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

To: Herbert van den Bergh <Herbert.van.den.Bergh@xxxxxxxxxx>
Subject: Re: [Xen-devel] vnif socket buffer mistaken for pagetable page causes major performance problem
From: Gianluca Guida <gianluca.guida@xxxxxxxxxxxxx>
Date: Tue, 09 Jun 2009 14:25:54 +0100
Cc: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>, "Xen-Devel \(E-mail\)" <xen-devel@xxxxxxxxxxxxxxxxxxx>
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Herbert van den Bergh wrote:
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.

This is with a Linux guest (HVM of course), right?

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.

Most probably the guest OS is recycling a page that used to be a pagetable and that is still shadowed.

Thanks,
Gianluca

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