Re: [Xen-users] when dom0 loadavg above 1, domUs not available

[Tim Post <tim.post@xxxxxxxxxxxxxxx>]

On Mon, 2006-07-03 at 12:45 +0200, Tomasz Chmielewski wrote:
> I have a xen 3.0.2 running on a 733 MHz server, 1 GB RAM.

That's part of the problem, but should work much better than it does. 

> It has two domU domains.
> dom0 has 512 MB, domUs have 256 MB each.
> 

Have you considered shrinking dom-0 to 128 MB and not using it for
anything but managing the dom-u's? I'm guessing you're also :

1 - Using file backed VBD's
2 - Using legacy IDE/EIDE drives

Your problem is I/O congestion due to a very slow CPU and slow drives. I
wouldn't use dom-0 to do anything, make yourself a dom-u to use. 

> Whenever load average on dom0 is above 1-2 (for example, compressing 200 
> MB file), I can't reach any domU domain.
> 

See above related to I/O. You're choking the dom-u's from accessing
their VBD's by doing that on dom-0. You could try setting scheduling
appropriately and see if that makes a difference, however even if you
used a dom-u to do that kind of work, I/O is going to be a problem on
that kind of legacy system.

> Ping replies to domUs take very long time (it's on LAN), and there are 
> packet losses:
> 
> 64 bytes from 192.168.11.61: icmp_seq=7 ttl=64 time=4140 ms
> 64 bytes from 192.168.11.61: icmp_seq=8 ttl=64 time=39548 ms
> 

Yes, i'd imagine so. :) The dom-u's can't access their own file system
because the disk is backed up tarring up your 200 mb file on dom-0 - and
dom-0 isn't scheduling cpu access because its very busy waiting for your
drives to write data.

> 
> When I ping dom0, it replies immediately, there are no packet losses:
> 
> 64 bytes from 192.168.111.173: icmp_seq=1 ttl=64 time=4.07 ms
> 64 bytes from 192.168.111.173: icmp_seq=2 ttl=64 time=0.406 ms
> 
> 
> It's problematic to get to the domU with "xm console domain": it takes 
> really long to get the login prompt, and even if I get it, it times out 
> when waiting for the password.
> 

See above about choking the dom-u from access to their FS.

> There are no problems logging to dom0.
> 
> 
> I found a similar problem here, but it didn't have any conclusions:
> 
> http://lists.xensource.com/archives/html/xen-users/2005-12/msg00728.html
> 
> 
> Is it a known problem, and if so, is there a remedy for this?
> 

Use slightly better hardware and stop doing intense stuff on dom-0.

> It makes things a bit unreliable when one system makes some job, and 
> other can't be accessed because of it.
> 
> 

However, your CPU cooks eggs beautifully! That's not a bug its a
feature! <kidding>

Try re-arranging things a bit, using partitions instead of file backed
VBD's and then try fine tuning cpu scheduling and priority based on what
you want each dom-u doing. Only run services that are absolutely
necessary on dom-0, and try to avoid i/o intense workouts on dom-0
itself.

HTH
Tim



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