Sluggish performance and debug messages from x11vnc

[jmechnich]

In Debian 11 testing as well as in Debian 10 since a recent update to libvncserver, x11vnc performance is quite bad when accessing a running X session via ssh tunnel using different VNC viewers (i.e. ssvncviewer on Linux, vnc-viewer/RealVNC on macOS).

The terminal running the ssh session shows repeated output lines from x11vnc, such as:

The VNC desktop is:      localhost:0
*** fb_push ublen NOT ZERO: 759582801
*** fb_push ublen NOT ZERO: 759582801
(...)

Those messages can be triggered for example by moving the mouse around the VNC screen.

From the recent appearance of this bug in Debian 10, I was able to attribute the bug to this change in libvncserver:
https://github.com/LibVNC/libvncserver/blame/f45672cdc9b749e100be2463b1a04f3a9fc0aa26/rfb/rfb.h#L570

UPDATE_BUF_SIZE has been changed from 30000 to 32768. This macro definition is used in x11vnc as well, more specifically in misc/turbovnc/tight.c.

Changing the value back to 30000 and recompiling fixes the problem.

The triggered messages are printed out here.

It would be great if someone with a deeper knowledge of x11vnc and associated protocols could comment as I am not certain if things are broken here or maybe even inside the viewer code.

References:

• https://forums.debian.net/viewtopic.php?t=153655
• https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1027043
• Please support larger screens libvncserver#521

Edit:
Apparently this problem is caused by something else and can be fixed by simply recompiling x11vnc locally.

[SunRain]

UPDATE: This issuse (the ** fb_push ublen NOT ZERO log in systemd) appears in ArchLinux x11vnc-1:0.9.16-5-x86_64. After I update with x11vnc 1:0.9.16.r12.g97d632c-1 built from aur it dispeared.

In Debian 11 testing as well as in Debian 10 since a recent update to libvncserver, x11vnc performance is quite bad when accessing a running X session via ssh tunnel using different VNC viewers (i.e. ssvncviewer on Linux, vnc-viewer/RealVNC on macOS).

The terminal running the ssh session shows repeated output lines from x11vnc, such as:

The VNC desktop is:      localhost:0
*** fb_push ublen NOT ZERO: 759582801
*** fb_push ublen NOT ZERO: 759582801
(...)

Those messages can be triggered for example by moving the mouse around the VNC screen.

From the recent appearance of this bug in Debian 10, I was able to attribute the bug to this change in libvncserver: https://github.com/LibVNC/libvncserver/blame/f45672cdc9b749e100be2463b1a04f3a9fc0aa26/rfb/rfb.h#L570

UPDATE_BUF_SIZE has been changed from 30000 to 32768. This macro definition is used in x11vnc as well, more specifically in misc/turbovnc/tight.c.

Changing the value back to 30000 and recompiling fixes the problem.

The triggered messages are printed out here.

It would be great if someone with a deeper knowledge of x11vnc and associated protocols could comment as I am not certain if things are broken here or maybe even inside the viewer code.

References:

• https://forums.debian.net/viewtopic.php?t=153655
• https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1027043
• Please support larger screens libvncserver#521

[venoom27]

I am also having this issue with Arch x11vnc-1:0.9.16-5-x86_64 since I updated.

[jmechnich]

UPDATE: This issuse (the ** fb_push ublen NOT ZERO log in systemd) appears in ArchLinux x11vnc-1:0.9.16-5-x86_64. After I update with x11vnc 1:0.9.16.r12.g97d632c-1 built from aur it dispeared.

...

Great to hear that. Could you investigate what fixed the problem in your case? Is there a specific patch applied to x11vnc or one of its dependencies in Arch AUR?

[svenssonaxel]

Just in case it helps, I'm often getting negative numbers in this error message:

*** fb_push ublen NOT ZERO: -1028847095
*** fb_push ublen NOT ZERO: -1028847095
*** fb_push ublen NOT ZERO: -1028847095
*** fb_push ublen NOT ZERO: -1028847095

[venoom27]

By building the aur version x11vnc-git 1:0.9.16.r12 it fixed the issue. Thanks

[SunRain]

See this, but this thread posted at 2013-04-03

https://sourceforge.net/p/libvncserver/mailman/libvncserver-common/thread/20130403122626.0e393c65%40chronoschnitte/

https://aur.archlinux.org/packages/x11vnc-git

And the x11vnc aur package in archlinux still used libjpeg-turbo, and libjpeg-turbo(version 2.1.4-2) contains libjpeg, here's the log

ldd /usr/bin/x11vnc
linux-vdso.so.1 (0x00007ffd489f1000)
libvncserver.so.1 => /usr/lib/libvncserver.so.1 (0x00007f2cbf14b000)
libvncclient.so.1 => /usr/lib/libvncclient.so.1 (0x00007f2cbf11e000)
libssl.so.3 => /usr/lib/libssl.so.3 (0x00007f2cbf07e000)
libcrypto.so.3 => /usr/lib/libcrypto.so.3 (0x00007f2cbec00000)
libcrypt.so.2 => /usr/lib/libcrypt.so.2 (0x00007f2cbf04c000)
libXtst.so.6 => /usr/lib/libXtst.so.6 (0x00007f2cbf5ea000)
libXext.so.6 => /usr/lib/libXext.so.6 (0x00007f2cbebeb000)
libXinerama.so.1 => /usr/lib/libXinerama.so.1 (0x00007f2cbf5e3000)
libXrandr.so.2 => /usr/lib/libXrandr.so.2 (0x00007f2cbebde000)
libXfixes.so.3 => /usr/lib/libXfixes.so.3 (0x00007f2cbebd6000)
libXdamage.so.1 => /usr/lib/libXdamage.so.1 (0x00007f2cbebd1000)
libXcomposite.so.1 => /usr/lib/libXcomposite.so.1 (0x00007f2cbebcc000)
libXcursor.so.1 => /usr/lib/libXcursor.so.1 (0x00007f2cbebc0000)
libX11.so.6 => /usr/lib/libX11.so.6 (0x00007f2cbea7d000)
libavahi-common.so.3 => /usr/lib/libavahi-common.so.3 (0x00007f2cbea6f000)
libavahi-client.so.3 => /usr/lib/libavahi-client.so.3 (0x00007f2cbea5b000)
libXi.so.6 => /usr/lib/libXi.so.6 (0x00007f2cbea47000)
libcairo.so.2 => /usr/lib/libcairo.so.2 (0x00007f2cbe923000)
libc.so.6 => /usr/lib/libc.so.6 (0x00007f2cbe73a000)
libsystemd.so.0 => /usr/lib/libsystemd.so.0 (0x00007f2cbe65e000)
libz.so.1 => /usr/lib/libz.so.1 (0x00007f2cbe644000)
liblzo2.so.2 => /usr/lib/liblzo2.so.2 (0x00007f2cbe623000)
libjpeg.so.8 => /usr/lib/libjpeg.so.8 (0x00007f2cbe5a0000)
libpng16.so.16 => /usr/lib/libpng16.so.16 (0x00007f2cbe567000)
libgcrypt.so.20 => /usr/lib/libgcrypt.so.20 (0x00007f2cbe41d000)
libgnutls.so.30 => /usr/lib/libgnutls.so.30 (0x00007f2cbe200000)
libsasl2.so.3 => /usr/lib/libsasl2.so.3 (0x00007f2cbe1e1000)
libXrender.so.1 => /usr/lib/libXrender.so.1 (0x00007f2cbe410000)
libxcb.so.1 => /usr/lib/libxcb.so.1 (0x00007f2cbe1b6000)
libpthread.so.0 => /usr/lib/libpthread.so.0 (0x00007f2cbe409000)
libdbus-1.so.3 => /usr/lib/libdbus-1.so.3 (0x00007f2cbe165000)
libdl.so.2 => /usr/lib/libdl.so.2 (0x00007f2cbe404000)
libm.so.6 => /usr/lib/libm.so.6 (0x00007f2cbe07d000)
libfontconfig.so.1 => /usr/lib/libfontconfig.so.1 (0x00007f2cbe02e000)
libfreetype.so.6 => /usr/lib/libfreetype.so.6 (0x00007f2cbdf5f000)
libxcb-render.so.0 => /usr/lib/libxcb-render.so.0 (0x00007f2cbdf4e000)
libxcb-shm.so.0 => /usr/lib/libxcb-shm.so.0 (0x00007f2cbdf49000)
libpixman-1.so.0 => /usr/lib/libpixman-1.so.0 (0x00007f2cbde9c000)
/lib64/ld-linux-x86-64.so.2 => /usr/lib64/ld-linux-x86-64.so.2 (0x00007f2cbf5f4000)
libcap.so.2 => /usr/lib/libcap.so.2 (0x00007f2cbde90000)
liblzma.so.5 => /usr/lib/liblzma.so.5 (0x00007f2cbde5d000)
libzstd.so.1 => /usr/lib/libzstd.so.1 (0x00007f2cbddb5000)
liblz4.so.1 => /usr/lib/liblz4.so.1 (0x00007f2cbdd91000)
libgcc_s.so.1 => /usr/lib/libgcc_s.so.1 (0x00007f2cbdd71000)
libgpg-error.so.0 => /usr/lib/libgpg-error.so.0 (0x00007f2cbdd4b000)
libp11-kit.so.0 => /usr/lib/libp11-kit.so.0 (0x00007f2cbdc18000)
libbrotlienc.so.1 => /usr/lib/libbrotlienc.so.1 (0x00007f2cbdb74000)
libbrotlidec.so.1 => /usr/lib/libbrotlidec.so.1 (0x00007f2cbdb64000)
libidn2.so.0 => /usr/lib/libidn2.so.0 (0x00007f2cbdb42000)
libunistring.so.5 => /usr/lib/libunistring.so.5 (0x00007f2cbd988000)
libtasn1.so.6 => /usr/lib/libtasn1.so.6 (0x00007f2cbd971000)
libnettle.so.8 => /usr/lib/libnettle.so.8 (0x00007f2cbd91d000)
libhogweed.so.6 => /usr/lib/libhogweed.so.6 (0x00007f2cbd8d4000)
libgmp.so.10 => /usr/lib/libgmp.so.10 (0x00007f2cbd82f000)
libXau.so.6 => /usr/lib/libXau.so.6 (0x00007f2cbd82a000)
libXdmcp.so.6 => /usr/lib/libXdmcp.so.6 (0x00007f2cbd822000)
libexpat.so.1 => /usr/lib/libexpat.so.1 (0x00007f2cbd7f7000)
libbz2.so.1.0 => /usr/lib/libbz2.so.1.0 (0x00007f2cbd7e4000)
libharfbuzz.so.0 => /usr/lib/libharfbuzz.so.0 (0x00007f2cbd6f9000)
libffi.so.8 => /usr/lib/libffi.so.8 (0x00007f2cbd6ee000)
libbrotlicommon.so.1 => /usr/lib/libbrotlicommon.so.1 (0x00007f2cbd6cb000)
libgraphite2.so.3 => /usr/lib/libgraphite2.so.3 (0x00007f2cbd6a9000)
libglib-2.0.so.0 => /usr/lib/libglib-2.0.so.0 (0x00007f2cbd56a000)
libpcre2-8.so.0 => /usr/lib/libpcre2-8.so.0 (0x00007f2cbd4cd000)

[jmechnich]

Hrmm...interesting. It turns out that simply recompiling the package locally fixed the problem for me as well (on Debian 11), without removing the patch mentioned above.

I have no idea what is going on here...but it is unlikely to be an issue with the x11vnc code, so I am going to close this. Thanks for the input!

Edit: in Debian 11, this has been fixed in x11vnc 0.9.16-9.