Re: pvUSB backend performance

[Juergen Gross <jgross@suse.com>]

On 06/29/2015 03:22 PM, George Dunlap wrote:
> On Wed, Jun 24, 2015 at 1:06 PM, Juergen Gross <jgross@suse.com> wrote:
>> Hi,
>>
>> my qemu integrated pvUSB backend is now running stable enough to do
>> some basic performance measurements. I've passed a memory-stick with
>> about 90MB of data on it to a pv-domU. Then I read all the data on
>> it with tar and looked how long this would take (elapsed time):
>>
>> in dom0:                     5.2s
>> in domU with kernel backend: 6.1s
>> in domU with qemu backend:   8.2s
>>
>> So the qemu backend is about 30% slower than the kernel backend. Is
>> this acceptable?
>
> Just to be clear, you mean having qemu act as a pvusb backend (a la
> qdisk), not emulated, is that correct?

Yes.

> I don't actually understand your question -- is the overhead
> acceptable for what?

Acceptable for the decision to build the backend in qemu only. When I
posted the first draft of my kernel backend to lkml the question was
raised why I couldn't do this in userland via libusb.

> I think in an ideal world the toolstack will use the kernel backend if
> it's available, and fall back to a qemu backend if it's not available.

In case the performance is regarded to be sufficient I won't retry to
push a kernel backend. So there will be none in the near future.

If the performance is not good enough I'll give the kernel backend
another try. If it's being accepted I probably won't do the qemu one.


Juergen