[for-4.9] Re: HVM guest performance regression

[Ian Jackson <ian.jackson@eu.citrix.com>]

Juergen Gross writes ("HVM guest performance regression"):
> Looking for the reason of a performance regression of HVM guests under
> Xen 4.7 against 4.5 I found the reason to be commit
> c26f92b8fce3c9df17f7ef035b54d97cbe931c7a ("libxl: remove freemem_slack")
> in Xen 4.6.
> 
> The problem occurred when dom0 had to be ballooned down when starting
> the guest. The performance of some micro benchmarks dropped by about
> a factor of 2 with above commit.
> 
> Interesting point is that the performance of the guest will depend on
> the amount of free memory being available at guest creation time.
> When there was barely enough memory available for starting the guest
> the performance will remain low even if memory is being freed later.
> 
> I'd like to suggest we either revert the commit or have some other
> mechanism to try to have some reserve free memory when starting a
> domain.

Oh, dear.  The memory accounting swamp again.  Clearly we are not
going to drain that swamp now, but I don't like regressions.

I am not opposed to reverting that commit.  I was a bit iffy about it
at the time; and according to the removal commit message, it was
basically removed because it was a piece of cargo cult for which we
had no justification in any of our records.

Indeed I think fixing this is a candidate for 4.9.

Do you know the mechanism by which the freemem slack helps ?  I think
that would be a prerequisite for reverting this.  That way we can have
an understanding of why we are doing things, rather than just
flailing at random...

Thanks,
Ian.

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