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Re: [Xen-users] Is this typical memory usage?

  • To: "James Pifer" <jep@xxxxxxxxxxxxxxxx>
  • From: "Jeff Lane" <sundowner225@xxxxxxxxx>
  • Date: Wed, 21 May 2008 13:54:28 -0400
  • Cc: Xen List <xen-users@xxxxxxxxxxxxxxxxxxx>
  • Delivery-date: Wed, 21 May 2008 10:55:01 -0700
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On Wed, May 21, 2008 at 9:45 AM, James Pifer <jep@xxxxxxxxxxxxxxxx> wrote:

> That would become:
>    kernel /boot/xen.gz dom0_mem=2048M
>    module /boot/vmlinuz- root=/dev/cciss/c0d0p2 vga=0x317 
> resume=/dev/cciss/c0d0p1 splash=silent showopts
>    module /boot/initrd-

I am not positive that the default for that parameter is in Megabytes,
BUT the regular mem= parameter for Linux kernels defaults to bytes if
no modifier is specified.  Given that, its good practice to explicitly
state whether you want M(egabytes), K(ilobytes) or G(gigabytes)...
that way you know for a fact what you are allocating and not just
assuming that the kernel or hypervisor knows what you mean.

> Are there any negative side affects with doing this?

Nope... the only thing that happens with this is that, at boot,
Domain0 is brought up at 2048M or 2GB.  I am pretty sure that this
will also overide the max-mem setting for domain 0, but to be sure,
you can set that as well to ensure Domain0 does not try to grow beyond
the 2GB limit you set in grub.

Now, as I said, I've seen all sorts of weirdness happen related to the
ballooning mechanism in Xen.  In some cases, I've seen where you can
start X number of guests, then you try starting X+1 and the hypervisor
spits out an error saying "Cannot allocate memory" even though you've
still got more than enough ram to share.  In others, I've seen where
the machine would simply freeze... this was a race condition where
starting a bunch of guests at the same time (using xm create from a
script within a loop, for example) would cause the system to freeze
because the guests were grabbing memory faster than Xen could take it
from Domain 0.  And there have been other things in between.  I think
the race condition got fixed, I haven't seen that one pop up in a

As for negative side effects, I've not noticed any.  I've run as many
as 80 guests on a system, all using 512 or 1024 MB each, wiht domain0
limited to either 2 or 4 GB at boot time.  Granted, I could play wiht
that a bit because I had a system with 256GB and 64 cores.  My general
rule with setting up a Xen server is Domain0 gets either 1GB (for
small systems with 16GB or less) and 2GB for > 16GB (and I do use 4GB
for the big systems (256+ GB).

I've run anywhere from 4 to 80 domains at one time in these configs
and while the guests start getting slower and more twitchy as the
number of them increases, I have found no real issues that seem to be
directly related to the amount of RAM that Domain0 uses.


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