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

[vans163]

Most of the CPU is coming from ppm_save(filename, surface, errp);

graphic_hw_update(con) takes an insignificant amount.

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

Title:
  qemu-monitor screendump very slow

Status in QEMU:
  New

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
  
--------------------------------------------------------------------------------
  56,592,286,124  PROGRAM TOTALS

  
--------------------------------------------------------------------------------
              Ir  file:function
  
--------------------------------------------------------------------------------
  40,288,379,712  ???:0x000000000000caa0 [/usr/lib64/libpixman-1.so.0.34.0]
   3,585,795,168  ???:0x000000000006df20 [/usr/lib64/libpixman-1.so.0.34.0]
   1,763,982,432  ???:0x0000000000052360 [/usr/lib64/libpixman-1.so.0.34.0]
   1,517,832,033  ???:__memcpy_sse2_unaligned [/usr/lib64/libc-2.23.so]
     993,997,885  ???:__GI_mempcpy [/usr/lib64/libc-2.23.so]
     484,059,456  ???:0x0000000000050430 [/usr/lib64/libpixman-1.so.0.34.0]
     460,109,168  ???:pixman_image_composite32 
[/usr/lib64/libpixman-1.so.0.34.0]

  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 I cant figure out where the raw display
  buffer/image is gotten from.

  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
  };

  Which as you can see already has the pixman_image_t. Maybe I should
  just work with that pixman_image_t?

  The most effective solution IMO seems to just memcpy from the display
  into a premade header for a .ppm or .bmp file assuming 24 or 32 bpp.
  No need for libpixman.

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