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RE: [Xen-users] Improve performance of domain U

To: "'saptarshi naha'" <naha80@xxxxxxxxx>, <xen-users@xxxxxxxxxxxxxxxxxxx>
Subject: RE: [Xen-users] Improve performance of domain U
From: "Steffen Heil" <lists@xxxxxxxxxxxxxxx>
Date: Sun, 3 Sep 2006 14:05:49 +0200
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> 1. I wanted to know if there are any configuration changes I 
> can make to improve the performance of domain U?

You will need to expose more of your setup.
LVM devices are typically faster than loopback file devices.
Other things might help as well. Give us more information.

> 2. Domain U have no /boot sector in /etc/fstab. It is all a 
> single partition in / . I wanted to know if there are any 
> shortcoming to just having / partition to performance?

I am also unsure about that point.

> 3. Also can the domain U be given swap space to improve 
> performance? I am guesssing thats the swap is used from 
> domain 0 which was added during inital installation.

Yes, just pass another device in and "swapon" it in domU. You should use a
real partition or LVM device for that.
Swap for dom0 is NOT used inside domUs. Every domain has it's own memory and
needs to swap on it's own.
(Yes, in my view this is a shortcoming of Xen, as it can force one domain to
swap out things it need frequently while another domain uses memory for
things it does not really need. However it seems to be the only way to be
fair. Otherwise any domain cound directly affect other domain's

> 4. Is the performance of teh domain U dropped due to internal 
> networking through bridging? I wanted to know if anyone knows 
> how to fine tune it to improve performance.

I am also unsure here. I don't think the bridge itself adds any noticeable
load (there are performance benchmarks for netfilter with 3.500.000
concurrent connections and 60.000 connects per second.), BUT the indirection
of the virtual network interfaces inside the domUs means that any packet
which is ready to send is only written to a buffer and will only be send,
when the control is given back to dom0, which is the only domain that can
really send...

So I think it might be usefull to have a own NIC in the domU using pci
passthrough, however that's not possible in my environment (especially with
a lot of domUs), and so I didn't (or couldn't) try...


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