Re: [Qemu-devel] Ideas wiki for GSoC 2010

Anthony Liguori
2010-03-15T13:56:02Z

On 03/15/2010 08:24 AM, Joerg Roedel wrote:
> On Mon, Mar 15, 2010 at 03:11:42PM +0200, Avi Kivity wrote:
>
>> On 03/15/2010 03:03 PM, Joerg Roedel wrote:
>>
>>>
>>>>> I will add another project - iommu emulation. Could be very useful
>>>>> for doing device assignment to nested guests, which could make
>>>>> testing a lot easier.
>>>>>
>>>>>
>>>> Our experiments show that nested device assignment is pretty much
>>>> required for I/O performance in nested scenarios.
>>>>
>>>>
>>> Really? I did a small test with virtio-blk in a nested guest (disk read
>>> with dd, so not a real benchmark) and got a reasonable read-performance
>>> of around 25MB/s from the disk in the l2-guest.
>>>
>>>
>>>
>> Your guest wasn't doing a zillion VMREADs and VMWRITEs every exit.
>>
>> I plan to reduce VMREAD/VMWRITE overhead for kvm, but not much we can do
>> for other guests.
>>
> Does it matter for the ept-on-ept case? The initial patchset of
> nested-vmx implemented it and they reported a performance drop of around
> 12% between levels which is reasonable. So I expected the loss of
> io-performance for l2 also reasonable in this case. My small measurement
> was also done using npt-on-npt.
>

But that was something like kernbench IIRC which is actually exit light
once ept is enabled.

Network IO is typically exit heavy and becomes something more of a
pathological work load (both for nested ept and nested npt).

Regards,

Anthony Liguori

> Joerg
>
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