Understanding X-Plane 11 Performance

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This topic is more for the beginners of flight simulation who chose X-Plane, or for those who have migrated form other simulators to see what all the hubbub is about.  A lot of what is written, has been interpreted from various sources on the net, as well as my own observations, experimentation and proven results.

There has been a lot of discussion lately, both here and at other forums, concerning our old friend FPS.  Back in FSX and P3D, monitoring framerates (or as it has been nicknamed "chasing frames"), has been quite the subject of interest.  Whether your system just makes the cut to the supercomputers overclocked to kingdom come, we all have at some point, chased those frames because it was the first indicator of how well a given sim was performing.  In X-Plane though, Laminar Research has been gracious enough in providing very detailed information, otherwise known as Data Output, to monitor just about every data-driven aspect to the sim.  More to the point though, is the subject of framerates and how they equate to the overall performance of your sim on your system.

Accessing this information has changed a bit from XP10 to XP11, via the new UI interface, though the feature heading is the same and the choices of data output is the same as well.  To do this, access your upper menu bar in X-Plane (which stays hidden during flight), and in the upper right, you will see some icons, and the one we want is the settings icon (resembles sliders on an equalizer).  Clicking on this will bring up the settings menu, and you can click on the Data Output tab.  This new screen will display a multitude of data features, starting with Frame Rate.  To the right of this are a series of checkboxes, allowing the user to click on any of these, depending on how the data output is to be delivered.  We want "Show In Cockpit" checked (not to dissimilar to FSX's red text displayed in the upper left corner of your screen).  After you have clicked on that, click on the Done button, in the lower right corner of your screen, to bring you back to your cockpit.

You should now see a grey rectangular box in the upper left corner of your screen, displaying a series of data output results.  Below is a list of the outputs, with simplified explanations for the purpose of this post:

  • f-act/sec - This is the actual (real time) frame rate being processed in X-Plane.  This number is the direct result of two other outputs, shown below
  • f-sim/sec - This is the frame rate the simulator is processing for the purpose of maintaining stability to the flight model.  If you haven't adjusted your flight model, in the settings, disregard this for now.
  • frame time - This is the time required to render one frame, in seconds, based on the current position of your aircraft in the sim, taking into account of weather, scenery objects and mesh.
  • cpu time - A rate (in seconds) at which the cpu is processing one frame in the sim, and outputting to the frame rate time.
  • gpu time - A rate (in seconds) at which the gpu is processing one frame in the sim, and outputting to the frame rate time.

The last two outputs, grnd ratio and flit ratio, have not been officially explained by LR, so for now, we will skip these.

Now that we have active data being communicated to the screen, we can now start interpreting what is going on in the simulator.  As a test, I encourage you to place yourself at a large airport, near a metroplex, such a KLAX.  The expansive area of scenery, scenery objects, as well as the large airport will help you to see what your simulator is doing (or not doing).  Deciphering the data outputs, will help you to tune your simulator and hopefully provide you with a smooth and stable experience.

Let us look at the f-act/sec value.  If the number is below 19, the simulator is not working efficiently at all, and you have a huge load on both the cpu and gpu.  To LR, a frame rate value of 19 or below, means that the sim is now running in "slow motion", attempting to process and render the output to your screen as fast as it can, with the settings values as you have set them, but not processing and rendering in real time.  If you have a value of this or below, you will need to check your settings and lower them a bit.  To achieve 25 or above, means that you are at least running in real time and high values equate to a very efficient and stable simulator, processing and rendering everything as it should.  Some have questioned if their frame times are 19-24, what would they expect and the short answer is that the sim is either on the recovery from or decline to slow motion processing.  if you find your frames are held at that range, you will still need to make further adjustments.  As for the f-sim/sec, this value (as explained above), can fluctuate as much as the f-act/sec, and if the f-act/sec is different than the f-sim/sec , this means your system is bogged down, and once again, you will need to adjust your settings.

The value from the f-act/sec output, is directly to your cpu time and gpu time, and in turn, are governed solely on their assigned responsibilities to process the simulator data.  In an ideal setting, you want both of these values to be as close to the same, and as low as possible, showing an even distribution of processing and rendering of the data in X-Plane.  These two values are pretty simple to figure out, as the numerical value is based on milliseconds of time taken to process and render one frame.  Lower numbers, equal better frames and of course, great performance and then with higher numbers, worse performance.  With these two values, they can  and will fluctuate, which are dependent on what is going on in the sim at the time.  For example, If you are flying at FL80 in the default C172SP, over the Los Angeles basin, you may find your frames running between 30-40, with your cpu and gpu times at around 0.037 or higher, and that can be understandable, considering the dense autogen in the area, as well as the high mountainous terrain that borders the area.  Now if we were to flying at the same flight level over the desert, your frames would jump higher and your cpu and gpu times will decrease...again, it's all about what is going on in the sim.  Of course, flying more complex aircraft with heavy weather also factors in to the your cpu and gpu times, so unless you have a beefy system with the latest hardware, you may find that you have to sacrifice some settings for the good of stable performance.  Finding balance is key here and important for everyone to understand.

Nailing down what the cpu and gpu do in X-Plane is important, and shouldn't be taken lightly.  As we are exposed to more and more add-on content, or content improvement in XP11, we will find ourselves having to make proper adjustments to the settings to maintain that balance we all want.  It isn't as bad as the days of FSX, where our major concerns over VAS and OOMs was a big issue.  With X-Plane being a 64-bit application now, it can take full advantage of all memory in our system, both gpu VRAM and system RAM, so rejoice that the 32gb of RAM in your system wasn't for not!  

Now that XP11 Beta is in full swing and soon will be wrapping up, Below is a rough list to which features are processed with the cpu and gpu.  Please note though that while XP11 is in Beta, the topic of adjusting the settings similar to the advanced options of older versions of XP, has been well discussed and could (though not confirmed) be changed before final release (for example, birds and wildlife, number of vehicles, etc) :

CPU Intensive

  • Number of world objects
  • Field of View (with or without "Allow windshield effects" checked below it can also be an additional factor)
  • Plug-ins

GPU Intensive

  • Visual Effects
  • Texture Quality
  • Antialiasing (with or without "Draw shadows on scenery" checked below it can also be an additional factor)
  • Reflection Detail (with or without "Draw parked aircraft" checked below it can also be an additional factor)

Unfortunately, when LR decided to streamline and simplify their settings dialog, they inadvertently made it too simple, so if you had experience with XP10 or lower, you remember that there were a lot more things you could control separately.  With that said though, it is this simplification that makes it a little easier to understand where our cpu and gpu times come into play.  Specific features, such as Antialiasing, do pose a significant cost to your gpu, so make wise adjustments.  Texture quality, while everyone would want everything to look as crisp and clean as the real world, also comes with a cost, so pay attention to the grey text below the slider, that indicates the amount of data loaded into your VRAM.  If you have a 8gb gpu with 10gb of textures loaded, it'll most certainly decrease your frames.  The Texture Quality is applied globally, meaning that your scenery, scenery objects and the textures within the VC are accounted for.

As I said previous about the cpu and gpu times, making settings changes help to stabilize the simulator. The main purpose of this post is to expose the transparency of X-Planes capabilities, and it is quite important to pay attention to what they provide.  Granted, it isn't necessary to display these values all the time, but they do make a great troubleshooting tool, if you experience issues in X-Plane.  As for optimal settings in X-Plane, this varies quite a lot from one system to another.  The internet is chock full of suggestions, but the right one can only be determined by your own experiences and testing.  If you feel that you need to be in the market for a hardware upgrade, utilizing these data output values, combined with the settings adjustments you make, will help determine what you need to get your sim running at peak performance.

In closing, I voluntarily posted this in hopes that perhaps the admins could look upon this as valuable information to help those who may be struggling with X-Plane, both 10.51 and the current Beta, to get their systems running better.  What was written above is a simplified explanation, with more detailed discussion and interpretation being available elsewhere, I know that AVSIM is a prime site for virtual pilots to visit, and any helpful information can be of benefit.

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Good info. On that breakout between CPU and GPU, note that there are two additional CPU tasks in the sim: scenery loading and AI aircraft. The AI aircraft run full flight models, same as your plane, and will use as many cores as you have available. Scenery loading is asynchronously distributed on multi-core.

Here's a quote from Ben on the dev blog (comments portion of the XP11 system requirements post):




Scenery loading while you fly is all multi-core, as our AI planes. There are some other random background functions that affect fps, but it’s a small percent of the fps-affecting code, but right now multiple aircraft and smooth flight while loading scenery are the main uses of multi-core.

Over time we hope to get more of the per-frame stuff to be multi-core, to bring the headline framerate up. This includes amortizing some of the cost of multiple monitors.




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Good post and good information, even for some of us older folks that have been around awhile. :-)

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Thank you Donald.  I tried my best (took quite a while to gather thoughts and get them in an easier to understand format), but maintain a general theme about it all.  I realize there are very technical specifics involved under the hood, but for the majority that need to figure things out at the onset, I figured I'd follow the KISS method.  

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Great post and  information. I've pinned this topic in the tips and tricks section

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