RE: [Xen-devel] [PATCH][HVM] remove qemu shadow_vram patchforperformance

["Dong, Eddie" <eddie.dong@xxxxxxxxx>]

Christian Limpach wrote:
> On 3/20/07, Keir Fraser <Keir.Fraser@xxxxxxxxxxxx> wrote:
>> I punted on this one. :-) Christian wasn't sure about it either,
>> hence it's not been checked in.
>
> My main concern is that with the patch we will be doing the transform
> from vga ram into the display memory everytime (vga_draw_line, some
> versions are quite expensive) and I don't see how that could be faster
> than doing the sse2 optimized compare and copy?
>
> When you test this, are the displays active or not, i.e. is the
> display changing a lot or are they at the screen saver?
>
> Maybe if the SDL frontend notices that its window is obscured, it
> should tell the rest of the code that a scan is not required...
>
> Same for VNC...
>
Christian:
        It doesn't matter if the screen is changing contents or not.
Comparing shadow vram with vram already consumes 2X memory bus
cycle. In case of 8 VMs, we get 16X memory bus cycle (and a lot
of cache flush , page fault for shadow vram and flush of other TLBs).
 While w/o memcmp, we only got 1 full screen update.
        Given that in multi core environment, different processor
running
different Qemu Windows will compete the memory access. So it seems
very nature the compare approach will become a bottle neck when VM #
increases.
        BTW, We didn't see performance issue with single VM either, and
in multiple VM environment, most Qemu windows are usually hidden.

        thanks, eddie

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