Re: qemu-dm performance

[Anthony Liguori <aliguori@us.ibm.com>]

There was a long thread about this topic already plus a patch floating 
around.  I don't think vram_dirty is the problem.

vram_dirty seems to be Xen-specific.  Presumably, since we map the 
framebuffer directly into the guest, we cannot use write-faulting 
anymore to track dirtying.  Instead, it looks like we rely on a double 
buffer to determine which portions of the screen change.

Regards,

Anthony Liguori

Tommie McAfee wrote:
> I've been investigating why qemu-dm is causing %CPU to be high when
> viewing fully vitalized guests with vncviewer( about 20% usage ).  
>
> I've looked at the code, and one area that I'm curious about is the
> vram_dirty() function in tools/ioemu/hw/vga.c.  Please correct me if I'm
> wrong, but vram_dirty() seems to be using SSE inline functions to
> optimize it's bit-shifting/masking/loading/storing/comparison operations
> to see if dirty bits need to be set for a page within the shadow table.
> Also, I used gdb to make sure that I'm really executing the SSE
> optimized version of vram_dirty() that utilizes xmm0 registers.
>
> So out of curiosity, I decided to comment out calls to vram_dirty() from
> vga_draw_graphic() and the guests still behave normally, but CPU% now
> drops to 8%.  I know this is silly, and that I should expect a CPU drop
> for commenting out code, but why is vram_dirty() adding 12% CPU
> utilization when it can be commented out without crashing my viewer, and
> without me even noticing a difference in the guests behavior?  Can
> someone help me to understand the purpose vram_dirty serves and perhaps
> why it seems 2 be so CPU intensive without really changing the way my
> virtual guest behaves?
>
> Also, where else should I look in the code for possible explanations to
> why qemu-dm uses 20% CPU simply to view a guest.  All comments and
> suggestions regarding this matter are appreciated,
>
> thx,
>
> T. McAfee
> Xen Testing
>
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