[Bug 1618122] Re: qemu-monitor screendump very slow

[Thomas Huth]

The QEMU project is currently considering to move its bug tracking to
another system. For this we need to know which bugs are still valid
and which could be closed already. Thus we are setting older bugs to
"Incomplete" now.

If you still think this bug report here is valid, then please switch
the state back to "New" within the next 60 days, otherwise this report
will be marked as "Expired". Or please mark it as "Fix Released" if
the problem has been solved with a newer version of QEMU already.

Thank you and sorry for the inconvenience.

** Changed in: qemu
       Status: New => Incomplete

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https://bugs.launchpad.net/bugs/1618122

Title:
  qemu-monitor screendump very slow

Status in QEMU:
  Incomplete

Bug description:
  qemu-monitor screendump often using 10-20% cpu usage of one core to
  take a small capture.

  Most of the CPU usage seems to come from libpixman. There were many
  reports of libpixman becoming 8 times slower in newer releases.

  
https://github.com/qemu/qemu/blob/0c56c6ab68902281094c7aac6305e2321c34c187/ui/console.c#L285

  Simple Valgrind Ir report.

  
--------------------------------------------------------------------------------
             Ir
  
--------------------------------------------------------------------------------
  9,994,313,959  PROGRAM TOTALS

  
--------------------------------------------------------------------------------
             Ir  file:function
  
--------------------------------------------------------------------------------
  4,883,603,360  
/usr/src/debug/pixman-0.34.0/pixman/pixman-access.c:store_scanline_b8g8r8 
[/usr/lib64/libpixman-1.so.0.34.0]
    282,312,800  
/usr/src/debug/pixman-0.34.0/pixman/pixman-implementation.c:_pixman_implementation_iter_init
 [/usr/lib64/libpixman-1.so.0.34.0]
    267,394,160  
/usr/src/debug/pixman-0.34.0/pixman/pixman-sse2.c:sse2_fetch_x8r8g8b8 
[/usr/lib64/libpixman-1.so.0.34.0]
    256,761,600  
/usr/src/debug/pixman-0.34.0/pixman/pixman-private.h:store_scanline_b8g8r8
    254,676,199  ???:0x0000000000011f40 [/usr/lib64/libjemalloc.so.2]
    199,990,526  ???:0x0000000000011a10 [/usr/lib64/libjemalloc.so.2]

  Calls to there start from.
  qemu_pixman_linebuf_fill
  pixman_image_composite
  ????
  pixman_image_composite32
  general_composite_rect

  I tried taking a look on how to fix this, but it seems pixman is
  deeply enrooted inside the monitor.  I wanted to try to simply take
  whats on the display and memcpy it into .ppm format manually creating
  the file header, but the image is all broken with colors being off or
  its smeared.

  For example this is DisplaySurface:

  struct DisplaySurface {
      pixman_format_code_t format;
      pixman_image_t *image;
      uint8_t flags;
  #ifdef CONFIG_OPENGL
      GLenum glformat;
      GLenum gltype;
      GLuint texture;
  #endif
  };

  
  It appears graphic_hw_update(con) renders the vram to a pixman_image.  Then 
ppm_save(filename, surface, errp) goes over it yet again, mutates the bytes in 
an expensive way to another pixman_image, and saves that row by row to a ppm.

  Using sdl graphics (the default) the drawing starts in
  
https://github.com/qemu/qemu/blob/7fa124b273acd22a808e742ead78c065ccd9b4c4/hw/display/vga.c#L1466

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