Total Unacked Data...? explaination needed

[Wallace Wirefly]

When moving around different places in SL from single estate sims vs. large joined estate sims I look at Total Unacked data statistics occasionally using FS viewer. From what I read on this it reflects: The size of the reliable packet data sitting on the server waiting to be acknowledged. A large number may indicate a thin pipe or other possible problems between the viewer and the simulator. What I would like to understand is this on Viewer side or Server side if one experiences LARGE (>1000Kb or more) Unacked? What is meaning of thin pipe? Does this occur because of ones type of computer used and/or internet settings?  The reason I am asking this is I have noticed what seems like slower rendering all over Second life. So is it me ? or are the servers just not handing off (waiting to be acknowledged) ? Has the pipe line thinned ? Is there a differences between SL VIewer and Firestorm viewer as it relates to Unacknowledged data? Any Information on this would be appreciated. 🙂

[animats]

Large values there are unusual, and will result in some moving objects moving somewhat jerkily. Check "ping sim" and "packet loss".

In most SL viewers, message processing is a spare-time activity, done once per frame. If the scene is complicated and the frame rate is low, fewer messages are processed. This is why ping time goes up for complex scenes. The network isn't slower or overloaded. The viewer is. If the viewer gets too far behind, it drops messages. Reliable ones get re-transmitted, unreliable ones are lost. When the viewer is overloaded, the "total unacked data" count can build up at the server end.

You can make this happen for test purposes by flying in the air so that you can see a large area and turning the draw distance way up. The frame rate will drop and the unacked data count will go way up. The server is trying to tell you about all that area in view but the viewer can't keep up. After a while, though, the viewer should catch up and the unacked data count should drop.

The other possibility is that you're losing lots of packets. If your frame rate is good, over 30 FPS, and you're seeing this, there probably really is a networking problem. Are you on WiFi? Mobile data? A slow network connection?

Are you seeing visual effects from all this? Objects missing?

Notes:

• The design of SL comes from an era of much slower networking and single-core CPUs. There are some deliberate bottlenecks built in to cope with that. The win is that SL continues to work on rather low-end PCs and over weak networks. The lose is that there are visual problems such as missing objects and jerky motion from packet loss.
• There's a game design decision - if a client is falling behind or has network problems, do you let it stay connected, or kick it off to prevent annoying other players? Most first person shooters will kick a lagging client off, because gameplay becomes confused and players get angry. SL will let laggards stay connected, even if some things are visually wrong.
• SL predates "eventually consistent" theory. If you lose packets, sim and viewer are out of sync, but should eventually come  back in sync. SL doesn't quite have that.  Each region server thinks it knows the exact list of objects each viewer knows about. If that gets out of sync, it's usually necessary to leave the region, or at least get beyond draw distance, to get the trouble spot re-synchronized. This is what many "interest list" bugs are about.
• There are a fixed number of retransmissions allowed for UDP packets. If there is too much unacked data, even "reliable" data is lost. This prevents the servers from choking on unacked data, but it creates out of sync conditions.

 

[Wallace Wirefly]

Hello thanks for information. To answer some of your questions. My packet loss is almost always 0%. My frame rate varies of course depending on where I am but would say in most calm areas I run on average  30 - 40 fps at ~ 200 m draw. Sim ping also varies 85 - 120 on average. I am hard wired to router my speed test from east coast to Oregon (Portland) runs 91 ms ping 600 Mbps download 88 ms ping upload 23 Mbps. What I am noticing is just standing in one area the unacknowledged data goes from 0 to sometimes 1200- 1500. What I am wondering has the way the servers are setup in talking to the client changed as to how  the pipes are used?  I am not experiencing packet loss that has been 99.5% of the time 0. Sometime back the issue of avatar rendering was a problem I think Linden supposedly moved this to server side. Recently many have noticed that the old "orange cloud" has returned where it is all that one sees for sometime until avatar appears. Also I was wondering how much difference there is between SL viewer and Firestorm. Firestorm uses a Cache system to supposedly help not sure if SL viewer has cache. One would think that storing data in cache would help with the issue. Does it?

Thanks again for helping. I am getting a better understanding but not sure if SL is getting worse or my computer is issue or something else. I am pretty sure its not my network.It's a rendering problem which involves the viewer "talking" with the server through "pipes" over  network and it seems that unacknowledged data is varying a lot along with "orange clouds" .🥺

[Ardy Lay]

I have always assumed that "Total Unacked Data", as a simulator statistic, includes data in flight to all agents connected to the simulator.

[animats]

Practical answer: you're probably trying to draw too much. Reduce your draw distance and see if the unacknowledged data value goes down. Try turning off shadows. That sort of thing.

A key point with classic SL viewers is that both networking and drawing run in the same thread, and they compete for resources. This is why rendering bottlenecks affect networking.

[Gwyneth Llewelyn]

On 4/24/2023 at 4:15 AM, animats said:

A key point with classic SL viewers is that both networking and drawing run in the same thread, and they compete for resources. This is why rendering bottlenecks affect networking.

I know, I know, I'm bumping an old thread, but I found this comment incredibly interesting!

Not having taken a look at the viewer code (well, I just glimpsed a few lines here and there), I thought that the single-thread solution designed for single-core systems of the late 1990s had been deprecated... oh, at least when the SL Viewer was bumped to 2.0. We're now close to 7.0... and they're still on just one thread?!  

So, are there non-classic SL viewers then? Which ones?