Xen Performance Monitor
-----------------------

The xenmon tools make use of the existing xen tracing feature to provide fine
grained reporting of various domain related metrics. It should be stressed that
the xenmon.py script included here is just an example of the data that may be
displayed. The xenbake demon keeps a large amount of history in a shared memory
area that may be accessed by tools such as xenmon.

For each domain, xenmon reports various metrics. One part of the display is a
group of metrics that have been accumulated over the last second, while another
part of the display shows data measured over 10 seconds. Other measurement
intervals are possible, but we have just chosen 1s and 10s as an example.


Execution Count
---------------
 o The number of times that a domain was scheduled to run (ie, dispatched) over
 the measurement interval


CPU usage
---------
 o Total time used over the measurement interval
 o Usage expressed as a percentage of the measurement interval
 o Average cpu time used during each execution of the domain


Waiting time
------------
This is how much time the domain spent waiting to run, or put another way, the
amount of time the domain spent in the "runnable" state (or on the run queue)
but not actually running. Xenmon displays:

 o Total time waiting over the measurement interval
 o Wait time expressed as a percentage of the measurement interval
 o Average waiting time for each execution of the domain

Blocked time
------------
This is how much time the domain spent blocked (or sleeping); Put another way,
the amount of time the domain spent not needing/wanting the cpu because it was
waiting for some event (ie, I/O). Xenmon reports:

 o Total time blocked over the measurement interval
 o Blocked time expressed as a percentage of the measurement interval
 o Blocked time per I/O (see I/O count below)

Allocation time
---------------
This is how much cpu time was allocated to the domain by the scheduler; This is
distinct from cpu usage since the "time slice" given to a domain is frequently
cut short for one reason or another, ie, the domain requests I/O and blocks.
Xenmon reports:

 o Average allocation time per execution (ie, time slice)
 o Min and Max allocation times

I/O Count
---------
This is a rough measure of I/O requested by the domain. The number of page
exchanges (or page "flips") between the domain and dom0 are counted. The
number of pages exchanged may not accurately reflect the number of bytes
transferred to/from a domain due to partial pages being used by the network
protocols, etc. But it does give a good sense of the magnitude of I/O being
requested by a domain. Xenmon reports:

 o Total number of page exchanges during the measurement interval
 o Average number of page exchanges per execution of the domain


Usage Notes and issues
----------------------
 - Start xenmon by simply running xenmon.py; The xenbake demon is started and
   stopped automatically by xenmon.
 - To see the various options for xenmon, run xenmon -h. Ditto for xenbaked.
 - xenmon also has an option (-n) to output log data to a file instead of the
   curses interface.
 - NDOMAINS is defined to be 32, but can be changed by recompiling xenbaked
 - Xenmon.py appears to create 1-2% cpu overhead; Part of this is just the
   overhead of the python interpreter. Part of it may be the number of trace
   records being generated. The number of trace records generated can be
   limited by setting the trace mask (with a dom0 Op), which controls which
   events cause a trace record to be emitted.
 - To exit xenmon, type 'q'
 - To cycle the display to other physical cpu's, type 'c'
 - The first time xenmon is run, it attempts to allocate xen trace buffers
   using a default size. If you wish to use a non-default value for the
   trace buffer size, run the 'setsize' program (located in tools/xentrace)
   and specify the number of memory pages as a parameter. The default is 20.
 - Not well tested with domains using more than 1 virtual cpu
 - If you create a lot of domains, or repeatedly kill a domain and restart it,
   and the domain id's get to be bigger than NDOMAINS, then xenmon behaves badly.
   This is a bug that is due to xenbaked's treatment of domain id's vs. domain
   indices in a data array. Will be fixed in a future release; Workaround:
   Increase NDOMAINS in xenbaked and rebuild.

Future Work
-----------
o RPC interface to allow external entities to programmatically access processed data
o I/O Count batching to reduce number of trace records generated

Case Study
----------
We have written a case study which demonstrates some of the usefulness of
this tool and the metrics reported. It is available at:
http://www.hpl.hp.com/techreports/2005/HPL-2005-187.html

Authors
-------
Diwaker Gupta   <diwaker.gupta@hp.com>
Rob Gardner     <rob.gardner@hp.com>
Lucy Cherkasova <lucy.cherkasova.hp.com>