HybridGraphics – Support for hybrid graphics laptops.

Please check the Psychtoolbox website and Wiki for up to date
information about the state of HybridGraphics. This document
may contain obsolete information quite quickly on this fast
changing topic, especially on Linux.


Hybrid graphics laptops are laptops which have two built-in
graphics cards (GPUs). One card is usually an integrated
graphics chip (iGPU) which has a low power consumption and
heat production, allowing for a cool laptop and long battery
runtime, but it also has relatively low performance. This chip
is sufficient for typical desktop GUI use, watching videos and
other light graphics applications. The 2nd card, called discrete
GPU (dGPU), provides much higher performance but consumes more
power and produces more heat, so the laptop runs hotter and/or
needs more cooling, all resulting in shorter battery runtimes.

The idea is to use the dGPU for performance hungry graphics
intense applications, and the iGPU for day to day tasks,
selecting either for high performance or long battery runtimes.

These laptops come in two basic flavors, hardware multiplexed
(muxed) and non-multiplexed (muxless). Muxed laptops have an
electronic switch that can connect the iGPU or dGPU to the video
outputs (internal flat panel and external video connectors like
MiniDisplayPort or HDMI or DVI or VGA). The switch is software
controlled and allows selection of which gpu is driving the
displays, the other gpu is powered down for maximum power saving.

OSX, Linux on Apple hardware and on other muxed Laptops

As of 2016, all past and current laptops from Apple (MacBookPro are muxed. When using Mac OSX, the operating system controls
the mux to select an appropriate gpu. For light desktop and 2D use,
the iGPU is active. Whenever a 3D application starts up, e.g.,
Psychtoolbox, the iGPU gets disconnected and powered down and the
dGPU gets powered up and connected. From the perspective of
Psychtoolbox there essentially is only one GPU, which is the dGPU,
either a NVidia or AMD graphics card. Apart from a couple of rather
horrible bugs, e.g., for the 2010 MacBookPro with some recent versions
of OSX, this means that hybrid graphics under OSX usually “just works”.

If one uses an Apple MacBookPro under Linux then the machine will run
with either the dGPU active and the Laptop behaves like a machine with
one gpu, or the active gpu can be switched via the Linux “vgaswitcheroo”
mechanism. The same applies for other PC laptops which are equipped
with a mux. Iow. one can manually select the performance vs. power
consumption tradeoff.

Most modern common PC laptops are muxless though. The iGPU is hard-wired to
the video outputs, both to the laptop flat panel and the external outputs.
The iGPU is always active and drives the displays and takes care of drawing
the GUI and handling 2D applications. The dGPU can be powered up as needed
to handle GPGPU computations and the rendering for more demanding 2D, Video
and 3D applications. As the dGPU is not connected to the display outputs, it
has to copy its rendered images into the RAM of the iGPU and the iGPU then
displays the images on behalf of the dGPU. This involves some significant
overhead: Multiple milliseconds of time are needed for each Screen(‘Flip’) to
copy image data from the dGPU to the iGPU, and converting the data into a format
the iGPU can display. For this reason, display latency on a muxless laptop
will always be longer and absolute graphics performance lower than on a laptop
which only has a dGPU, or on a muxless laptop. A big problem is the need to
properly synchronize the rendering of the dGPU with the display of the iGPU.
Depending on how this synchronization or handshake is implemented, visual
stimulus onset timing and timestamping can be highly unreliable and inaccurate.


On Microsoft Windows a handshake method is used which maintains good framerates
for video games and similar applications, but causes visual stimulus onset timing
and timestamping to be almost always completely wrong, with observed errors in
the range of +/- 33 msecs on a 60 Hz panel. That means that the dGPU is unuseable
if visual timing matters in any way. The best you can do on a muxless laptop under
Microsoft Windows is to configure the driver to disable the dGPU and only use the
iGPU for all rendering, and then hope that the iGPU graphics driver isn’t too buggy,
a hope that is often unjustified, especially if the iGPU is an Intel graphics chip.
Intel graphics has timing bugs itself quite often on MS-Windows, even on regular
single gpu laptops.


On Linux, as of August 2017, good progress has been made in implementing methods
which provide both good performance *and* reliable, trustworthy, accurate visual
timing and timestamping. Some - but not all! - types of Laptop hardware should
work well, but for all of them some special configuration or software upgrades
are needed.

We recommend XServer version 1.19.3 or later, and Mesa version 13 or later, and
Linux 4.10 or later, as this combination provides best performance and ease of
setup for all supported types of hybrid graphics laptops. Users of Ubuntu Linux
can simply install Ubuntu 16.04.6 LTS (or Ubuntu 18.04-LTS and later) from fresh
installation media, or upgrade to 16.04.6 LTS from earlier Ubuntu releases and then
install the new hardware enablement stack (HWE) via …

sudo apt install –install-recommends linux-lowlatency-hwe-16.04 xserver-xorg-hwe-16.04

… if it isn’t already automatically installed after an upgrade to 16.04.4 LTS.

The following sections describe the current level and quality of support for different
types of hybrid graphics laptops, and required configuration steps, assuming you have
sufficiently up to date kernel, X-Server and Mesa as explained in the previous paragraph.
Psychtoolbox would tell you if you need to upgrade your kernel, if you’d run it on a
muxless hybrid graphics Laptop.

* Laptops with an Intel iGPU combined with a NVidia dGPU (“NVidia Optimus” models):

These should work perfectly if you use the “nouveau” open-source graphics driver,
at least as far as testing with three different laptops went. Stimuli are displayed
without any artifacts, and timing and timestamping is accurate and trustworthy.
Performance is highly dependent on the model of NVidia gpu though, with the latest
generations currently providing only relatively low performance, and ongoing work
to improve the performance for recent models.

If you want to use the NVidia proprietary display driver for Linux instead, there
now exists a solution which works with correct timing and timestamping, as verified
on two Optimus Laptops, a Lenovo Lenovo Ideapad Z50-70 with GeForce 840M and a Razer
Blade 2016 with GeForce 1060M. However, the solution is less flexible and power-efficient
than use of the “nouveau” open-source driver. It requires some setup work, and it needs
a NVidia proprietary driver of at least version 375.66. If you can’t select a recent
enough driver of at least version 375.66, you need to enable the proprietary graphics
driver ppa to get a convenient update to NVidia driver version 375.66 or later versions.
Also, you must use a X-Server of the 1.19.x series, the v1.20 servers are not yet supported.
Follow the following steps to get Optimus set up:

  1. Install the proprietary graphics drivers ppa by typing in a terminal:
    sudo add-apt-repository ppa:graphics-drivers/ppa
    sudo apt-get update

  2. Then launch the 3rd party driver manager GUI to select the NVidia proprietary driver
    for use on your system. You must select a NVidia proprietary driver of the 375 series,
    with a version number of 375.66 or later. This will also automatically setup standard
    Optimus / PRIME support for tear-free stimulus display, but getting proper visual
    stimulation timing and timestamping for Psychtoolbox requires some more steps.

  3. Edit the file /etc/modprobe.d/nvidia-graphics-drivers.conf. Modify the last line
    in that file and replace the assignment modeset=0 with modeset=1 to enable drm
    modesetting support.

  4. Execute “sudo update-initramfs -u -k all” in a terminal.
    NOTE: You may need to repeat both steps 3 and 4 every time after a significant upgrade
    of your distributions software. Otherwise Psychtoolbox may complain about timing and
    synchronization problems after such an upgrade.

  5. Copy the custom Psychtoolbox modesetting driver into the system driver directory.
    There are two variants, the nolag variant and the highlag variant. In theory, the
    nolag variant would be preferrable, but it sometimes gives inconsistent performance:

    sudo cp /pathto/Psychtoolbox/PsychHardware/LinuxDrivers/NVidiaOptimus/modesetting_drv.so /usr/lib/xorg/extra-modules/modesetting_drv.so

    For use of the variant with higher lag but consistent performance, use the highlag driver

    sudo cp /pathto/Psychtoolbox/PsychHardware/LinuxDrivers/NVidiaOptimus/modesetting_drv.so_highlag /usr/lib/xorg/extra-modules/modesetting_drv.so

  6. Reboot. Now your system should be ready for research compatible Optimus.

On Ubuntu, the “nvidia-settings” GUI tool allows you to switch between Optimus (PRIME) and
standard Intel graphics. The section “PRIME profiles” allows to click on a toggle button
to switch between “NVidia” gpu for power hungry but fast Optimus, and “Intel” for low power
consumption lower performance mode.

If you want to use a different distribution than Ubuntu, “Fedora 25” and later, “Debian unstable”,
“Arch Linux” and “SuSE Tumbleweed” are known to ship required X-Server, Linux kernel and NVidia
driver options for Optimus. However, these are not tested with PTB, and setup may be different
from Ubuntu’s approach. Alternatively you could also download and compile your own X-Server 1.19
if you are not afraid of compilers and Makefiles and willing to spend a workday doing this. The
following paragraph assumes you are not using Ubuntu 17.04:

Once you have a X-Server 1.19 up and running, you will need the NVidia proprietary
display drivers of version 375.66 or later for 64-Bit Intel processors. Then you need
to copy various configuration files into various places, and adapt some of these files
to your specific system. Finally you need to install a custom xf86-video-modesetting
display driver onto your system. This modesetting driver is specifically made to
interoperate with Psychtoolbox to provide research grade precision timing and
timestamping. Then, after a reboot, you may be rewarded with a NVidia Optimus laptop
which can efficiently use your discrete high-performance NVidia gpu with research grade
timing. However, research grade timing is only provided for pure single-display setups,
not for any kind of multi-display operation. That means your Laptop can have exactly
one display enabled, either the Laptop internal flat-panel, or one externally connected
display. Also timing is only reliable and trustworthy for a Psychtoolbox fullscreen
window. You will find all the needed config files and custom made display driver and
setup instructions in the Psychtoolbox subdirectory Psychtoolbox/PsychHardware/LinuxDrivers/NVidiaOptimus/

See the following thread for the current state of the NVidia proprietary implementation
and for some more nice background info on the challenges of proper handshaking and
synchronization on muxless laptops:


* Laptops with an Intel iGPU combined with an AMD dGPU (“AMD Enduro” models):

These should work very well out of the box on Ubuntu 16.04.3 LTS and later, as explained

On other Linux distributions make sure to install Linux 4.8.11 or later versions of the Linux
kernel, together with X-Server 1.18 or later, and Mesa version 17.0 or later.

AMD Enduro hybrid graphics was tested with two PC setups:

  • Intel HD “Haswell desktop” graphics chip + AMD Radeon R9 380 Tonga Pro.
  • Intel HD “Ivybridge desktop” graphics chip + AMD FireGL “Cedar”.

Stimuli are displayed without any artifacts and timing and timestamping is accurate and

* Laptops with dual NVidia gpus NVidia iGPU + NVidia dGPU:

Muxless would not work with any current official solution [1]. However, i am not aware of
any recent muxless laptops - or any such muxless laptops actually - which use dual-NVidia
gpus. All known dual-NVidia laptops are rather old and use a hardware mux, so Linux
“vgaswitcheroo” mechanism can be used to switch between gpus for perfect results.

* Laptops with dual AMD gpus AMD iGPU + AMD dGPU (“AMD Enduro” models):

Muxless won’t work with any current official solution, so you can only use the
AMD iGPU [1].

For those combinations that should work (Intel iGPU + NVidia/AMD dGPU “Optimus/Enduro”),
after you’ve upgraded to all the required software, the following setup steps are
needed for muxless PRIME mode. Note that these *do not apply* to Optimus with the proprietary
graphics driver from NVidia:

  1. Run the “XOrgConfCreator” script to create a proper XOrg configuration file,
    and then “XOrgConfSelector” to switch to that configuration file, logout and
    login again.

2. Tell Matlab or Octave to use the dGPU for rendering with Psychtoolbox:

  • Either use the setenv(‘DRI_PRIME’,’1’); command before calling the first Screen()
    command, e.g., by adding it to Matlabs startup.m script or Octaves ~/.octaverc
    startup script.

  • Or copy the Psychtoolbox/PsychHardware/LinuxX11ExampleXorgConfs/_.drirc to the file
    ~/.drirc and then customize it for your hardware to always select the dGPU for
    rendering with Octave or Matlab. The file itself contains customization instructions.
    The <device> … <\device> section in that file can also be included into the
    global /etc/drirc file if it should apply to all users on a machine.

  1. Optionally verify the handshaking and synchronization actually works. Psychtoolbox must not
    report any timing or timestamping related errors or warnings, or other warnings
    relating to hybrid graphics problems. Typical tests like PerceptualVBLSyncTest or
    VBLSyncTest must work correctly. All demos should display without any visual artifacts,
    tearing artifacts etc.

    Additionally you can use the Linux ftrace script i915_optimus_trace.sh in the
    Psychtoolbox/PsychHardware/ folder. Instructions on how to use it are inside the
    script. Running it while a Psychtoolbox stimulation script runs will measure the
    timing of functions relevant for proper timing. The printout after 20 seconds should
    show the function “reservation_object_wait_timeout_rcu” using a significant amount of
    time, e.g., multiple thousand microseconds (usecs), e.g.,

    3) intel_mmio_flip_work_func i915 {
    3) # 3060.318 us reservation_object_wait_timeout_rcu();
    3) # 3070.039 us }

    Here the iGPU waited for 3060.318 usecs until the dGPU was done with its part
    of the job. That’s a realistic waiting time for simple visual stimuli, although
    numbers could easily go up into the > 9 msecs range for more demanding stimuli or
    slower gpus. Just to give you a perspective on the potential performance loss or
    added latency compared to a single gpu laptop.

[1] There also exist some muxless Laptop models where the iGPU is hard-wired to the internal
Laptop flat panel, whereas the dGPU is hard-wired to (some of) the external video outputs.
On these models one can configure a dual-x-screen setup for visual stimulation and then
assign the iGPU to drive X-Screen 0 on the internal panel and assign the dGPU to drive
X-Screen 1 on the external video outputs.

This would work with high performance and timing precision even on hybrid graphics laptops  
which otherwise wouldn't work, e.g., dual [NVidia](NVidia) or dual AMD laptops. Such a setup  
wouldn't require any of the setup steps mentioned above. Instead it would require to  
create a dual-x-screen setup via [XOrgConfCreator](XOrgConfCreator), but then to manually customize the  
created config file, as [XOrgConfCreator](XOrgConfCreator) can't automatically handle such dual-gpu setups  
yet. [Ask](Ask) for assistance on the Psychtoolbox user forum if you happen to have such a  
laptop. One example xorg.conf file for handling such a setup (Intel iGPU + AMD dGPU)  
can be found under the name xorg.conf\_SeparateScreensDualGPUIntelAndAMD in the  
Psychtoolbox/[PsychHardware](PsychHardware)/[LinuxX11ExampleXorgConfs](LinuxX11ExampleXorgConfs)/ folder. It would need customization  
though for a given Laptop, specifically adapting the "[BusID](BusID)" parameter for your hardware.  
Another example X-Config file for such a laptop can be found for year 2016 Razer Blade gaming  
laptop with Intel HD-530 Skylake iGPU + [NVidia](NVidia) [GeForce](GeForce) 1060M Pascal dGPU, where the Intel  
iGPU is hardwired to the laptop panel and USB-C output, whereas the [NVidia](NVidia) dGPU is hardwired  
to a HDMI output. The filename in the Psychtoolbox/[PsychHardware](PsychHardware)/[LinuxX11ExampleXorgConfs](LinuxX11ExampleXorgConfs)/  
folder is xorg.conf\_RazerBlade-2-[XScreens](XScreens)-intel+nouveau  
Another X-Config example file for the Razer Blade 2016 is the file ...  
... This file is for use with the [NVidia](NVidia) proprietary driver instead of the nouveau open-source  
driver. It uses the [NVidia](NVidia) gpu to drive two separate X-Screens 0 and 1. X-[Screen](Screen) 0 is driven  
via Optimus PRIME output, displaying via the Intel HD 530 iGPU on either the laptop flatpanel  
or the USB-C video output, but not both at the same time if visual stimulation timing matters.  
X-[Screen](Screen) 1 is driven directly via the HDMI output connected to the [NVidia](NVidia) dGPU.  
NOTE: If you copy these files into the /etc/X11/xorg.conf.d/ folder you must rename them to  
      end with the suffix .conf otherwise they won't be actually used!  
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