[Spread-users] Ordering

[John Schultz]

The token based protocol of Spread 3 does incur some extra latency 
because a daemon only sends a message on the network when it has the 
token.  In the worst case, the daemon just released the token and has to 
wait for it to traverse the entire ring before it can send any more 
messages.  Also, if your network is lossy, then delivery may be delayed 
at recipients (at least for AGREED msgs) until earlier messages are 
recovered and delivered to maintain a local total order.

Also, remember that with Spread you have a client-server architecture. 
So when you send a message across the network the one way latency is the 
total time it takes for: sender -> local Spread daemon -> remote Spread 
daemon -> receiver.

This multi-step architecture incurs some additional latency even when 
the sender and receiver are local w.r.t. their Spread daemons. The best 
way to reduce this type of latency would probably be to tune your 
kernels to swap processes and respond to I/O requests faster somehow.

As to your main question, I wouldn't expect CPU power to affect latency 
numbers for this protocol as much as the size of your ring, the 
latency/lossiness of your network, the tuning of Spread and the tuning 
of your kernel, probably in that order of importance.

P. Krishna wrote:
> Hello Yair,
> Thank you for the prompt response. I read the 2 pager. 
> Spread maintains only one ring of spread daemons, and the seq# is global
> across all groups.
> 
> Looking @ Figure 1.4 which plots mesg latency versus number of groups.
> If I look at the curve for 10000B packets, the latency is about 2.8ms.
> The xmission latency due to pkt over 100Mb link is 0.8ms - what is the
> breakup of the remaining 2ms? How much of it is due to CPU (P3 in this
> expt) and how much is due to the ordering protocol - i.e., token based
> access?
> 
> I am just trying to get a gauge of performance expectation if we use a
> different setup for example, faster CPUs but continue using 100Mb/s
> links.
> 
> Thanks,
> Krishna
> 

-- 
John Lane Schultz
Spread Concepts LLC
Phn: 443 838 2200