
> > Not sure how that should work.
> 
> 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?

Letting kernel parse a data structure in user defined memory is also
always somewhat tricky.

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)

> > >
> > >         Packet sizes are determined by the number of  packets in a single sendmsg() unless
> >
> > Number of bytes surely?
> 
> Sorry yes you are right.
> 
> > >         overridden by the RM_SET_MESSAGE_BOUNDARY socket option.
> >
> > 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.

> > >
> > >         A. Setting the window size / rate.
> > >
> > >                 struct pgm_send_window x;
> > >                 x.RateKbitsPerSec = 56;
> > >                 x.WindowSizeInMsecs = 60000;
> > >                 x.WindowSizeinBytes = 10000000;
> > >
> > >                 setsockopt(fd, SOCK_RDM, RM_RATE_WINDOW_SIZE, &x, sizeof(x));
> > >
> > >                 Default is sending at 56Kbps with a buffer of 10 Megabytes and buffering for a minute.
> >
> > That's a very large buffer for a socket. It would be better to use the usual
> > auto shrinking/increasing mechanisms.
> 
> 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?

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

> Never used it. I'd rather skip for now. Maybe later.
> 
> >
> > > /* Socket API structures (established by M$DN) */
> > > struct pgm_receiver_stats {
> > >         u64     NumODataPacketsReceived;        /* Number of ODATA (original) sequences */
> >
> > It's difficult to maintain 64 bit counters on 32bit hosts on all targets.
> > But I guess it would be ok to only fill in 32bit in this case.
> 
> 32 bit counters have the awful habit of overflowing.

There's just no portable atomic64_t. Ok maybe you can use the socket lock
to synchronize all the counts if they are only per socket.

-Andi

-- 
[email-masked] -- Speaking for myself only.