CentOS - Aug 2007 - "near native" performance with xen?

Just wondering if there was a howto or other URL that 
explains what is needed to achieve "near native" performance 
on a xen domU -- for this purpose, I am thinking about a 
single domU running on a physical server, in comparison to 
that same physical server running the same kernel but 
non-xenified.

For instance, using a physical partition for VBD v. using a 
file-backed one is one of the more obvious ones. Assigning 
all the VCPUs. And as much RAM as you can get away with 
(maybe leaving the dom0 with 512MB).

But are there others? Since I'm doing paravirtualization, I 
assume I don't need to turn on VT in the BIOS? What about 
32-bit v. 64-bit OS, for the dom0 and for the domU? (I'll be 
using CentOS-5.) Anything else?

johnn

> -----Original Message-----
> From: centos-bounces at centos.org 
> [mailto:centos-bounces at centos.org] On Behalf Of Johnn Tan
> Sent: Monday, August 06, 2007 2:56 PM
> To: CentOS mailing list
> Subject: [CentOS] "near native" performance with xen?
> 
> Just wondering if there was a howto or other URL that 
> explains what is needed to achieve "near native" performance 
> on a xen domU -- for this purpose, I am thinking about a 
> single domU running on a physical server, in comparison to 
> that same physical server running the same kernel but 
> non-xenified.
> 
> For instance, using a physical partition for VBD v. using a 
> file-backed one is one of the more obvious ones. Assigning 
> all the VCPUs. And as much RAM as you can get away with 
> (maybe leaving the dom0 with 512MB).
> 
> But are there others? Since I'm doing paravirtualization, I 
> assume I don't need to turn on VT in the BIOS? What about 
> 32-bit v. 64-bit OS, for the dom0 and for the domU? (I'll be 
> using CentOS-5.) Anything else?
You don't need to allocate all the CPUs if the app running in
the domU doesn't need that horse power, same goes for the
memory.

Where you want to concentrate on is storage and network.

Of course you need physical volumes but look for a storage
solution that will also run as direct to disk as possible,
maybe something based on the newer SATA/SAS where the RAID
logic is built into the enclosure and a plain SATA/SAS
card is in the server which can be actively shared between
multiple domUs.

If that is out of your budget then use some type of hardware
RAID for your volumes to get rid of any use of software RAID
in dom0 for guest storage.

Same goes for the network side of things. Install the latest
PV drivers in the domUs to get the latest advances.

-Ross

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On Mon, 2007-08-06 at 14:55 -0400, Johnn Tan wrote:
> Assigning all the VCPUs.
Having more than one vcpu currently emits non-fatal error messages, at
least with the C5 domU kernel. I didn't have time to look into that yet.
Though, it's probably useful for threaded applications if it works. Note
that you can always specify which CPUs can be used by a domU (seen from
the hypervisor), regardless of the number of vcpus. E.g.:

cpus="0-1"
vcpus=1
>  And as much RAM as you can get away with (maybe leaving the dom0 with
512MB).
The Xen hypervisor requires 64MB RAM. If you don't do much work in dom0
you can set the dom0 memory fairly low. I have seen people setting it to
64 MB RAM. But remember that this is the minimum, suppose that a system
has 1024 MB RAM, and only one 512MB domU guest. dom0 can use
approximately 1024 - 512 - 64 = 448MB RAM, even if dom0-min-mem is set
to 64.
> But are there others? Since I'm doing paravirtualization, I 
> assume I don't need to turn on VT in the BIOS?
No.
>  What about 32-bit v. 64-bit OS, for the dom0 and for the domU? (I'll
be
> using CentOS-5.) Anything else?
A 64-bit dom0 with 32-bit domUs isn't currently supported in CentOS 5,
but it it will in 5.1.

-- Daniel