Re: Subject: RE: [Xen-users] Poor disk io performance in domUs

["David Brown" <dmlb2000@xxxxxxxxx>]

> Actually, if you expect IOMMU to solve the problem, you can do the same
> (with slightly less security, admittedly) in Para-virtual domains today
> - since IOMMU can only translate and protect on a per-device level, so
> you need to have one device per domain.
Well yeah the same thing can be accomplished today but there's that
little kernel process that has to do the mapping in software and that
takes away cpu time from doing other work.

Ideally, you would bring up a box with say 6+ domains on it plus a
dom0 and no matter what you did with the domains it wouldn't adversely
affect the other domains, even dom0. So this theoretical box would
probably have 7 network devices (with IOMMU) mapped to the various
domains in hardware so no software has to do the mapping to the
different address spaces. Similarly with disk devices, there would
have to be 7 scsi devices so that each could be mapped the the various
domains. Then there would be only one box and it could really look
like 7 different machines and have the same performance as it would
normally.

> So if you have a disk-controller with disk for each domain, you could do
> that today. Same with network controllers [there are even some network
> controllers which are "multihead", meaning that they present themselves
> as multiple individual devices, even though it all goes onto a single
> network connection].
>
> The other point that immediately comes to mind here is that the Dom0
> should definitely have it's own CPU if you're doing a lot of
> disk/network IO through it.
Well, I should have said hardware IOMMU, but you're right, its on a
device based level which doesn't help I/O so much since that's not
based on disks. Really what we need is a scsi based IOMMU controler of
some kind that can do the translation on the device itself per disk,
now that'd be cool :)

- David Brown

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