Re: [Xen-devel] [Xen-users] Grant reference batch transmission

[Gareth Stockwell <Gareth.Stockwell@xxxxxxx>]

On Wed, Mar 11, 2015 at 10:54:25, Ian Campbell wrote:
> > > What sorts of batch sizes are you expecting to see in your use case?
> >
> > We need to share hundreds of MB, so (assuming a 4kB guest page size)
> > the batch size can be thousands of grant references.
>
> FWIW, the granularity of a gref is 4kB irrespective of the guest's
> page size (so e.g. a 64Kb page would need 16 grefs to cover it), so
> your calculation is correct everywhere.
>
> We have been considering allow "superpage" grants of higher order
> mappings, but no work has been done on that yet. You such a feature be
> useful or are your hundreds of MB scattered?

They will be contiguous in 2MB chunks - so higher order grants would be useful.

What happens to the gref granularity if the stage-2 page size is larger than 
the stage-1 page size?

> I suppose the 100s of MB is changing reasonable often (e.g. not just a
> start of day setup thing) otherwise the performance/efficiency of gref
> communications wouldn't be such an issue.

It's not start of day (i.e. at driver probe time), but it's not particularly 
frequent either.  The memory being mapped into the remote domain contains data 
for a workload whose processing time is typically quite extended.  The time 
available for establishing the mapping should be at least in the tens of 
milliseconds.

Gareth


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