RE: [Xen-devel] qemu-dm performance

["Dugger, Donald D" <donald.d.dugger@xxxxxxxxx>]

Since I wrote the code all I can do is say that Anthony is completely
correct, there's a shadow frame buffer that `vram_dirty' compares to see
if the HVM guest has modified it's frame buffer.  Deleting this function
should result in a blank VGA screen for your HVM guest, I'm really
curious how you got this to work.  Are you accessing the guest from the
serial line rather than the VGA console?

--
Don Dugger
"Censeo Toto nos in Kansa esse decisse." - D. Gale
Donald.D.Dugger@xxxxxxxxx
Ph: (303)443-3786 

>-----Original Message-----
>From: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx 
>[mailto:xen-devel-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of 
>Anthony Liguori
>Sent: Wednesday, September 20, 2006 2:13 PM
>To: Tommie McAfee
>Cc: xen-devel@xxxxxxxxxxxxxxxxxxx
>Subject: Re: [Xen-devel] qemu-dm performance
>
>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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>> Xen-devel mailing list
>> Xen-devel@xxxxxxxxxxxxxxxxxxx
>> http://lists.xensource.com/xen-devel
>>
>>   
>
>
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