[Spread-users] consequences of making a large group
prubel at bbn.com
Wed Jul 26 14:53:19 EDT 2006
Thanks for your help, this is exactly what I was hoping for.
John Lane Schultz writes:
> Paul Rubel wrote:
> > I'm considering adding some functionality to my system that would
> > entail adding every spread-using application in my system into a
> > single group. This group would be used to occasionally disseminate
> > name-service like information.
> This is actually a fairly common practice.
> > Does the size of a group
> > really matter or is the number of daemons and the network topology the
> > real limiter? Thinking about the ring topology it seems the total size
> > could be more important than the size of any group.
> In Spread the more limiting factor tends to be the number of daemons and the
> network topology.
That's more or less what I was hoping for.
> Spread can support groups of up to about 2000 members out of the box. The
> limiting factor is that membership view changes have to fit into a single Spread
> message and (in the worst case) each member (32 bytes) can be listed twice. If
> you increase the size of the max message, then you (I believe) increase the
> limit on the number of group members as well.
> The main effect of having groups with very many members is that the group
> membership list for each group gets large (obviously). Therefore, if your
> clients track membership, then the Spread daemon has to push large messages (e.g
> - 32-64KB for 1000 members) to each of the members every time there is a
> membership. Furthermore, often the more clients you have in a group, the more
> often group memberships occur. If you have a lot of clients connecting to the
> same daemon, then you can begin to see quadratic performance degradation in
> membership changes because the group list grows linearly w/ the size of the
> group and if you have (a fraction of) linear clients connecting to a single
> daemon that causes it to generate linear number of membership messages; so the
> effects multiply to become quadratic.
Good point, I will keep that in mind.
> > Does the group structure have any effect on recovery time when a host
> > is lost? I'm guessing that it doesn't as the underlying structure
> > needs to be repaired regardless of any group membership.
> The only effect would be on in memory data structure operations would should be
> very fast (i.e. - not noticeable) relative to the network protocol. Of course,
> the load from the operations discussed above (delivering membership messages to
> clients) could affect performance noticeably.
> > Currently we have 9 hosts with 10s of groups. Would 30 hosts 100
> > groups change the answer?
> It shouldn't. The more important questions are how many clients are going to be
> in your system, what is the maximum number of clients connecting through a
> single daemon and how many clients are going to sit in that one mega group?
The clients should be fairly evenly distributed and only have a few
per host so things look pretty good.
thanks again for your help,
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