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

[Herbert van den Bergh <Herbert.van.den.Bergh@xxxxxxxxxx>]

Gianluca Guida wrote:
> 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,
Network throughput was reduced to 10% of normal.
> in what workload,
A network send throughput test using netperf.
> and what heuristic you found to be wrong in this case?
The access pattern to the memory page that was recognized as a pagetable 
access was a regular memcpy doing 4 byte aligned writes into a newly 
allocated page.  I'm not that familiar with the shadow pagetable code, 
so I don't know how "wrong" this is, just that it caused a false 
positive on this type of memory access.
Thanks,
Herbert.


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