[Beowulf] What services do you run on your cluster nodes?y

Robert G. Brown rgb at phy.duke.edu
Tue Sep 23 12:39:45 EDT 2008

On Tue, 23 Sep 2008, Perry E. Metzger wrote:

> "Robert G. Brown" <rgb at phy.duke.edu> writes:
>> You can run xmlsysd as either an xinetd process or forking daemon
>> (the former is more secure, perhaps, the latter makes it stand alone
>> and keeps one from having to run xinetd:-).
> Arguably, running processes under inetd can make them more secure, not
> less, in so far as they do not need their own network listening and

As I said.

> daemon management code (reducing code size means less code to audit),
> and the processes can be run as non-root even if they need to listen
> on so-called "privileged" ports (a vile invention, but never mind, one
> has to live with its existence.) All this presumes inetd runs
> correctly, of course, which clearly is an assumption that may or may
> not be warranted.

And xinetd can be configured to use at least TCP wrappers and limits on
incoming addresses to restrict access to specific LAN or WAN addresses.
Not much by way of security, admittedly, but par for the course.

As a forking daemon you can just run it by hand from userspace and it
works just fine as it needs no special privileges to run and do its (far
more limited) job (than e.g. ganglia or bproc).  Being able to "just run
it" -- especially with a verbose debugging mode turned on -- makes
development and debugging far more "pleasant" than it might otherwise
be, and it means I can instantly monitor (linux) systems on which I
don't have root privileges without having to install packages.

The two modes share most of their code.  xinetd mode just reads/writes
to stdin/stdout/stderr instead of to/from sockets, so a single CL flag
can easily switch between the distinct setup modes.  Not really a lot of
extra code to audit, actually, and most of the socket code is pretty
much boilerplate straight out of e.g. Stevens.

>> It costs you one fork to run the initial daemon in the latter case, and
>> a fork per connection BUT the connections are persistent TCP connections
>> and hang out indefinitely.
> Actually, it need not cost a fork per connection to run a daemon under
> inetd. One can run a TCP wait service instead of the usual TCP nowait
> service. That means that the daemon still needs to know how to do
> accept, of course.

It need not, but it does...;-) Boilerplate out of Stevens (and a couple
of other references that I started with), as I said.

As also noted, there are things I'd mess with if I were to take up
working on it again because it became wildly popular or if somebody else
also adopted it as their personal hobby and joined the club of
developer/maintainers.  This is one of them.  I honestly don't know
whether my current solution scales optimally; it certainly is adequate
for the limited number of connections one creates under normal usage,
which is usually "one" but has been as high as "four or five" on the
clusters I've deployed on.  However, since the connection is persistent,
the overhead of the fork itself is irrelevant.  One fork amortized over
hours to days of socket utilization?  Not like a UDP daemon,
connectionless, constantly making and breaking.  There's a nontrivial
amount of work involved in intializing the daemon -- opening files in
/proc, creating data structures -- so constantly creating and freeing,
opening and closing is to devoutly be avoided and bundled in with the
one piece of startup "overhead" so it too becomes irrelevant.  However,
doing it all REALLY just one time might be even better, although one
would probably have to rewrite the core information parser to avoid
collisions (two requests for information on different ports before one
is complete) and so on.  Be interesting to try it, especially on a
really big cluster where one could actually see scaling problems with
the design.

The other place wulfstat is weak (not so much xmlsysd per se) is in
managing dropped connections in wulfstat because e.g. an xmlsysd host it
is monitoring crashes.  This is, of course, a common problem in parallel
application design.  With TCP it is not easy to distinguish a hung or
crashed host from an asynchronous delay, and one has to manage e.g.
TIME_WAIT states and so on that can eat into your socket budget on
retries.  One really would like to be able to tell positively if a host
is up "out of band", as well.  Alas ICMP systems/socket commands (one
easy way to see if a host is up and its network is responsive) seems to
be privileged (hence ping is suid root) and creating a monitoring tool
that was suid root seemed unwise, to me anyway.  nmap supposedly
contains a "non-ping ping" command that doesn't require privileges and
one day I'll have time to dig it out and see if I can use it in
wulfstat.  Otherwise every ATTEMPT to connect to the TCP service itself
costs one at least a timeout of some sort, which leads in turn to
blocking problems or the need to thread the application and try
reconnections in the background while the monitoring tool runs the
existing "good" connections in the foreground.

This is where xmlsysd/wulfstat is interesting.  It is an example -- a
fairly rare example -- of a master-slave parallel application outside of
HPC per se, that uses raw networking and could be adapted to doing
certain classes of hpc computation.  After all, the nodes could be doing
ANY work requested of them by the requesting client; they just happen to
be doing the work of rewinding the files that are needed to produce the
requested message, encapsulating the results in XML, and writing them to
the output socket.  Could just as easily compute a strip of the
mandelbrot set, do a random number generator test, render a graphical
frame, or any other nicely partitioned master-slave structured task.
Not as easy to code as e.g. PVM, but arguably less overhead and more if
rawer control.



Robert G. Brown                            Phone(cell): 1-919-280-8443
Duke University Physics Dept, Box 90305
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Web: http://www.phy.duke.edu/~rgb
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