Auto-suspend and Presence Service

[John Gilmore]

In 9.1.0, auto-suspend is going to be on by default, which we estimate
will roughly triple our battery life.  We're working on fixing all the
bugs that would prevent that.

One of them is not strictly a bug -- it's a performance issue.  The
Presence Service sends packets frequently, for tracking the presence
of your friends and any shared activities.  Many of these are
multicast packets (some are probably unicast jabber packets, too).

In 8.2.0, with auto-suspend enabled, you have to run a specific command
to enable suspended laptops to awaken when multicast packets arrive;
see bug #6818 and run "ethtool -s msh0 wol um".  This will become the
default in 9.1.0.

The problem I can foresee is that the Presence Service will send 
packets so frequently that everyone's laptop will always be waking up,
negating the power-stretching abilities of auto-suspend.  This seems
to occur with only a few laptops around (in my tests at home); yet
we need to make it work with dozens of laptops within range, for
our deployments.

I hope that the Collabora team is running in this mode now, and
working out ways to reduce packet traffic to let sleeping laptops lie.
The cellular phone industry went through several cycles of design
improvements like this, enabling today's remarkable phone battery
life, without impact on our ability to rapidly communicate whenever
desired.  OLPC hasn't yet done the first round of such improvements,
and I hope it WILL do so before 9.1.0.  We could all use those long
battery lives -- especially the kids in the rural deployments.  We
don't want to tell them to turn off collaboration if they want their
laptop to last all day; it's one of our key features.

        John