[mythtv-users] mythfrontend performance over x11vnc

[Stephen Worthington]

On Wed, 16 Mar 2022 18:34:53 -0500, you wrote:

>On 3/16/22 16:38, Dave Ulrick wrote:
>> I have two Intel NUC boxes that I use to run mythfrontend under Fedora 
>> 34. I like to do most administration remotely using x11vnc. (That way 
>> I can use an ergonomic keyboard and sit in front of a monitor that's 
>> within good viewing distance...much more comfortable than using a 
>> wireless keyboard while trying to look at a distant TV.)
>>
>> Sometimes remote control is nearly as fast as it would be to control 
>> the box locally--the OpenGL fade effects look pretty good and 
>> recording playback is very close to optimal--but other times it's 
>> extremely slow: > 1 second delay for response to any keystroke. Once 
>> the slow behavior kicks in, it persists from one x11vnc session to 
>> another until mythfrontend is restarted or the box's monitor (TV) is 
>> turned on.
>>
>> Note that the x11vnc slowness occurs even if I implement the solutions 
>> recommended for slowness issues: set -noxdamage, disable desktop 
>> compositing, disable vblank, etc..
>>
>> Just now I figured out how this issue occurs:
>>
>> 1. The remote PC's monitor is turned off.
>> 2. The screensaver on the remote PC has blanked the screen.
>> 3. x11vnc is invoked.
>> 4. The VNC viewer connects and shows a blank screen.
>> 5. Pressing a key brings the screen to life but from then on 
>> mythfrontend is very slow via x11vnc.
>>
>> My guess as to what's going on is that when the PC turns off the 
>> monitor, the X server loses sync so x11vnc falls back on a 
>> timeout-driven refresh scheme.
>>
>> I have come up with a simple solution.
>>
>> In my use case, I only have a monitor--that is, an HDTV switched to an 
>> HDMI input--turned on when I'm watching MythTV content. Otherwise, the 
>> monitor is turned off. Therefore, there's no need to run a screen 
>> saver. I didn't find a quick way to stop the screensaver 
>> (xfce4-screensaver) from running so I just went ahead and removed it 
>> from the boxes (dnf remove xfce4-screensaver).
>>
>> Since removing the screensaver, the screen hasn't blanked so 
>> mythfrontend has retained fast performance over VNC.
>>
>> Hope this helps someone out...
>>
>> Dave

@Dave

I have recently come across a more extreme version of your problem - I
was using a cheap HDMI KVM switch that just turns off the keyboard,
video and mouse connections to PCs that are not currently selected as
the active PC.  I found that a VNC connection to x11vnc on a PC that
was disconnected like that would not refresh at all - I just got a
blank screen.  I have just received a much more expensive HDMI KVM
switch that provides an emulated keyboard, video and mouse connection
to all PCs connected to it at all times, and that fixes the problem.

>Interesting. Without knowing this dependency, I have screensaver 
>uninstalled because the only monitor is TV and it is off in my case when 
>I am not actively using it. However, my x11vnc is always slow from one 
>machine and always reasonable from another machine (connected to two 
>different TVs)
>
>I noticed that when I boot without turning on TV, my X is 640x480. I 
>have to do xrandr --fb 1920x1080 to get it big or turn on the TV. When 
>this happens I have seen slowness also. So best to turn on TV, remove 
>screensaver and boot the PC.
>
>Regards
>Ramesh

@Ramesh

When you boot without a monitor/TV, X is unable to get the EDID data
from the monitor and so will boot up using whatever defaults it uses.
For a lot of monitors and TVs, there is a big difference between
standby and off.  If it is in standby (still has power from the
mains), a monitor/TV will often respond to EDID requests.  And my old
Sony 1080p TV actually responds to EDID requests even when it is
unplugged from the mains - it must be getting enough power from its
HDMI cable to be able to do that.  It only does not respond if the
HDMI cable is unplugged.

With the Nvidia drivers, it is possible to tell X to read a file where
the EDID data has been stored and use that instead of reading the real
EDID data.  That is very useful when your monitor/TV has bad EDID data
that does not correctly map to its actual capabilities - you can read
the real EDID data to a file and edit it, and then get the Nvidia
drivers to read your edited EDID data.  That is usually a much easier
alternative than telling the Nvidia drivers to ignore the EDID data
and creating modelines for all the modes your monitor/TV has.  Bad
EDID data is not at all uncommon.

There is also an ability to tell the kernel on its command line to
read an EDID file and provide that data when asked.  This is really
useful on headless server PCs (and presumably on headless mythbackend
PCs to allow mythtv-setup to be run), but I have not actually tried it
out.

Headless systems often use an EDID emulator plugged on to the HDMI
port of the PC.  That is programmed to respond with appropriate EDID
data and allows a headless system to easily be set up so that VNC
works.

My new HDMI KVM switch, like my old VGA KVM switch (and unlike my
cheap HDMI KVM switch) provides EDID passthrough/emulation, where it
takes a copy of the EDID data from the monitor every time it is read
by a PC, and stores that data.  When a PC not actually connected to
the monitor requests the EDID data, the KVM switch provides a copy of
the stored data.  You can also buy passthrough EDID emulators that
plug onto the HDMI cable and do the same job for a single HDMI
connection.  However, there are apparently quite a few out there that
do not work very well, so you need to choose a good one.

On Ubuntu, to read the EDID data from a monitor/TV, try installing the
read-edid package and using a command like this:

get-edid >edid.dat

If it works, it was able to find your monitor/TV and read its EDID
data.  If it does not work, then that will mean that X is also unable
to read the EDID data and if it needs to do that, it will revert to a
default screen mode.

It is also possible to just create a full set of modelines for your
monitor/TV and tell X to ignore the EDID data.  Then it does not
matter if the monitor/TV is connected or not at boot time.  I would
hope that it would also mean that VNC would work properly at all
times, but I have not tried that.  But creating the correct modelines
is a *lot* of hard work.