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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