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xen-devel
Re: [Xen-devel] vnif socket buffer mistaken for pagetable page causes ma
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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