Re: [Xen-devel] [Hackathon minutes] PV block improvements

[Roger Pau Monnà <roger.pau@xxxxxxxxxx>]

On 25/06/13 17:57, Ian Campbell wrote:
> On Sat, 2013-06-22 at 09:11 +0200, Roger Pau Monnà wrote:
>> First because grant_copy is done by the hypervisor, while when using
>> persistent grants the copy is done by the guest.
> 
> This is true and a reasonable concern.
> 
>> Also, grant_copy takes
>> the grant lock, so when scaling to a large number of domains there's
>> going to be contention around this lock.
> 
> Does grant copy really take the lock for the duration of the copy,
> preventing any other grant ops from the source and/or target domain?
> 
> If true then that sounds like an area which is ripe for optimisation!
> 
> However I am hopeful that you are mistaken... __acquire_grant_for_copy()
> takes the grant lock while it pins the entry into the active grant entry
> list and not for the actual duration of the copy (and likewise
> __release_grant_for-copy). I hope Jan can confirm this!

Sorry, probably I haven't detailed enough here. I didn't want to mean it
takes the lock for the duration of the whole copy, but it is used at
some places during the grant copy operation so it might introduce
contention when the number of domains is high (although I have not
measured it).

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