[Opensim-dev] Behaviour of adaptive throttles under high load

[Mic Bowman]

As you mention... cutting the throttle by 50% was modeled on the TCP
congestion control approach. It is very aggressive as a congestion control
mechanism and certainly could be tuned.

That being said... do you know why the packets were considered un-acked? If
its because the simulator is having problems (which given your description
that it happens under load seems to be the case) then we can probably do
something more intelligent about throttling over all simulator BW. That
is... maybe the problem is that the top end of the overall simulator bw is
the problem, not the per connection throttles.

Manual throttles & adaptive throttles are not exclusive. You can use both.
Adaptive manages the top end, but the manual throttles set an absolute max.

--mic


On Tue, Nov 25, 2014 at 5:15 PM, Justin Clark-Casey <
jjustincc at googlemail.com> wrote:

> Hi Mic (primarily),
>
> Two years ago [1] we had a discussion about the enable_adaptive_throttles
> setting.  Just for background, this is a setting that adapts the amount of
> data sent to the viewer depending on whether reliable packets sent from the
> simulator are acked or not.  As such, it looks to make sure that a viewer
> which sets a downstream bandwidth higher than its network connection can
> cope with is not permanently hosed with too much data.  We enabled it on an
> experimental basis [2].
>
> As you said at the time, this is modelled on the congestion approach used
> in TCP.  I see that for TCP, the rate is halved on every unacked segment.
> In OpenSimulator, it's halved on every unacked reliable packet.
>
> However, under fairly modest load conditions in the conference grid, I saw
> a behaviour where sometimes for a connection a sequence of packets would
> expire for some connections in a very short time period (< 1 sec).  This
> would halve the throttle many times, in my observations right down to the
> absolute minimum.  This caused the behaviour from the user's point of view
> to degrade considerably for an extended period of time.  The throttles
> takes quite a long period to grow again.
>
> I didn't get much further with the diagnostics since a lack of time forced
> us to switch back to manual throttling instead (with a 1 mbit per viewer
> and 400 mbit total on the keynotes).  This seemed to work okay in testing
> and in the event itself.  However, this leaves one vulnerable to the
> problem adaptive_throttles looks to tackle in the first place.
>
> I'm still reading up about this stuff, but it strikes me that halving the
> throttle on every missed packet is much harsher than the TCP approach, as
> with UDP a whole sequence can expire at once rather than a single segment
> that is subsequently retried before another segment can be missed.
>
> One idea is to ignore all expiries in a certain period (e.g. next 2
> seconds) if an expired packet has already caused the throttle to be
> halved.  Of course, this is a bit more complicated to do but hopefully not
> too much so.  What do you think?  Any other ideas?
>
> [1] http://opensimulator.org/pipermail/opensim-dev/2011-
> October/023017.html
> [2] http://opensimulator.org/pipermail/opensim-dev/2011-
> October/023063.html
>
> Best Regards,
>
> --
> Justin Clark-Casey (justincc)
> OSVW Consulting
> http://justincc.org
> http://twitter.com/justincc
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>
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