Re: [Xen-devel] [PATCH, RFC] i386: highmem access assistance hypercalls

["Jan Beulich" <jbeulich@xxxxxxxxxx>]

>>> "Jan Beulich" <jbeulich@xxxxxxxxxx> 17.10.08 17:28 >>>
>While looking at the origin of very frequently executed hypercalls I
>realized that the high page accessor functions in Linux would be good
>candidates to handle in the hypervisor - clearing or copying to/from
>a high page is a pretty frequent operation (provided there's enough
>memory in the domain). While prior to the first submission I only
>measured kernel builds (where the results are not hinting at a
>meaningful improvement), I now found time to do a more specific
>analysis: page clearing is being improved by about 20%, page copying
>doesn't seem to significantly benefit (though that may be an effect of
>the simplistic copy_page() implementation Xen currently uses) -
>nevertheless I would think that if one function is supported by the
>hypervisor, then the other should also be.

Actually, the workload I used for measuring the effect on page copying
was not meaningful. On an (artificial) workload that really exercises
page copying in a meaningful way, the improvement is about 15%.

Also, the improvements on a 64-bit hypervisor are, as expected, even a
little better (close to 25% for page clearing and close to 20% for page
copying - with the original numbers already having been slightly better
than on a 32-bit hypervisor).

Jan


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