Coherent GT and Performance decreasing during flight

OK, issue only partially resolved.

Now the SIM use 100% my GPU
After more than 90 minutes of fly I see a performance drop.

Please ASOBO, check this…

There was a thread on this months ago, it was suggested to turn render scaling down mid flight and then to increase it again shortly after, whether it works or not I haven’t tested

Same thing happened to me I was doing a 6 hour flight on the 787-10 the first 3 hours i was stable at 60 by the 4th hour I was capped at 30 fps, and by the 5th hour I was capped at 22 fps with a significant gpu utilization drop. Tbh this has been happening for a really long time, I remember back in November 2019 I tried to do an overnight 12 hour flight and everything was fine until i woke up next day and checked that I was at 9fps. And you would think ASOBO would fix this with SU5, but they didn’t.


This is an interesting thread but if you don’t show your in-game FPS tool screenshot plus the plane you’re flying the comments are worthless. PilotDaniIP seems to be skipping from one problem to another.

Three key points:

  • If you are limited by CPU, your GPU utilisation will be low.
  • Generic FPS/performance tools are of no real use whatsoever to diagnose MSFS issues compared to the in-game tool.
  • If you don’t use the in-game FPS tool, you will have no clue whether the GPU or the CPU is limiting your framerate, so your post will be pretty pointless.

I suspect there is an issue with framerate impact after MSFS as been running for a while, but this is a very specific issue that will take detailed info to clarify. As a total guess, this issue will be CPU-related, but we’ll see.

There was a problem with the Developers FPS tool calculating FPS incorrectly for some users. I submitted a ticket to ZenDesk but I haven’t checked it since SU5 to see if it has been fixed. Since the FPS tool is not an integral part of MSFS, I have no idea if it is supported.

Nevertheless, when I used the Developers FPS tool after SU5, it showed that I was limited by the RdrThread. My CPU, GPU, and memory were all under 30% utilized. It appears that there are no bottlenecks except for the RdrThread. And I don’t have any idea if it can be fixed or how it can be fixed.

Sure, fair point - this is much better than the common “my FPS is low here’s the Afterburner screenshot” posts.

There’s a detail in the in-game FPS tool which I think is new and might be a clue - for me when the FPS falls to maybe a stuttery 20-25 during a flight the in-game FPS tool says “limited by Main Thread” but the Main Thread milliseconds is still maybe 14ms. Previously this Main Thread ms for limited by Main Thread would correlate with the frame rate (i.e. 14ms plus maybe another 4 ms of overhead = 18ms per frame = 55 FPS). So if MSFS said “limited by Main Thread” and FPS was 25, I’d see a Main Thread ms more like 35ms.

But with my current performance drop after a while, this is no longer the case - the in-game FPS tool says “limited by main thread” but the main thread milliseconds is still ok.

May be that one is the results of the other…
It’s not clear also for me, if both are correlated.

I’m actually (after SU5) have a good FPS improvement
I lock the sim at 30FPS cause mine is not an high end configuration for gaming (RX590 eGPU connected to a business laptop i7-8565 with 32GB), and anyway I’m connected ad monitor to an economic LED TV

It seems not in my case, considering an average CPU utilization is under 60%

I’m re activate again the developer mode to use in game FPS tool.
ASAP I will update also the SDK

My configuration is particolar (eGPU connected by TB3 port), but it seems that I’m not the only one that have a performance drop.

Departed by about 35 mins.
All seems ok, except that my old problem is back (sometimes it freeze for about 1/1.5 mins - and flying online that sounds really bad).

This is about the issue of game freeze for a min…
Today (in this flight) happen often (but this seems another story)

Perfomance drop after a couple of hours (only for few seconds)
temporary sistuation, but longer is fly, higher is the problem.

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In the last two photos, your performance is limited by CoherentGTUIthread and CoherentGTDraw. There doesn’t seem to be any correlation between these and CPU or GPU utilization or any MSFS graphics settings. There must be something slowing things down in the code.

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I think your calculation is a liitle off. If 25 FPS, the render time should be 40 ms or a render time of 35 ms should provide about 28.6 FPS.

The Developers FPS Counter tool was significantly changed in SU5. None of these changes were documented in the release notes. In addition, a number of performance improvements (changes) were made in SU5. These changes were not documented very well. Making performance changes and Developers FPS Counter tool changes without giving us specific tuning guidance leaves us scrambling to find assumptions on the new bottlenecks. Not good IMHO.

I understand what you mean but that would only be correct if there was no other overhead involved in rendering each frame outside the time recorded for the main thread (assuming ‘limited by CPU’). In practice you have to add a little bit to the ‘limited by CPU’ ‘main thread milliseconds’ to get the actual FPS, e.g. add 5ms (that seems about right on my 9700K system but I’m not pretending it’s accurate or valid for all frame rates)

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@PilotDaniIP you definitely have a CPU problem (i.e. not GPU).

Your CPU is clearly getting whacked by the code in the flat panel instruments (CoherentGT Draw) and running the sim itself (Main Thread). Maybe it will be worth looking at the Windows Task Manager when you see that to confirm how busy it says your multiple CPU’s are - the average CPU usage might be quite low but one CPU might be pegged at 100% - at least that will clarify how Task Manager reporting differs from the in-game reporting.

For comparison when I load the A320neo my approx in-game FPS tool CPU numbers are
Main Thread 10.5ms (9700K stock)
RdrThread 4.5ms
Manipulators 6.2ms
CoherentGTDraw 0.3ms
CoherentGTUIThread 1…6ms

A possible test would be to fly the Cessna 152 which has plain animated ‘steam’ gauges and compare the FPS tool performance there.

But my CPU doesn’t appear overstressed.
By my point of view seems something that it’s not optimized in GoherentGT lib.

Anyway, I will test the C152 as suggested.

Departed by less than 10’
My default settings are VSync ON / 30fps (my TV - I’m using it as monitor) does not support more than 30fps

Just for test porpouse
There after 15 minutes VSync OFF

We can see total GPU utilization and FPS increase.
Now I revert to my default.


After 90’

The “main thread” overhead is already taken into account. Any delay (overhead) in the GPU getting what it needs to render a frame decreases FPS. If the GPU has everything it needs to render a frame GPU utilization will go up. The FPS Counter is only measuring the difference between the time a frame render “enters” the GPU to the time it exits the GPU (latency). These numbers are fairly accurate before SU5 verified with other FPS counters.

Daily screenshot, after a couple of hours of flight, generally if I do not touch anything my sim run at bout 29-30fps, (using about 70% of GPU)
if I open a web page (as example to read approach charts) or (as in this case) I get a screenshot…

Definitively it’s not a lack of resources (CPU/GPU/MEM), there is something wrong with the CoherentGT UI Thread.

After SU5 when it happen my Audio “crackles” and “drops out”.

In External View…

Actually, I can only hope that with DX12 will change something… (and that’s sometimes is more annoying that FPS drop…)

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Also getting this on long flights, with audio crackle and single digit FPS drops when panning. Have tried fiddling with almost every setting possible - traffic, online services, render scale, graphics settings, PFD refresh rate etc etc, made no difference - well PFD refresh rate changed the behaviour of the bug slightly (as you’d expect) but didn’t eliminate it. Appears to be the same bug that was happening with the 787 previously?

I noticed my last core (core 5 on a 6 core) was higher than the rest, but not maxed out. Overall CPU was around 50-60% and GPU ~40%. 9600KF & 2070S.

Also it appears CoherentGTUIThread is the issue, not CoherentGTDraw.

I wonder if this is relevant? Improve your game UI performance with Coherent GT asynchronicity


In the previous version of dev mode there was only one counter.
Anyway actually I agree with you

With the today update (17 September) is partially back the issue.
Sim seems really stable during the flight,

From External view 30FPS (locked by SW)
From Internal view (with a glass cockpit aircraft, so Coherent GT lib) 20fps and GPU locked at 65% as max…

Not bed, but not optimized.
I hope in DX12

I will test it better in the next days

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At the moment DX12 seems more a problem that a solution.
SU7 degrade my performance…

Hoping in next(s) SUs and WUs…