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

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

Keir Fraser wrote:
> On 20/3/07 09:36, "Li, Xin B" <xin.b.li@xxxxxxxxx> wrote:
>
>> The major problem this patch is trying to fix is the performance drop
>> when multiple HVM guests are running, and I think we need a better
>> solution for this.
>
>> From my own reading of the code I would have thought that by
>> removing that
> SSE memcmp we'd be replacing periodic SSE block memcmp with periodic
> pixel-by-pixel comparison (which is what I think we end up doing if
> we get past the SSE memcmp?). I'm thinking particularly of the VNC
> side of things rather than SDL here, by the way: you guys use SDL
> most of the time, right?
>
Keir:
        I confirmed with Don that this memcmp will happen no matter the
Qemu
window is active or not (i.e. the screen is covered by other Windows).
So to
answer the question Anthony and people have, using memcmp all Qemu
window
need to do periodic memory comparation and may have many page fault
when accessing to shadow vram buffer used for comparation, and thus
eat a lot of CPU cycle and system bus cycles. Without the memcmp,
only the top Qemu window (usually most other windows are hidden)
will do full screen update, while others won't.
        In benchmark where we run 8 VMs, it is very obvious that we will
benefit from no memcmp. Given that using shadow page table to indicate
a dirty page needs to solve the gpn or mfn to multiple gva backlink
issue,
pulling in this solution for 3.0.5 will increase HVM scalability
obviously.
thanks,eddie

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