[Xen-devel] Multiple priviliged domains

[John L Griffin <jlg@xxxxxxxxxx>]

During my test machine's idle time I'm running the Linux Test Project on 
multiple unprivileged Xen domains (i.e., more domains than the number of 
real processors on the machine), for general stress testing & to see 
whether any problems crop up.

While running this workload I noticed occasional horrendously slow 
interactive performance in Domain-0 (in which I was not running the LTP). 
Although I haven't yet looked in depth at the source of the slowdown, my 
hypothesis is that Domain-0 blocks handling the high I/O load generated by 
the unprivileged domains, leading to slow keyboard responses.  This brings 
up several questions:

1. What is the model for allocating processor time to Domain-0?  Based on 
my read of the Xen docs to date, I would expect it to [at least be 
intended to] have an unbounded priority share of the total processing 
resources, with some attempt at allocating unprivileged-domain-specific 
processing (e.g., handling I/O or memory allocation requests) to the 
requesting unprivileged domain.  Along these lines, should there be a 
parameterizable configuration file for Domain-0?

2. Have there been discussions about allowing multiple simultaneous 
privileged domains, among which the physical resources are split?  Or 
perhaps "semiprivileged" domains -- for example, a domain that handles all 
the I/O requests to a particular storage device, or alternatively handles 
all the I/O requests for a particular class/subset of unprivileged 
domains?  I envision a desire for a master control partition (with 
priority resource allocation) that forms the root of a hierarchical domain 
structure, under which one or more I/O partitions execute.  (I recall 
reading about this sort of design in one of the older VMM papers, or 
perhaps a recent Denali paper?)

3. I don't seem to be able to create more than 8 VBDs. [I am using 
xen-2.0.1-src.tgz.]  While trying to start 5 domains, each of which had 
two "scsi" disks (/ and swap), I discovered that the 5th domain wouldn't 
start until I removed the swap disks from both the 4th and 5th domains' 
configuration files -- i.e., not exceeding 8 VBDs total.  A cursory search 
through the code didn't reveal any relevant #define's, and I didn't see 
anything about this in the Xen docs, so before I look further I thought 
I'd ask the list if this is a known limit.  (If so, I am surprised the VBD 
structures aren't allocated dynamically to prevent this?)  A search of the 
mailing list revealed a message from Ian on 2004-03-10 stating "Mark wrote 
a pretty good readme on VBDs and put it in the tree", but I couldn't find 
the readme; is it still part of the tree?

4. On a loosely related note, what regression tests are used by the Xen 
developers?  (I.e., what should be run before generating patches?)

-- 
Dr. John Linwood Griffin
Research Staff Member, Secure Systems Department
IBM T.J. Watson Research Center, Hawthorne, New York, USA
JLG at us.ibm.com, http://www.research.ibm.com/people/j/jlg/



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