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


From: vans163
Subject: [Qemu-devel] [Bug 1618122] Re: qemu-monitor screendump very slow
Date: Wed, 31 Aug 2016 12:53:00 -0000

** Description changed:

  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 I 
cant figure out where the raw display buffer/image is gotten from.
+ 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
  };
  
- 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.
+ 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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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
  
--------------------------------------------------------------------------------
  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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