RE: [Xen-users] Large server, Xen limitations

["Robert Dunkley" <Robert@xxxxxxxxx>]

Hi Steve,


Do you have a link or details of an example for the cygwin compiles you
use as a sort of benchmark?

I use Centos 5.2 X64 setup running Xen 3.30 and don't see any big
differences between 32 and 64bit HVM performance. Disk and network
performance is faster on Para but not much else.

We are using some Quad Quad Core Opterons (16 Cores Total) here and it
works very well. Especially for Quad CPU and above I would recommend AMD
as long as you don't need PCI passthrough. 

For AMD dual and quad setups it is important that NUMA is enabled.
Pinning cores to VCPUs seems to help performance in most cases as well,
so does disabling memory ballooning and working with fixed RAM VMs
(Although some people may not have enough ram to do this).


Rob
   
-----Original Message-----
From: xen-users-bounces@xxxxxxxxxxxxxxxxxxx
[mailto:xen-users-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of Steve
Thompson
Sent: 01 January 2009 15:09
To: morten@xxxxxxxxxxxxxx
Cc: xen-users@xxxxxxxxxxxxxxxxxxx
Subject: Re: [Xen-users] Large server, Xen limitations

On Thu, 1 Jan 2009, morten@xxxxxxxxxxxxxx wrote:

> we're contemplating getting a large new server, where we will run a 
> number of virtual servers.  Are there any things we need to keep in
mind 
> in that case?  Are there limitations on what a Xen system can manage?
>
> We're talking about a 4 x Quad core CPU server with 64 GBs of RAM and
a
> couple of terabytes of RAIDed SATA storage.

I have a 2 x quad core Dell PE2900 server with 24 GB memory and a couple

of TB of RAID-1 SATA disk running CentOS 5.2 x86_64 and xen 3.0.3, and 
currently have 33 file-based guests running on it (mostly 32-bit and 
64-bit linux, some Windows). All guests have 1 vcpu and 512 MB memory. 
This setup has been running solidly for about 6 months. Some things I
have 
noticed:

- HVM guests were a lot slower than PV guests, and there is a _lot_ of 
qemu-dm overhead on Dom0. In particular, 32-bit HVM guests were much 
slower than 64-bit HVM guests. Avoid HVM as much as possible, and if you

can't (Windows), use the Xen PV drivers. My workload consists mostly of
a 
software development environment, so I run a lot of make's (on Windows, 
under cygwin, sources and objects on Samba shares). I found that the Xen

PV drivers on Windows improved performance by over three times (reduced 
one compilation from 6 hours to 100 minutes); Windows compile
performance 
with cygwin is now about 80% of native speed.

- I converted most of my Linux guests to diskless guests with an NFS
root, 
with the Dom0 as NFS server. Not only do the formerly-HVM guests run
much 
faster, but they also run faster than file-based PV guests, and now
32-bit 
guests are a little faster than 64-bit guests (less stuff to read from 
NFS, I presume). The qemu-dm overhead on Dom0 is now essentially zero.

- With a large number of guests, you have to be more careful with their 
start/stop/start order to avoid the 'out of memory' error.

- Set xen.independent_wallclock on Linux guests if possible (Xen
kernel), 
otherwise it will be next to impossible to keep the clocks in sync.

Steve

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