Re: [Xen-devel] Custom build the Xen0 kernel && miscellany

["Mark A. Williamson" <mark.williamson@xxxxxxxxxxxx>]

> I'm new to the list.

Welcome on board :-)  That's another country to add to the list!

> Can I tell the default build system to use a modified .config file for the
> kernel build? From the looks of the makefile it appears that much more than
> patching and compiling is going on with the kernel, so if I do a make
> menuconfig and make my changes, am I able to install the kernel in the
> normal way?

As Keir said, you should put your XenLinux config files into install/boot/ and 
they will be used when you rebuild.  You could even stick your old Linux 
configs into there (with the appropriate names, to match the Xen build 
system's expectations) and they should work (though the Linux build system 
might have to confirm a few Xen-specific options interactively).

> The support I'm adding is for SMP, hypterthreading, big memory, and ATI
> framebuffer support. I'd also like to pull out various bits and pieces --
> mostly drivers for hardware I don't have, as this would speed up the build
> process.

UP guests (that includes dom0) only at the moment.  You'll use both CPUs by 
running (multiple) uniprocessor domains on each CLU. SMP guests will be a 
feature of a future version (perhaps a 3.0 release???).

This also means guests won't see the hyperthreading but instead you can run a 
domain on each hyperthread - you may want to see if this is better or worse 
for performance than disabling HT - it'll depend somewhat in your workload.

> Thank you for your time. I've been watching Xen with some interest for a
> while now and I'm thrilled that my employer has given me the go ahead to
> put together this cluster. I would like to feed the results back to the Xen
> project somewhere, especially where it pertains to performance of virtual
> web and database servers. It seems that the wiki has gone west (been dead
> for a while now), so perhaps it would be appropriate for me to submit stuff
> back to this list for archival purposes.

I update the Wiki every so often but it doesn't see that much activity at the 
moment (maybe more after the 2.0 release...).  Contributions to the user 
guide are useful, too ;-)  You may like to look at it, although it's not 
fully up to date at the moment.

> One final question (and this is bound to be unpopular). I would like to
> have some idea as to how many virtual web servers I might be able to run on
> dual Xeon 2.8GHz machines with 4-12GB RAM. Our sites are typically LAMP,
> with medium sized databases. On average, our current web servers are 900MHz
> PIII machines that don't often get pushed very hard. Perhaps just some
> anecdotal accounts would satisfy my curiosity. I realize I'm not being too
> practical here -- my apologies! ;o).

Not sure about actual numbers, I'm afraid.  Since Xen has a low overhead you 
should get a large proportion of the available power for running virtual 
machines...  You'll also get some "statistical multiplexing" benefit, since 
slack time from idle domains can be used by active ones, rather than wasted 
(as in the case of a dedicated machine).  The scheduler should ensure fair 
sharing when multiple domains are busy.

Cheers,
Mark

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