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[Xen-devel] 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


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