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

[Mister Blue]

Could be that network congestion is throwing away a block of UDP packets
which becomes a block of un-acked messages which causes multiple divide
bandwidth by two? Most measurements I've seen show that UDP packet
discarding happens in bursts.


On Tue, Nov 25, 2014 at 6:39 PM, Mic Bowman <cmickeyb at gmail.com> wrote:

> 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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