Limiting Framerate kills framerate - why?

[darem 84]

I often read that limiting the framerate in P3D V4.3 has benefits - for me it only kills framerates.

Let me give you an example for the same scenery, being static on the ground (let's take LOWW for example):

• Unimited framerates result in app. 40-50fps
• Framerate limit to 40fps results in app. 25-30fps and choppiness
• Framerate limit to 30fps results in app. 20fps and choppiness
• Framerate limit to 20fps results in app. 12-15fps and choppiness

Why? For me, limiting the framerate has no benefit at all...

[jabloomf1230 2,445]

I may wrong about this but isn't this all about math? The frame rate limiter clips any frame rates above the limit, so in your case 40 fps. Hence, frame rate will vary between the minimum and 40. If you want a mean frame rate of 40, the limit would have to be higher.

If one continually lowers the frame rate limit, the range between the minimum and limit will become smaller and smaller and hence the mean frame rate will approach a central value.

[darem 84]

That doesn't explain for me why I have stutter and dips to low frames when I limit the framerate - If I have framerates between 40 and 50 and limit them to 40, shouldn't I have stable 40 with more CPU time for textures and autogen? When I limit to 30fps in a situation when I have 40-50fps, shouldn't I have rock solid 30fps?

[odourboy 1,288]

I don't understand it either, but I get my best performance limiting my rates using vsync only and setting my monitor refresh at 30 (or 60) via Nvidia control panel and leaving frames unlimited.

Edited August 13, 2018 by odourboy

[GHarrall 494]

Its all about the look ahead when you limit FPS. The CPU has to process frames in advance.

 

Steve W talks all about this in another thread....I think the blurries one amongst others.

https://www.avsim.com/forums/topic/522852-locking-frame-rate/?do=findComment&comment=3736447

Edited August 13, 2018 by GHarrall

[patrice_bambou 85]

The FFTF (Fiber Frames Time Fraction) enters in action when you limit frame-rates. The standard value is 33% (0.33) (if not defined in p3D.cfg, you may set it to a lower value at the expense of scene and autogen generation)

So, with the standard value, limiting to 30 fps (for instance) gives you an effective 20 fps (10 fps are lost due to scene and autogen generation)

FFTF is not active when setting unlimited framerates, but the time passing, the scenery becomes blurry and autogen loads in batches or even disappear completely

Patrice.

[GHarrall 494]

2 minutes ago, patrice_bambou said:

The FFTF (Fiber Frames Time Fraction) enters in action when you limit frame-rates. The standard value is 33% (0.33) (if not defined in p3D.cfg, you may set it to a lower value at the expense of scene and autogen generation)

So, with the standard value, limiting to 30 fps (for instance) gives you an effective 20 fps (10 fps are lost due to scene and autogen generation) 

FFTF is not active when setting unlimited framerates, but the time passing, the scenery becomes blurry and autogen loads in batches or even disappear completely

Patrice.

Its not quite that simple.

[Albin 7]

I liked to use in app frame limiter because for me that's smoother but I also lose about 20 fps which make some place like Brisbane unplayable 

[patrice_bambou 85]

10 minutes ago, GHarrall said:

Its not quite that simple.

Could you develop; please?

I have made several tests and read also several topics on this site that demonstrate this clearly.

[GHarrall 494]

1 minute ago, patrice_bambou said:

Could you develop; please?

I have made several tests and read also several topics on this site that demonstrate this clearly.

Because depending on the load, if you limit to 30 fps you can achieve 30 fps. Locking the FPS doesn't mean in any given condition you will lose 1/3 of your frame rate. Its about the overhead room that is available in any given situation and the effect that has on the look ahead frames,.

 

[lownslo 592]

41 minutes ago, GHarrall said:

Its all about the look ahead when you limit FPS. The CPU has to process frames in advance.

Steve W talks all about this in another thread....I think the blurries one amongst others.

This!  Locking the framerate in P3D in essence can cause the CPU to do more work (based upon the overhead).  Much of that work should be handled by the GPU.

Greg

[patrice_bambou 85]

55 minutes ago, GHarrall said:

Because depending on the load, if you limit to 30 fps you can achieve 30 fps. Locking the FPS doesn't mean in any given condition you will lose 1/3 of your frame rate. Its about the overhead room that is available in any given situation and the effect that has on the look ahead frames,.

 

I agree. If you fly above the sea, nothing to generate regarding the scenery apart the clouds and in these conditions you will get the limited framerate.

But this is not the case in general. Flying above a complex terrain with a lot of autogen will surely affect your framerate and you cannot get the limit you defined in the settings.

 

[GHarrall 494]

9 minutes ago, patrice_bambou said:

I agree. If you fly above the sea, nothing to generate regarding the scenery apart the clouds and in these conditions you will get the limited framerate.

But this is not the case in general. Flying above a complex terrain with a lot of autogen will surely affect your framerate and you cannot get the limit you defined in the settings.

 

Of course you can.

If I set 30 FPS limit in FTX germany even with the aersoft bus I can hit the 30 fps limit. If I set it at 40 or 60 then i wont.

The bottom line here is that to hit x FPS with the limiter on, you need to be able to do approx double the frames with limiter off. This is because of the look ahead frames. It does not mean that if I set a limit I will always be 1/3 below it. It really depends on WHAT limit you set and how much room you have in CPU overhead. FPS limiter is hard on the CPU and it does so for a reason. See this thread.

 

Edited August 13, 2018 by GHarrall

[Bobsk8 12,580]

I have never had any luck with limiting frame rates except to lower my average frame rate. I have used unlimited for years with no problems. 

[GHarrall 494]

2 minutes ago, Bobsk8 said:

I have never had any luck with limiting frame rates except to lower my average frame rate. I have used unlimited for years with no problems. 

This is true Bob. But as you aren't a v4 user (v4.3 specifically) you may not be aware of the 'no autogen loading' and blurry issue that is a real problem running unlimited on this platform. There are a couple of long threads about it. The only fix right now is a locked frame rate in the sim.

This kind of screws everyone who was using unlimited FPS target with vsync and triple buffer on with  a 30hz refresh rate.