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qemu-monitor screendump very slow
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 pixmap 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 starts.
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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