RE: [Xen-devel] [PATCH] fix memory allocation from NUMA node for VT-d.

[Espen Skoglund <espen.skoglund@xxxxxxxxxxxxx>]

You only require more memory if you duplicate the structures per
IOMMU.  While this is indeed possible (and may even be the desired
solution) it is not what I suggested.

And you're making the assumption here that the guest is assigned to
the node of the IOMMU.  As Allen points out, how does a user make this
decision?  And in many cases I would expect that you would not want to
assign many guests to the same node anyway.  By at least keeping the
IOMMU page tables local to the node you'll get lower latencies for the
page table walker.

        eSk


>> -----Original Message-----
>> From: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx 
>> [mailto:xen-devel-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of 
>> Yuji Shimada
>> Sent: Wednesday, November 19, 2008 12:26 AM
>> To: Espen Skoglund
>> Cc: xen-devel@xxxxxxxxxxxxxxxxxxx; 'Keir Fraser'
>> Subject: Re: [Xen-devel] [PATCH] fix memory allocation from 
>> NUMA node for VT-d.
>> 
>> Hi Espen,
>> 
>> Your suggestion allocating memory from one of the nodes where the
>> IOMMU is attached improves performance more. But more memory is
>> needed, because structures are needed per IOMMU.
>> 
>> My patch keeps the current implementation, one Device Assignment
>> Structure and Address Translation Structure per guest.
>> 
>> Xen's user will assign a device to a closer guest. So, node of the
>> guest and node connected to IOMMU will be the same.
>> As a result, the memory performance will be improved with my patch.
>> 
>> Thanks,
>> --
>> Yuji Shimada
>> 
>> On Tue, 18 Nov 2008 12:00:37 +0000
>> Espen Skoglund <espen.skoglund@xxxxxxxxxxxxx> wrote:
>> 
>>> Given an FSB based system the IOMMUs sit in the north-bridge.  How
>>> does this work qith QPI?  Where in the system do the different IOMMUs
>>> sit?  Wouldn't it make more sense to allocate memory from one of the
>>> nodes where the IOMMU is attached?  Having the memory allocated from
>>> the node of the guest only helps when the guest needs to update its
>>> page tables.  I'd rather optimize for page table walks in the IOMMU.
>>> 
>>>     eSk
> >
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