[Tech] Backoff considered harmful?

[Michael Rogers]

toad wrote:
> In which case, can you detect at what point the throttle becomes the
> limiting factor rather than the submission of requests?

The network with no flow control gets better throughput than the 
networks with flow control even at very low request rates - this 
probably just reflects the fact that it's hard to get 100% utilisation 
from an adaptive throttling mechanism. Anyway the difference is small at 
low request rates.

The difference gets larger as the request rate increases - then 
somewhere between 10 and 12 requests per node per minute, the throughput 
of the network without flow control drops practically to zero as the 
message queues get long enough to cause most requests to time out. This 
is with 15 kbyte/s upstream and downstream bandwidth per node. I don't 
think asymmetric bandwidth will make a big difference, because upstream 
bandwidth will remain the bottleneck - the point where the collapse 
occurs might shift a bit due to reduced queueing delays on the 
downstream side, but the collapse will still happen.

By 16 requests per node per minute, the throughput of the various flow 
control mechanisms seems to be tailing off slightly - I need to run more 
simulations to see whether this is due to random variation or whether it 
represents a trend, but either way there's no dramatic collapse like 
there is without flow control.

Cheers,
Michael