Thanks :)
I'll try this and report back.

On Tuesday, October 10, 2017 at 4:31:44 PM UTC+11, DRC wrote:
>
> Ah, OK.  That sounds like a DRI permissions issue.  Normally the DRI 
> device permissions will be set such that only the user who logged into 
> the X server will be able to use direct rendering.  If you configure the 
> 3D X server as recommended in the VirtualGL User's Guide, including: 
>
> -- Installing a display manager such as GDM and starting it when the 
> system starts 
> -- Running vglserver_config to grant limited access to the X server when 
> the display manager is active 
>
> this will automatically configure the DRI permissions appropriately for 
> nVidia GPUs.  You will not be able to share the 3D X server if you start 
> it using startx.  You need to configure the 3D X server as specified in 
> the User's Guide in order for it to be sharable. 
>
> DRC 
>
> On 10/9/17 11:33 PM, Jafaruddin Lie wrote: 
> > Thank you for your clarification. 
> > See if I get this correct: 
> > So what I have is two accounts connected to the same machine, each 
> > connected with two VNC sessions (the 2D X server) and this is working 
> > fine as I can see the desktops. 
> > I have one 3D X server instance configured using xorg.conf, with 4 
> > Nvidia GPUs and as far as I can tell from the document, this xorg.conf 
> > has been configured as headless. 
> > 
> > My issue now, it seems, only one user is able to access the 3D X server 
> > properly. The other account, when running vglrun, will throw errors like 
> > [VGL] WARNING: The OpenGL rendering context obtained on X display 
> > [VGL]    :0.3 is indirect, which may cause performance to suffer. 
> > [VGL]    If :0.3 is a local X display, then the framebuffer device 
> > [VGL]    permissions may be set incorrectly. 
> > 
> > I am just trying to get the two accounts to use the 3D X server, so I 
> > was wondering if the issue lies with my xorg.conf or somewhere else? 
> > Again, thank you for your replies. I am learning (albeit slowly). 
> > 
> > 
> > 
> > 
> > On Tue, Oct 10, 2017 at 3:14 PM DRC <[email protected] <javascript:> 
> > <mailto:[email protected] <javascript:>>> wrote: 
> > 
> >     You are either still fundamentally misunderstanding how to configure 
> a 
> >     VirtualGL server, or you are not describing your configuration 
> properly. 
> >      "I have two VNC servers (from two different users) running on an 
> >     instance of X on :0" makes no sense.  As I explained before, VNC is 
> the 
> >     2D X server, not the 3D X server.  xorg.conf is used to configure 
> the 3D 
> >     X server.  In a normal VirtualGL environment, :0.0, :0.1, :0.2, etc. 
> are 
> >     screens on the 3D X server, each assigned to a different GPU.  In a 
> >     normal VirtualGL environment, :1.0, :2.0, :3.0, etc. are X proxy/2D 
> X 
> >     server instances. 
> > 
> >     On 10/9/17 9:09 PM, Jafaruddin Lie wrote: 
> >     > Thanks, DRC. 
> >     > I commented out the InputDevice(s) lines, now I have two VNC 
> servers 
> >     > (from two different users) running on an instance of X on :0 (ps 
> aux 
> >     > shows "X: 0") 
> >     > 
> >     > The first user to login via the job controller is allocated the 
> first 
> >     > two graphic cards, and this user has no issue 
> >     > running vglrun glxgears (just to test). 
> >     > The second user is allocated the remaining graphic cards, and got 
> this 
> >     > message: 
> >     > 
> >     > [VGL] WARNING: The OpenGL rendering context obtained on X display 
> >     > [VGL]    :0.3 is indirect, which may cause performance to suffer. 
> >     > [VGL]    If :0.3 is a local X display, then the framebuffer device 
> >     > [VGL]    permissions may be set incorrectly. 
> >     > 
> >     > And this error from glxinfo: 
> >     > name of display: :2 
> >     > X Error of failed request:  GLXBadContext 
> >     >   Major opcode of failed request:  152 (GLX) 
> >     >   Minor opcode of failed request:  6 (X_GLXIsDirect) 
> >     >   Serial number of failed request:  43 
> >     >   Current serial number in output stream:  43 
> >     > 
> >     > That error persists with all the display number that I've tried 
> (-d 0.0 
> >     > to -d 0.3). 
> >     > Is there anything I am missing from my X configuration here?? 
> >     > 
> >     > On Tue, Oct 10, 2017 at 12:18 PM DRC <[email protected] 
> <javascript:> <mailto:[email protected] <javascript:>> 
> >     > <mailto:[email protected] <javascript:> <mailto:
> [email protected] <javascript:>>>> wrote: 
> >     > 
> >     >     You're missing the point.  In a VirtualGL environment, there 
> are two X 
> >     >     servers: 
> >     > 
> >     >     -- The "3D X server" is a shared resource.  It is used by 
> multiple users 
> >     >     simultaneously in order to access the GPU, so it needs to 
> remain 
> >     >     running.  It is only necessary to run one instance of the 3D X 
> server, 
> >     >     although multiple GPUs can be assigned to different screens on 
> that X 
> >     >     server.  This allows VirtualGL to address the GPUs separately, 
> which can 
> >     >     be useful for load balancing (assigning different users to 
> different 
> >     >     GPUs) in large multi-user environments. 
> >     > 
> >     >     -- The "2D X server" (X proxy) is per-user and can be 
> started/stopped as 
> >     >     needed.  TurboVNC is our recommended 2D X server/X proxy 
> solution, but 
> >     >     VirtualGL can also be used with other X proxy solutions, 
> including 
> >     >     TigerVNC, Xpra, FreeNX, etc.  The 2D X server receives and 
> displays the 
> >     >     rendered 3D images from VirtualGL, as well as rendering the 
> output of 
> >     >     other (non-3D) X applications.  It is this X server that you 
> would want 
> >     >     to start/stop using your job controller. 
> >     > 
> >     >     The basic purpose of VirtualGL is two-fold: 
> >     > 
> >     >     (1) To allow multiple users to share a single GPU (via the 3D 
> X server 
> >     >     instance) 
> >     >     (2) To split 3D and 2D rendering to different X servers, so as 
> to enable 
> >     >     hardware-accelerated OpenGL in an X proxy environment 
> >     > 
> >     >     The relationship between the 3D X server and the 2D X server 
> is thus 
> >     >     one-to-many.  Your test results made me remember a discussion 
> about this 
> >     >     on the old virtualgl-users mailing list, and IIRC someone told 
> me that 
> >     >     you have to disable the keyboard and mouse drivers in 
> xorg.conf in order 
> >     >     for the 3D X server instance to be truly headless, i.e. for 
> you to be 
> >     >     able to disconnect it from the console without encountering 
> errors.  Try 
> >     >     commenting out 
> >     > 
> >     >         InputDevice    "Mouse0" "CorePointer" 
> >     >         InputDevice    "Keyboard0" "CoreKeyboard" 
> >     > 
> >     >     and see if that changes the situation.  If so, I'll add that 
> to the 
> >     >     how-to on VirtualGL.org. 
> >     > 
> >     >     On 10/9/17 7:03 PM, Jafaruddin Lie wrote: 
> >     >     > Thanks, our xorg.conf is already set to headless according 
> to that page. 
> >     >     > Here's part of the section: 
> >     >     > 
> >     >     > Section "Screen" 
> >     >     >  Identifier "Screen0" 
> >     >     >  Device "Device0" 
> >     >     >  Monitor "Monitor0" 
> >     >     >  DefaultDepth 24 
> >     >     >  Option "UseDisplayDevice" "none" 
> >     >     >    SubSection "Display" 
> >     >     >    Depth 24 
> >     >     >  EndSubSection 
> >     >     > EndSection 
> >     >     > 
> >     >     > Section "Device" 
> >     >     >     Identifier     "Device0" 
> >     >     >     Driver         "nvidia" 
> >     >     >     VendorName     "NVIDIA Corporation" 
> >     >     >     BoardName      "Tesla K80" 
> >     >     >     BusID          "PCI:0:6:0" 
> >     >     > EndSection 
> >     >     > 
> >     >     > Section "Monitor" 
> >     >     >     Identifier     "Monitor0" 
> >     >     >     VendorName     "Unknown" 
> >     >     >     ModelName      "Unknown" 
> >     >     >     HorizSync       28.0 - 33.0 
> >     >     >     VertRefresh     43.0 - 72.0 
> >     >     >     Option         "DPMS" 
> >     >     > EndSection 
> >     >     > 
> >     >     > I'll test it with a single X instance and see how we go. 
> >     >     > The reason we want to have multiple X is that these desktop 
> sessions are 
> >     >     > started by a job controller (user will submit their request 
> for a 
> >     >     > desktop, which includes starting X). 
> >     >     > We would like the X server to be killed whenever a user 
> finishes their 
> >     >     > job, but one step at a time I suppose :) 
> >     >     > 
> >     >     > 
> >     >     > On Tue, Oct 10, 2017 at 5:12 AM DRC <[email protected] 
> <javascript:> <mailto:[email protected] <javascript:>> 
> >     <mailto:[email protected] <javascript:> <mailto:[email protected] 
> <javascript:>>> 
> >     >     > <mailto:[email protected] <javascript:> <mailto:
> [email protected] <javascript:>> 
> >     <mailto:[email protected] <javascript:> <mailto:[email protected] 
> <javascript:>>>>> wrote: 
> >     >     > 
> >     >     >     Yes, it is possible.  You don't actually even need 
> multiple X servers 
> >     >     >     for multiple users to use VirtualGL at the same time. 
>  Multiple users 
> >     >     >     can share the same GPU using VirtualGL.  That is one of 
> its purposes. 
> >     >     >     In order to use VirtualGL with multiple GPUs, generally 
> the easiest way 
> >     >     >     to do it is to configure a single X server with multiple 
> screens, so GPU 
> >     >     >     0 would be accessible by setting VGL_DISPLAY=:0.0 and 
> GPU 1 would be 
> >     >     >     accessible by setting VGL_DISPLAY=:0.1, etc.  The reason 
> why you are 
> >     >     >     getting the pixel readback error is because, unless the 
> X server is 
> >     >     >     headless, it has to be attached to the physical display 
> in order for 
> >     >     >     pixel readback to work.  The only way to use VirtualGL 
> with multiple X 
> >     >     >     servers (:0.0, :1.0, etc.) is if one or more of them is 
> configured to be 
> >     >     >     headless.  (
> https://virtualgl.org/Documentation/HeadlessNV explains how 
> >     >     >     to configure a headless X server with an nVidia GPU.) 
> >     >     > 
> >     >     >     On 10/9/17 5:35 AM, Jafaruddin Lie wrote: 
> >     >     >     > Hi all 
> >     >     >     > Simple question is if it is possible, on a single 
> machine with multiple 
> >     >     >     > GPUs, running different X servers on those GPUs, for 
> multiple users to 
> >     >     >     > use vglrun at the same time? 
> >     >     >     > This is our current setup: 
> >     >     >     > 
> >     >     >     > 1 machine with 4 Nvidia K80 cards (latest drivers), 
> running CentOS 7, 
> >     >     >     > VirtualGL 2.5.2, TightVNC, and Mate Desktop. 
> >     >     >     > 
> >     >     >     > We have 2 xorg.conf (2 cards configured on each 
> xorg.conf), and 
> >     >     >     > currently we are testing whether we can bring up 2 
> desktop sessions via VNC. 
> >     >     >     > The desktop loads, and we can see two different X 
> servers running on 
> >     >     >     > those 4 cards and the VNC servers running on different 
> displays. 
> >     >     >     > 
> >     >     >     > The issue is with VirtualGL. The first user can do 
> startx and run vglrun 
> >     >     >     > glxgears, this will work fine. 
> >     >     >     > When the second user startx, the first user's vglrun 
> session will be 
> >     >     >     > terminated with this error: 
> >     >     >     > 
> >     >     >     > [VGL] ERROR: OpenGL error 0x0502 
> >     >     >     > [VGL] ERROR: in readpixels--- 
> >     >     >     > [VGL] 439: Could not read pixels 
> >     >     >     > 
> >     >     >     > The first user can run vglrun again once the second 
> user stops their X 
> >     >     >     > session. 
> >     >     >     > My understanding is that with our setup, this should 
> be do-able, right? 
> >     >     >     > 
> >     >     >     > Thanks. 
> >     >     > 
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