Re: [Xen-users] disk performance about half in domU? + question about XenSource

[Mark Williamson <mark.williamson@xxxxxxxxxxxx>]

> There's also a fairly big hit on network I/O.

Network IO is particularly hard to virtualise with good performance.  The 
trick you suggested with a multi-processor system does help.  Actually, even 
just dedicating a hyperthread to dom0 helps (if your system has HT).

> I don't believe you can get it any faster with the current architecture
> as it doesn't support any kind of DMA.. it's all software copying of
> data from one place to another.

Disk data is DMAed to /from the correct location for paravirt devices.  I'm 
not sure how it works for HVM emulated devices.

Received network data is copied these days because it actually gives better 
average case performance than the previous copy avoidance scheme.  Ideally 
the network card would know to DMA straight to the correct virtual machine 
without dom0 getting involved.

> loopback partitions are faster because a lot gets cached in RAM,
> although because of that they're not so great if you get a sudden power
> loss.  Never had a kernel that supported blktap so I don't know what
> kind of speed you can get out of that.

Loopback devices can be a bit hairy though, both because of the power outage 
issue you mentioned and also because they're heavy RAM users.  The caching 
improvements are probably better provided by reducing the size of dom0 and 
increasing the size of the domUs so that they can cache more stuff 
themselves.

Blktap is supposed to be a nicer solution for file-backed devices.  Still, 
using raw partitions should really be plenty fast enough, there's not really 
a lower overhead solution than that.  And performance problems with one 
solution are probably going to crop up with another, anyhow.

Cheers,
Mark

-- 
Dave: Just a question. What use is a unicyle with no seat?  And no pedals!
Mark: To answer a question with a question: What use is a skateboard?
Dave: Skateboards have wheels.
Mark: My wheel has a wheel!

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