[Spread-users] Ordering

Yair Amir yairamir at cnds.jhu.edu
Tue Jun 22 12:08:55 EDT 2004


P.> Hello Yair,
P.> Are you using the TOTEM-type (token-passing) algorithm for ensuring
P.> agreed/safe ordering of messages?

Yes - the Totem algorithm is the basis for agreed and safe ordering
during regular operation in the open source version.

P.> Assuming thats true, then I presume you are doing the following:

Unfortunately, the description in your e-mail below is *not* a good
description for Spread. The recent 2-pager we put on the web
(that will appear as part of Ken Birman book's new edition) can
answer most of your questions and provide some representative
performance measurements.



      :) Yair.  http://www.cs.jhu.edu/~yairamir

P.> - For each Spread group, Spread creates a "token ring" and each daemon
P.> basically keeps track of the TOTEM protocol params (seq#, aru etc) for
P.> each of the rings it is part of.
P.> - Each ring has a single seq# which gets incremented upon each message
P.> getting inserted into the ring. This effectively serializes the messages
P.> thru a "global fifo" and hence ensures total ordering.
P.> - the token is the media-access protocol. So, the Token Ring traversal
P.> time is at least the media access time. 
P.> - Either the last node on the ring for a message discards the message
P.> (based on zero TTL detection) or maybe the source itself discards the
P.> message upon getting a match on the src ID.

P.> The question I have is how many messages can be tagged along with the
P.> token? Say, I have ten nodes on the ring and say Node 1 initiates the
P.> token, and then all the nodes along the ring path consume all the
P.> messages in the token and also transmit a message. What is the max pkt
P.> size? Of course, it cannot be more than the ethernet-frame-size - what
P.> if the # of messages being tagged along causes multiple ethernet pkts to
P.> be sent across, how do you ensure the "group of pkts = single
P.> token+messages" relationship?

P.> Do you have results of latencies of agreed ordering w/ respect to
P.> message traffic, number of members/group. I dont understand how the
P.> latencies are not much different between agreed and say basic/fifo
P.> ordering?

P.> Thanks & Regards,
P.> Krishna

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