Re: vnif socket buffer mistaken for pagetable page causes major performance problem

[Herbert van den Bergh <Herbert.van.den.Bergh@oracle.com>]

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.