[Opensim-dev] Behaviour of adaptive throttles under high load
Justin Clark-Casey
jjustincc at googlemail.com
Thu Nov 27 00:04:09 UTC 2014
This was actually happening at quite low loads (< 40 connections over all 4 keynotes). Once adaptive throttles was
disabled and other unrelated issues fixed the system had no obvious issues coping with higher loads in both testing and
the conference itself (e.g. the 159 peak keynote avatars in the conference). So I don't think it was a server bandwidth
issue.
That said, it was somewhat strange behaviour as affected only maybe 10-20% of connections. Once it did affect a
connection (I saw this happening by logging downward adjustments which one can still do with the console command "debug
lludp throttles log 1"), the connection would not recover - at some point a bunch of expires would reduce the throttle
again. Connections seemed to be affected randomly - I experienced the issue myself at one point and I have pretty solid
fibre.
You're right in that I don't know why this happened or why problematic connections stayed problematic instead of slowly
recovering. Because of time constraints we had to disable adaptive instead of investigating further. But I don't
advocate doing this by default at all because, as you say, it's an important mechanism for congestion control.
I do plan further investigation will happen at some point but it's time consuming work and I'd really love to get a
release out soon-ish. So for the moment I would like to do tune the adapation mechanism tuning as you've mentioned,
which I believe should probably be done anyway. Because of the nature of the problem, my plan would be not to change
the adaption divisor but rather to adapt downwards only every 2 seconds or so if packets are expiring rather than on
every packet expire. I believe this should still achieve the adaption effect without massively penalising the
connection if there has been a momentary connection issue or similar.
On 26/11/14 02:39, Mic Bowman 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 <mailto: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
> <http://opensimulator.org/pipermail/opensim-dev/2011-October/023017.html>
> [2] http://opensimulator.org/__pipermail/opensim-dev/2011-__October/023063.html
> <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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--
Justin Clark-Casey (justincc)
OSVW Consulting
http://justincc.org
http://twitter.com/justincc
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