[Opensim-dev] Sim and Phys Frames per Second (FPS) Issues

Tue Mar 3 09:08:02 UTC 2015

If my memory is correct, SL sims run at a default of 45 frames/second.
OpenSimulator runs at 11. I'm not certain exactly why 11 was chosen but I
do know that increasing it increases the amount of work the simulator must
do. E.g., if you go from 11 to 45 you quadruple the work the simulator must
do

The only signal I'm aware of that affects the viewer is "time dilation".
This value varies from 0.0 to 1.0 where 1.0 indicates the simulator is
running at full speed. If for some reason the physics engine cannot keep up
with the simulation it can signal the viewers to slow movement. This value
is sent with many UDP packets that are involved with the scene and moving
objects, so the viewers will know this value as timely as possible.

Running the simulation at 11 fps shoudl *not* affect devices such as the
Rift as the camera is controlled entirely viewer side except under special
circumstances such as a user surrendering camera control to a script. The
viewer frame rate is *entirely independent* of the simulator frame rate and
will run as fast as the hardware allows if configured to do so.

Choosing an appropriate simulation frame rate would involve weighing the
tradeoffs between simulation workload and responsiveness to user controls
such as those that control avatar position movement. Bear in mind that
there are also network delays involved; a simulation running at 11
frames/second will respond to a user control every 89 ms but there will
also be round-trip network delays which may typically be 100-200 ms but as
high as 1.5 seconds or more. Increashing the simulation frame rate will
likely not produce a perceivable difference in performance but will
*significantly reduce* the capacity of the simulator to do things like host
more avatars and scripted objects.

Also bear in mind that much of the code assumes 11 frames/second and
raising that may create motion-related and other bugs that may not be
apparent until much later. OpenSimulator is tuned to function properly at
11 frames/second.

To recap: the user camera is for the most part entirely controlled within
the viewer and is unaffected by simulation frame rate. As such devices such
as the Rift which manipulate the camera will not be affected.

On Tue, Mar 3, 2015 at 12:09 AM, Tom Bess <tom.willans at bessacarr.com> wrote:

>
> I am reporting a comparison between sl and opensim and did not realise
> this. Does sl run at a true 55fps? If so why bother?  Presumably the viewer
> needs 55fps sent to it to get its calculations correct as at 11fps opensim
> does the same at sl albeit in larger steps.
>
> Would a faster fps improve the accuracy of devices such as the rift by
> having to interpolate over a shorter period of time? Admittedly I suspect
> viewer rendering needs to be improved as well as this aspect is holding my
> experience back.
>
> I understand that other aspects may assume that 11fps is a fixed constant
> and not allow for this to change presumably that can be changed but you
> guys know more here.
>
> Thanks for the guide btw.
>
> Tom Willans  BSc(Hons)  MBCS  CITP
> Chartered IT Professional
>
> Managing Director Bessacarr Publications Ltd
> +44 (0)121 288 0281
> email: tom.willans at bessacarr.com
> skype: tom.willans
> Second Life and OSGrid: Tom Tiros
>
> Sent from my mobile
>
>
> On 3 Mar 2015, at 05:18, Sean M <mondesire.sean at gmail.com> wrote:
>
> Thanks Justin, Dahlia, Michael, and everyone. I now have a better
> understanding of the way FPS is calculated and of the correction factor.
>
> -Sean M.
>
> On Monday, March 2, 2015, Justin Clark-Casey <jjustincc at googlemail.com>
> wrote:
>
>> I think this has already been said but just to summarize.
>>
>> * The 11 fps are the number of scene frames processed - opportunities
>> where avatars may be notified about changes in the scene.
>>
>> * Each of these scene frames is associated with a physics process where 5
>> physics frames are processed in each frame. Hence 11 * 5 = 55 fps.
>>
>> * Why this number?  Others may know better but my guess is that it's
>> related to the frequency of updates expected by the viewer.  Teravus may
>> well know more if he's still around.
>>
>> * Changing m_reportedFpsCorrectionFactor will do nothing except change
>> the server FPS stat.
>>
>> * Changing MinFrameTime will change the number of scene frames.  From my
>> work in the past, you would also need to adjust other parameters like
>> physics frames to stop things going haywire (this was with ODE, Bullet
>> might work differently).  I expect you also wouldn't gain much if anything
>> in scene fidelity.
>>
>> On 02/03/15 18:23, Sean M wrote:
>>
>>> Greetings,
>>>
>>> We at the MOSES project have noticed Simulation and Physics frames per
>>> second (FPS) have a few issues that we are trying
>>> to resolve. The issues are producing suspicious performance statistics
>>> for the analysis of the current version of
>>> OpenSim that we are running.
>>>
>>> First, there is a correction factor (m_reportedFpsCorrectionFactor)
>>> that the raw SimFPS is multiplied by. The comment in
>>> the following line is a bit curious because it indicates that the FPS is
>>> artificially inflated to "lie" about the actual
>>> FPS being so low:
>>>
>>> OpenSim/Region/Framework/__Scenes/SimStatsReporter.cs: Line 317
>>> // We're going to lie about the FPS because we've been lying since
>>> 2008.  The actual FPS is currently
>>> // locked at a maximum of 11.  Maybe at some point this can change so
>>> that we're not lying.
>>> int reportedFPS = (int)(m_fps * m_reportedFpsCorrectionFactor);
>>>
>>> Also, lines 174 and 227 mention the use of this correction factor.
>>>
>>> Second, this multiplier also comes into play in the Scene where there is
>>> a MinFrameTime, which seems to be the minimum
>>> reported amount of time to process a frame:
>>> OpenSim/Region/Framework/__Scenes/Scene.cs:Line 723
>>>
>>> Both of these variables, the correction factor and MinFrameTime, are
>>> concerning from a statistics view point as they are
>>> generating skewed and massaged numbers; therefore, I have a few
>>> questions:
>>>
>>> 1) Is it commonly known that Sim and Phy FPSs are inflated to maintain
>>> the "lie"? And if so, will it be corrected to be
>>> an accurate reporting of processed frames per second?
>>>
>>> 2) What exactly are the definitions for OpenSim's Simulation (Sim) FPS,
>>> Physics (Phy) FPS and a frame (I have found
>>> conflicting and vague definitions on the wiki)?
>>>
>>> 3) What are the known performance consequences of setting the
>>> m_reportedFpsCorrectionFactor to 1 and MinFrameTime to 0?
>>>
>>> Thanks,
>>> Sean M.
>>>
>>>
>>> _______________________________________________
>>> Opensim-dev mailing list
>>> Opensim-dev at opensimulator.org
>>> http://opensimulator.org/cgi-bin/mailman/listinfo/opensim-dev
>>>
>>>
>>
>> --
>> Justin Clark-Casey (justincc)
>> OSVW Consulting
>> http://justincc.org
>> http://twitter.com/justincc
>> _______________________________________________
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>>
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