Re: [Xen-devel] vnif socket buffer mistaken for pagetable page causes major performance problem

[Gianluca Guida <gianluca.guida@xxxxxxxxxxxxx>]

Hi,

Sorry for the late reply,

Dan Magenheimer wrote:
> A recent posting reminded me of this and, though the
> information is a bit vague, someone familiar with the
> shadow code might know just where to look to fix this,
> hopefully in time for 3.4 (if its not already fixed).
>
> One of our performance experts discovered a strange
> major network performance hit seen only under certain
> circumstances, IIRC mostly in HVM but sometimes in
> PV when migrating.
>
> With some help from Intel, it was determined that
> the heuristics used by shadow paging to determine
> whether a guest is modifying a pagetable page were
> getting fooled by the access pattern used by the
> code that copies data into a newly allocated socket
> buffer.  As a result, many unnecessary vmenter/vmexits
> were happening.
>
> The workaround was to preallocate all socket buffer
> memory.
>
> That's all I've got, but we can try to answer questions
> if this isn't already a known fixed problem.
Can you be a little more specific about what is the performance loss, in 
what workload, and what heuristic you found to be wrong in this case?
Thanks,
Gianluca

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