[Opensim-dev] networking issues

[Dahlia Trimble]

the viewer discards small changes anyway if avatar imposters are enabled

On Mon, Mar 28, 2011 at 11:54 AM, Melanie <melanie at t-data.com> wrote:

> No, we can't discard small changes. As the avatar comes closer, they
> would be seen out of place, e.g. someone building in the distance
> would move prims and then you come closer to look and all prims
> would be out of place.
>
> Melanie
>
> Dahlia Trimble wrote:
> > a couple thoughts..
> >
> > Perhaps resend timeout period could be a function of throttle setting
> and/or
> > measured packet acknowledgement time per-client? (provided we measure
> it).
> > That may prevent excessive resend processing that may not be necessary.
> >
> > On the distance prioritization, could small changed in object
> translations
> > be discarded from the prioritization queues/resend buffers for distant
> > objects when new updates occur for those objects? Small changes may not
> be
> > noticeable from the viewer perspective anyway.
> >
> >
> > On Mon, Mar 28, 2011 at 10:48 AM, Teravus Ovares <teravus at gmail.com>
> wrote:
> >
> >> Here are a few facts that I've personally discovered while working
> >> with LLClientView.
> >>
> >> 1. It has been noted that people with poor connections to the
> >> simulator do consume more bandwidth, cpu, and have a generally worse
> >> experience.   This has been tested and profiled extensively.    This
> >> may seem like a small issue because what it's doing is so basic...
> >> however the frequency in which this occurs is a real cause of
> >> performance issues.
> >>
> >> 2. It's also noted that the CPU used in these cases reduces the CPU
> >> available to the rest of the simulator resulting in a lower quality of
> >> service for the rest of the people on the simulator.
> >> This has been seen in the profiling and has been qualitatively
> >> observed by a large number of users connected and everything is OK and
> >> then a 'problem connection' user connecting causing a wide range of
> >> issues.
> >>
> >> 3. It's also noted that lowering the outgoing UDP packet throttles
> >> beyond a certain point results in perpetual queuing and resends.
> >> This was tested by using a throttle multiplier last year that was
> >> implemented by justincc.  I'm not sure if the multiplier is still
> >> there.   It's most easily seen with image packets.   Again, I note
> >> that the packets are not rebuilt going from the regular outbound queue
> >> to the resend queue.    The resend queue is /supposed/ to be used to
> >> quickly get data that is essential to the client after attempting to
> >> send once already.   The UDP spec declares the maximum resend to be 2
> >> times, however there has been some considerable debate on whether or
> >> not OpenSimulator should follow that specific specification item
> >> leading to a configuration option to enable perpetual resends
> >> (Implemented by Melanie).  The configuration item was named similar
> >> to, 'reliable is important' or something like that.   I'm not sure if
> >> the configuration item survived the many revisions however I suspect
> >> that it did.
> >>
> >> 4. It's also noted that raising the packet throttles beyond what the
> >> connection can support results in resending almost every packet the
> >> maximum amount of times before the limit is reached.
> >> This is easily reproducible by setting the connection (in the client)
> >> to the maximum and connecting to a region that you've never been to
> >> before on a sub par connection.   Before the client adjusts and
> >> requests a lower throttle setting there's massive data loss and
> >> massive re-queuing.
> >>
> >> 5. The client tries to adjust the throttle settings based on network
> >> conditions.   This can be observed by monitoring the packet that sets
> >> the throttles and dragging the bar to maximum.   After a certain
> >> amount of resends, the client will call the set throttle packet with
> >> reduced settings (some argue that it doesn't do that fast enough).
> >>
> >> 6. A user who has connected previously to the simulator will use less
> >> resources then a user who has never connected to the simulator.  (this
> >> is mostly because of the image cache on the client).    Any client
> >> that uses CAPS images will use less resources then one that uses
> >> LLUDP.
> >>
> >> When working with the packet queues, it's essential to understand
> >> those 6 observations.   Even though, the place where you tend to see
> >> the issues with queuing is the image queue over LLUDP, the principles
> >> apply to all of the udp queues.
> >>
> >> Regards
> >>
> >> Teravus
> >>
> >>
> >> On Mon, Mar 28, 2011 at 1:00 PM, Mic Bowman <cmickeyb at gmail.com> wrote:
> >> > Over the last several weeks, Dan Lake & I have been looking some of
> the
> >> > networking performance issues in opensim. As always, our concerns are
> >> with
> >> > the problems caused by very complex scenes with very large numbers of
> >> > avatars. However, I think some of the issues we have found will
> generally
> >> > improve networking with OpenSim. Since the behavior represents a
> fairly
> >> > significant change in behavior (though the number of lines of code is
> not
> >> > great), I'm going to put this into a separate branch for testing
> (called
> >> > queuetest) in the opensim git repository.
> >> > We've found several problems with the current
> >> > networking/prioritization code.
> >> > * Reprioritization is completely broken for SceneObjectParts. On
> >> > reprioritization, the current code uses the localid stored in the
> scene
> >> > Entities list but since the scene does not store the localid for SOPs,
> >> that
> >> > attempt always fails. So the original priority of the SOP continues to
> be
> >> > used. This could be the cause of some problems since the initial
> >> > prioritization assumes position 128,128. I don't understand all the
> >> possible
> >> > ramifications, but suffice it to say, using the localid is causing
> >> > problems.
> >> > Fix: The sceneentity is already stored in the update, just use that
> >> instead
> >> > of the localid.
> >> > * We currently pull (by default) 100 entity updates from the
> entityupdate
> >> > queue and convert them into packets. Once converted into packets, they
> >> are
> >> > then queued again for transmissions. This is a bad thing. Under any
> kind
> >> of
> >> > load, we've measured the time in the packet queue to be up to many
> >> > hundreds/thousands of milliseconds (and to be highly variable). When
> an
> >> > object changes one property and then doesn't change it again, the time
> in
> >> > the packet queue is largely irrelevant. However, if the object is
> >> > continuously changing (an avatar changing position, a physical object
> >> > moving, etc) then the conversion from a entity update to a packet
> >> "freezes"
> >> > the properties to be sent. If the object is continuously changing,
> then
> >> with
> >> > fairly high probability, the packet contains old data (the properties
> of
> >> the
> >> > entity from the point at which it was converted into a packet).
> >> > The real problem is that, in theory, to improve the efficiency of the
> >> > packets (fill up each message) we are grabbing big chunks of updates.
> >> Under
> >> > load, that causes queuing at the packet layer which makes updates
> stale.
> >> > That is... queuing at the packet layer is BAD.
> >> > Fix: We implemented an adaptive algorithm for the number of updates to
> >> grab
> >> > with each pass. We set a target time of 200ms for each iteration. That
> >> > means, we are trying to bound the maximum age of any update in the
> packet
> >> > queue to 200ms. The adaptive algorithm looks a lot like a TCP slow
> start:
> >> > every time we complete an iteration (flush the packet queue) in less
> than
> >> > 200ms we increase linearly the number of updates we take in the next
> >> > iteration (add 5 to the count) and when we don't make it back in
> 200ms,
> >> we
> >> > drop the number we take quadratically (cut the number in half). In our
> >> > experiments with large numbers of moving avatars, this algorithm works
> >> > *very* well. The number of updates taken per iteration stabilizes very
> >> > quickly and the response time is dramatically improved (no "snap back"
> on
> >> > avatars, for example). One difference from the traditional slow
> start...
> >> > since the number of "static" items in the queue is very high when a
> >> client
> >> > first enters a region, we start with the number of updates taken at
> 500.
> >> > that gets the static items out of the queue quickly (and delay doesn't
> >> > matter as much) and the number taken is generally stable before the
> >> > login/teleport screen even goes away.
> >> > * The current prioritization queue can lead to update starvation. The
> >> > prioritization algorithm dumps all entity updates into a single
> ordered
> >> > queue. Lets say you have several hundred avatars moving around in a
> >> scene.
> >> > Since we take a limited number of updates from the queue in each
> >> iteration,
> >> > we will take only the updates for the "closest" (highest priority)
> >> avatars.
> >> > However, since those avatars continue to move, they are re-inserted
> into
> >> the
> >> > priority queue *ahead* of the updates that were already there. So...
> >> unless
> >> > the queue can be completely emptied each iteration or the priority of
> the
> >> > "distant" (low priority) avatars changes, those avatars will never be
> >> > updated.
> >> > Fix: We converted the single priority queue into multiple priority
> queues
> >> > and use fair queuing to retrieve updates from each. Here's how it
> works
> >> > (more or less)... the current metrics (all of the current
> prioritization
> >> > algorithms use distance at some point for prioritization) compute a
> >> distance
> >> > from the avatar/camera to an object. We take the log of that distance
> and
> >> > use that as the index for the queue where we place the update. So
> close
> >> > things go into the highest priority queue and distant things go into
> the
> >> > lowest priority queue. Since the area covered by a priority queue
> grows
> >> as
> >> > the square of the radius, the distant (lowest priority queues) will
> have
> >> the
> >> > most objects while the highest priority queues will have a small
> number
> >> of
> >> > objects. Inside each priority queue, we order the updates by the time
> in
> >> > which they entered the queue. Then we pull a fixed number of updates
> from
> >> > each priority queue each iteration. The result is that local updates
> get
> >> a
> >> > high fraction of the outgoing bandwidth but distant updates are
> >> guaranteed
> >> > to get at least "some" of the bandwidth. No starvation. The current
> >> > prioritization algorithm we implemented is a modification of the "best
> >> > avatar responsiveness" and "front back" in that we use root prim
> location
> >> > for child prims and the priority of updates "in back" of the avatar is
> >> lower
> >> > than updates "in front". Our experiments show that the fair queuing
> does
> >> > drain the update queue AND continues to provide a disproportionately
> high
> >> > percentage of the bw to "close" updates.
> >> > One other note on this... we should be able to improve the performance
> of
> >> > reprioritization with this approach. If we know the distance an avatar
> >> has
> >> > moved, we only have to reprioritize objects that might have changed
> >> priority
> >> > queues. Haven't implemented this yet but have some ideas for how to do
> >> it.
> >> > * The resend queue is evil. When an update packet is sent (they are
> >> marked
> >> > reliable) it is moved to a queue to await acknowledgement. If no
> >> > acknowledgement is received (in time), the packet is retransmitted and
> >> the
> >> > wait time is doubled and so on... What that means is that a resend
> >> packets
> >> > in a scene that is rapidly changing will often contain updates that
> are
> >> > outdated. That is, when we resend the packet, we are just resending
> old
> >> data
> >> > (and if you're having a lot of resends that means you already have a
> bad
> >> > connection & now you're filling it up with useless data).
> >> > Fix: this isn't implemented yet (help would be appreciated)... we
> think
> >> that
> >> > instead of saving packets for resend... a better solution would be to
> >> keep
> >> > the entity updates that went into the packet. if we don't receive an
> ack
> >> in
> >> > time, then put the entity updates back into the entity update queue
> (with
> >> > entry time from their original enqueuing). That would ensure that we
> send
> >> an
> >> > update for the object & that the data sent is the most recent.
> >> > * One final note... per client bandwidth throttles seem to work very
> >> well.
> >> > however, our experiments with per-simulator throttles was not
> positive.
> >> it
> >> > appeared that a small number of clients was consuming all of the bw
> >> > available to the simulator and the rest were starved. Haven't looked
> into
> >> > this any more.
> >> >
> >> > So...
> >> > Feedback appreciated... there is some logging code (disabled) in the
> >> branch;
> >> > real data would be great. And help testing. there are a number of
> >> > attachment, deletes and so on that i'm not sure work correctly.
> >> > --mic
> >> >
> >> >
> >> >
> >> >
> >> >
> >> > _______________________________________________
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> >> >
> >> >
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> >>
> >
> >
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