To be or not to be : CPU limited Or GPU limited, that is the question

I have a situation that makes me think it is not so straightforward. I have an old, 2600k i7 CPU from years ago and a new RTX 2070 GPU. I see ‘Limited by GPU’ when I would expect to see the CPU being the bottleneck. Could it be a slow PCIE bus on the motherboard or something? Still, my CPU usage is about 40% and GPU is around 90%. I just can’t make sense of it.

Sorry for the late response. I didn’t receive a notification as you did not reply directly to my post. If you didn’t know already, you can reply to a specific post by clicking the arrow located in the bottom right of that message :slight_smile:

Anyways, since it looks like you’re CPU limited, a newer processor may be your best shot. You may need to upgrade your motherboard as well, depending on what processor you decide on, if any. Doing some research, it looks like your CPU is in fact underpowered for your GTX 1080. Not in every scenario, but in a lot of them. I would definitely look into a newer processor to help ease the bottlenecking you are experiencing.

The decision is up to you. It sounds like this is an Alienware pre-built PC? A CPU upgrade will likely benefit you, but theres not a 100% guarantee you will see the results you are looking for. Generally speaking though, a better processor should give you a good boost when you are CPU limited.

Hello, the problem may actually not be the CPU, here is my configuration:

AMD Ryzen 9 3900X
RTX 2080 TI
64 GB Ram at 3600 MHz
500 GB SSD M2 Windows
1 TB SSD M2 flight simulator

I think the error is somewhere in the program code, my graphics card is 90% busy, my CPU is only between 10% - 20%.

It runs smoothly, but my graphics card is loaded more, the computer has been completely rebuilt this year. I think it’s time for Direct X12 to arrive.

Strangely enough, in the alpha and beta phase everything was faster, also from the point of view of loading, now it takes a long time because he calculates more with the GPU than with the CPU, which is hardly used.

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If you are running at 1080p - CPU upgrade.
If you are running at 1440p / higher - GPU upgrade.

CPU limited can mean that stutter will occur.
GPU limited can mean that there will be significant input lag.

Best of both worlds is to cap framerate so you get stable smooth framerates without the input lag.

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It really depends, if you fly A320 or 747, you will probably getting CPU limited even at 4k. But if you fly smaller planes, at 1440P and above, you will most likely GPU limted.

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Great thread guys !

Very relevant to me as I have an old system and debating whether its worth upgrading the GPU or just get an entirely new system. This is mine at the moment.

Core i7-920 @ 4Ghz (OC)
Asus P6T Deluxe BCLK @ 200Mhz
12GB RAM @ 1600Mhz
Asus GTX960 OC @ 1350Mhz
FS2020 Settings: High-End

With that I am finding ~25fps in low density areas eg. Cessa Training area and ~15fps in high density areas eg. JFK

Cessna Training Area

KJFK

What’s interesting is that for low-density Cessna area the GPU is clearly the bottleneck, while flying KJFK to KORD pretty much the whole time it constantly jumps back and forth between Limited by Main Thread vs GPU at around 15-20fps.

But looking closely at the JFK numbers it seems overall the GPU is still the greater bottle neck with an average GPU of 65ms (~15fps) vs CPU of 45ms (~20fps).

Am I reading those numbers right ?

If so it would seem if I could get say a 1080 Ti on the cheap it might just get me to a playable 20fps average even in high density which I would happily accept rather than upgrade the entire system for $$$$$$

PS. No matter how low the density or graphical settings it always maxes out around 25fps ala Cessna Training - so not sure if thats simply a bottleneck of the old MB technology or the GPU - do the FPS numbers reveal anything here ?

I’d go for a brand new CPU + Motherboard (R5 3600 + B450?). You will start to notice RAM starving at 12GB of ram. Infact in your devmode screenshot, it already uses 7.5GB / 12GB just for the sim alone. You probably don’t want to upgrade to more of the same slow DDR3 RAM,

A 960 should be perfectly fine for 1080p on medium-low.

Yes indeed the RAM is an issue especially loading into KFJK it takes a while to re-page memory and settle down into an acceptable 15fps framerate … but once you get going its fine.

The P6T x58 Nehalem architecture is (officially) limited to 12GB tri-channel memory ie. 6 x 2GB and even then I shouldn’t be clocking it all at 1600Mhz (probably why my system is a little unstable lol) …

So yes of course a brand new MB/CPU is the ideal situation but to get something futureproof (like I did with the P6T/920 in first place which has lasted me 12 years !) I’d need to spend A$1000+ which I wanna avoid for now as the rig still plays all other recent games at full framerates on Ultra (eg. Jedi Fallen Order, F1 2019, Watchdogs 2) so if I could get away with a $200 spend to get it over 20fps I would take it as it’s still got a few good years left in it …

Eh? GPU or CPU upgrade answer is clear there this is 1440p btw


What you’re saying is true but not the case with the airliners.

Exactly, CPU bound there.

A 960 should be able to keep up in the airliners.

This is Sandy Bridge with only PCI 2.0, so yes that will limit the data sets of your 2070 for sure. And you have DDR3, which is way too slow to shovel the earth around… The performance of that Cpu with 8 Threads isn’t that bad (so you see how far Intel has come with its development “efforts”), as you should have 16 cores to be in recommendations… or to stay in medium settings @ least modern 8-12 threads.
i would think it’s time to say goodbye.

Cheers :blue_heart:

GTPILOT & TheOriginalBlue - its’ not so clear to me.

If you look at the JFK FPS counters the GPU is actually running slower than the CPU ?

Also when you say ‘a 960 should be able to keep up with the airliners’ your saying the GPU ie. GTX960 or CPU eg. i7-960 ?

If you fly in JFK especially in an 747 or A320, even 10900k will have hard time getting over 30fps. If you only fly small aircrafts and avoid big cities, GPU upgrade should be fine and you can play at higher settings, however, if you want good fps in airliner and big airport/cities you need large ram and good CPUs.

Thank you for that conscise analysis. I knew it was outdated but didn’t know exactly why or how. Very good to know. I always did look at the CPU only and go, well, that seems ok. Thanks again.

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I meant GTX 960, not i7 960. My bad, I should have been clearer :slightly_frowning_face:

If you fly an airliner at that sort of resolution and settings, you will probably run into a CPU bottleneck. You can’t really go wrong with a CPU or GPU upgrade, you will notice the GPU upgrade with higher graphics settings + higher framerates in VFR planes, and you will notice the CPU upgrade with less stuttering / hitching + higher framerates in IFR planes.

Also yes, 25-26 FPS is rather low for a 960 @ 1080p, but you mentioned you are running at “High End” settings. If you look here, you can find GPU intensive settings to tone down.

Also, I forgot to ask what 960 you have. I assumed you had a 4GB model, if you have a 2GB model I reccomend you replace the GPU instead. Just my two cents.

Ahhh, the venerable 920 CPU! Had one for years. It delivered sterling service.

Sadly, even your splendid 50% clock cannot overcome the fact that the 920 is hidebound, in modern
terms, by it’s internal cache’s and registers.

I am afraid you are long overdue a complete system upgrade.
If it’s any consolation you can always do what I did: I semi retired my old 920 based system by putting her ‘out to grass’ with the occasional, light word processing, duties!

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That CPU has 8 cores.

The display loop is mostly single threaded on one core, hence 12.5% usage (1/8th)

If your cpu had 2 cores, it would run around 50% etc.

There is no fault in the code, and adding more cores will not change anything - only one core talks to the gpu.

Look carefully at single core speed when choosing a cpu.
Currently we want 2 to 4 very fast cores, not lots of medium speed cores.

It gets complicated to do multi CPU core drawing. That’s a future enhancement.

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You’re sort of on the correct line of though, but you’re also completely wrong.

The game currently requires a minimum of 4cores/8threads to run smoothly. 6cores/12threads is optimal right now.

There will be a single core that’s hit the hardest, the core running the so called ‘main thread’. This is the thread that keeps all other tasks that are outsourced to other threads in sync. Those other cores are still utilized though, but don’t expect them to be loaded to 100%.

If you try running the sim on a 4core/4thread CPU you’re already running into a huge bottleneck. Try running it on a 2core CPU and it will simply grind to a halt if it will run at all.

So yes, the sim will benefit from fast single core performance (high clocks + high IPC). But to say it’s only utilizing 1 core is just plain wrong.

I agree on all the fine technical details.

The original message asked for a non-technical explanation of 10-20% cpu with 8 cores.

I did my best to chop out the hardware/software jargon with the simplest explanation possible (of an average below 2 cores).

Just got my first PC AMD5900X and 3080TI and DDR4 32mb 3600.
Although I first just went to try 4k ultra and LOD 400 my FPS was 30-40 but it was in red saying limited by main thread and GPU alternating between the both. Is this normal? Do I try lowering settings till everything is green? Thanks in advance from a pc newbie )