Re: PCI passthrough with VT-d - native performance

Avi Kivity
2008-07-16T15:22:35Z

Ben-Ami Yassour wrote:
>>> (with higher CPU utilization).
>>>
>>>
>> How much higher?
>>
>
> Here are some numbers for running iperf -l 1M:
>
> e1000 NIC (behind a PCI bridge)
> Bandwidth (Mbit/sec) CPU utilization
> Native OS 771 18%
> Native OS with VT-d 760 18%
> KVM VT-d 390 95%
> ﻿KVM VT-d with direct mmio 770 84%
> KVM emulated 57 100%
>
> Comment: its not clear to me why the native linux can not get closer to 1G for this NIC,
> (I verified that its not external network issues). But clearly we shouldn't hope to
> get more then the host does with a KVM guest (especially if the guest and host are the
> same OS as in this case...).
>
> e1000e NIC (onboard)
> Bandwidth (Mbit/sec) CPU utilization
> Native OS 915 18%
> Native OS with VT-d 915 18%
> ﻿KVM VT-d with direct mmio 914 98%
>
> Clearly we need to try and improve the CPU utilization, but I think that this is good enough
> for the first phase.
>
>

Agree; part of the higher utilization is of course not the fault of the
device assignment code, rather it is ordinary virtualization overhead.
We'll have to tune this.

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