Re: [Xen-users] How can PV be slower than PVHVM?

[Olivier Lambert <lambert.olivier@xxxxxxxxx>]

Hi Steffen,

This is because (in short and roughly):

1. Linux guest kernel is detecting Xen drivers on boot and then it will bypass any emulation (hypercalls)

2. Modern hardware is "cabled" to use virt instructions (faster than software), which is using HVM (especially for memory allocation)

So it's not a "fully emulated system" like you imagine, but instead calls to Xen and using direct hardware capabilities. This is why it's faster than PV for majority of the load (in short, almost everything related to memory translations)

Note this a short explanation without getting deeper into details :)

Best,

Olivier.

On Thu, Jan 17, 2019 at 1:14 PM Steffen Einsle <einsle@xxxxxxxxxx> wrote:

Hi there,

after reading the PV vs PVHVM performance comparison at

https://xen-orchestra.com/blog/pv-vs-pvhvm-on-next-xenserver/

but I still cannot accept how a fully emulated system can even come
close to the performance of a PV-only system. Every translation layer
must add some overhead - right? So a true PV system without translation
layer should (imho) be at the top of the performance list?

Or does the mentioned comparison apply only to debian on xen-orchestra?

My system is a barebone gentoo xen 4.10 Dom0 and multiple gentoo PV
DomUs in PV mode - and now I'm confused...



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