AW: mulitcast copy or snowball copy

Donald Becker becker at
Mon Aug 18 16:00:04 EDT 2003

On 18 Aug 2003, Mitchell Skinner wrote:
> On Mon, 2003-08-18 at 09:50, Donald Becker wrote:

> > This is costly.  "Open loop" multicast protocols work by having the
> > receiver track the missing blocks, and requesting (or interpolating)
> > them later.  Here you are discarding that information and doing much
> > extra work on both the sending and receiving side by later locating the
> > missing blocks.
> There's an ietf working group on reliable multicast that wasn't making a
> whole lot of progress the last time I checked.

It's a hard problem, and when they agree on a protocol it likely won't
apply to clusters.

The packet loss characteristic and cost trade-off is much different on a
WAN than with a local Ethernet switch on a cluster.  On a WAN every
packet is costly to transport, so it's worth having both end stations
doing extensive computations.

On a cluster we might talk about doing more computation to avoid
communication, but that's only for a few applications.  In reality we
prefer to do minimal work.  Thus we prefer OS-bypass for application
communication, and kernel-only for file system I/O.  Notice the
attention given to zero copy, TCP offload, TOE/TSO and sendfile().

Multicast and packet FEC add exactly what people are trying to avoid,
extra copying, complexity and work.

Donald Becker				becker at
Scyld Computing Corporation
914 Bay Ridge Road, Suite 220		Scyld Beowulf cluster system
Annapolis MD 21403			410-990-9993

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