Thank you very much for the info, is there a good Infiniband reference
(other than the IBA spec, I mean) I should read?
-Peter
On 11/24/2015 05:08 PM, Anuj Kalia wrote:
I don't have experience with multicast, but here's some info.
InfiniBand flow control is done at the link layer, so UD does not drop
packets due to congestion.
AFAIK, UD only drops packets due to irrecoverable bit errors and
network device failures. Mellanox's FDR physical layer has BER less
than 10^(-15), and forward error correction on top of that, so an
irrecoverable bit error is extremeley extremely rare.
If the network topology does not have multipath, (Mellanox) UD will
not reorder packets to a particular destination sent from the same UD
QP. There is probably some guarantee in multipath topologies, too.
--Anuj (rdma_guy)
On Tue, Nov 24, 2015 at 10:10 PM, Peter Chinetti
<[email protected]> wrote:
I've been reading* through the IBA spec (Release 1.3 2015-03-03), trying to
understand IB multicast and its pitfalls.
I understand that IB multicast only supports Unreliable Datagram sends
(10.5.2.1), and that there are neither delivery guarantees nor
acknowledgments for UD sends. Furthermore, flow control is only available
for Reliable Connections. I thought I saw that there was an ordering
guarantee within a multicast group for a specific sender, but now I can't
find that section again.
How far is the send guaranteed to propagate? What would cause the data to be
dropped? Is it possible for an oversubscribed (e.g. a 80 Gb/s burst of
multicast bandwidth trying to fit through a link with only 40 Gb/s of
bandwidth) link to slow down multicast data on a different network path, or
will burst be "clipped" by dropping packets?
In testing some co-workers of mine have done we have found that when
multicast is run in parallel over IB and Ether, the IB is occasionally
slower, but does not seem to drop packets. This seems to suggest that flow
control /is/ working for multicast (which would be great, as long as we know
where the pathological cases are and avoid them).
Thank you,
Peter Chinetti
*Honestly, more like Control-F'ing for "multicast". Not super effective.
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