[Xen-devel] Re: performance regression from c/s 21647:cfba1560054a

[Gianluca Guida <glguida@xxxxxxxxx>]

Hi Jan,

On a personal note, thanks for asking me about shadow pagetables, I've
been missing this kind of fun and it's a good way to celebrate my
return to Xen (and Citrix) after exactly two years... :-)
Stefano imported an old patch of mine, so I am the culprit for this.

On Thu, Nov 10, 2011 at 3:23 AM, Jan Beulich <JBeulich@xxxxxxxx> wrote:
> in this c/s you did, besides the main purpose of the change, an
> adjustment to check_for_early_unshadow() in that L2...L4 entries
> would no longer be attempted to get unshadowed. Neither the
> patch description nor the added comment really make clear why
> this was done, and we now got a customer report regarding this
> causing considerable slowdown in process creation/destruction
> intensive workloads (e.g. shell scripts).

Unfortunately, I can't clearly remember the reason for this. I assume
that your mentioning of shell scripts implies that this is running
under a Linux guest. Back in the day, this patch was tailored very
strictly against Windows guests, and that probably meant that doing so
had a performance improvement over the way Windows handled pagetables
and process distructions.

Perhaps (I need some time to catch up) the PV-HVM thing has made this
patch affect Linux as well, at the point that this has became a
problem.

Can you tell me in what guest OS this is experiencing a slow down?
What is the order of magnitude of the performance decrease?

I have no problem in reverting that part of the change, if it makes you happy.

Thanks,
Gianluca

-- 
It was a type of people I did not know, I found them very strange and
they did not inspire confidence at all. Later I learned that I had been
introduced to electronic engineers.
                                                  E. W. Dijkstra

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