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Re: HVM performance once again
- To: Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>
- From: Roger Pau Monné <roger.pau@xxxxxxxxxx>
- Date: Wed, 31 May 2023 10:24:44 +0200
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- Cc: xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Wed, 31 May 2023 08:25:20 +0000
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On Wed, May 31, 2023 at 09:49:32AM +0200, Marek Marczykowski-Górecki wrote:
> Hi,
>
> I returned to HVM performance once again, this time looking at PCI
> passthrough impact evaluating network throughput.
> The setup:
> - Xen 4.17
> - Linux 6.3.2 in all domU
> - iperf -c running in a PVH (call it "client")
> - iperf -s running in a HVM (call it "server")
> - client's netfront has a backend directly in server
> - frontend's "trusted" is set to 0
> - HVM have qemu in a stubdomain in all the cases
> - no intentional differences about HVM besides presence of a PCI device
> (it is a network card, but it was not involved in the traffic)
>
> And now the results:
> - server is plain HVM: ~6Gbps
> - server is HVM and has some PCI passthrough: ~3Gbps
>
> Any idea why such huge difference?
Just a wild guess, when domains have a PCI device assigned the memory
cache types from the guest are enforced, otherwise it all defaults to
write-back (see epte_get_entry_emt()).
If you are not using the PCI device you might want to play with
epte_get_entry_emt() and see if that makes a difference.
Do you see the same performance regression when testing on AMD?
Thanks, Roger.
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