Exit Balancing Patch

Mike Perry mikeperry at fscked.org
Wed Jul 18 09:51:32 UTC 2007

Thus spake Roger Dingledine (arma at mit.edu):

> On Wed, Jul 18, 2007 at 02:18:56AM -0700, Mike Perry wrote:
> > Why does it matter if routers are CPU or network bound? From the Tor
> > network routing point of view, it shouldn't matter, capacity is
> > capacity. If it is a problem for node operators, they limit their
> > bandwidth in the config, problem solved. If they don't, then they just
> > run at 100% CPU, and Tor should still properly report their observed
> > bandwidth rate (unless they lie, but again, that is a another,
> > orthogonal matter).
> Well, the problem is that Tor in fact doesn't properly report capacity at
> the extremes. Our measure of capacity is the most you've seen yourself
> burst in the past day -- it pretty much assumes that the pipe and other
> resources you have are static throughout the day.
> So if you somehow managed to push a lot briefly but you're too busy to
> handle that level of traffic sustained, then you've overadvertised.

Again, this happens both with network load and CPU load. In fact, I
think the network load capacity differences are far more extreme and
far more common. On my box, Tor uses 10% CPU, but is constantly
prioritized below regular traffic, which is bursty and very irregular.
My node is also below the 1.5Mbyte/sec barrier, so it does not help
even out this problem for me, nor most others who share Tor with
normal traffic.

> Putting a cap on advertised bandwidth when load balancing is a crude way
> to account for this. Making our bandwidth estimate more complex may also
> work, but then we have to figure out what's better. :)

If it has to be fixed one place, it is probably best done by not
advertising burst traffic. That problem is independent of this patch,
though, IMO.

Mike Perry
Mad Computer Scientist
fscked.org evil labs
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