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From: Jeff Squyres (jsquyres_at_[hidden])
Date: 2007-08-15 13:50:38


I just upgraded my Cisco MPI development cluster to OFED 1.2 over the
weekend. This morning, I discovered a fun situation with regards to
uDAPL...

WHAT: We propose adding a check into the udapl configury to disable
automatically building the udapl BTL when on Linux/OFED. --with-
udapl can be specified to override the check and do the normal udapl
configury stuff.

WHY: The udapl BTL is built by default on OFED 1.2 clusters (because
the UDAPL libraries are in /lib), but the /etc/dat.conf file that
ships in OFED 1.2 is broken such that the UDAPL BTL will emit
warnings upon init.

WHERE: config/ompi_check_udapl.m4

WHEN: ASAP -- I want this for v1.2.4 because affects all OFED 1.2 users

TIMEOUT: Thursday COB (because I think Brian's out today?)

---------------------------------------------------------------

Short version:
--------------

Terry, George, and Jeff propose to add a check into
ompi_check_udapl.m4 that will disable building the udapl BTL by
default when on Linux. You can specify --with-udapl when on Linux to
force the normal check-for-headers-and-libraries udapl configure
stuff. When not on Linux (e.g., Solaris), the normal check-for-
headers-and-libraries configure stuff will always happen.

Long version:
-------------

Since OFED 1.2 [by default] installs into /usr, Open MPI's configure
script finds the header files/libraries for both verbs and uDAPL, and
therefore builds both the openib and udapl BTLs. Keep in mind that
on Linux/OFED, uDAPL is implemented as a layer on top of verbs, so it
is not the "preferred" transport to use -- we want to use verbs
(i.e., the openib BTL).

After some poking around (and checking with George/Galen), we found
that the BTL exclusivity parameter in the openib BTL is set to
MCA_BTL_EXCLUSIVITY_DEFAULT; the udapl BTL sets it to
(MCA_BTL_EXCLUSIVITY_DEFAULT-10). So that's good -- if Open MPI
loads both BTLs, it's going to effectively ignore the udapl BTL
(after initializing it) and use the openib BTL -- which is what we want.

The problem is that OFED 1.2 ships with an /etc/dat.conf that is
effectively broken (dat.conf is the text config file for DAT/DAPL).
The udapl BTL attempts to open all DAPL providers, but by the default
dat.conf in OFED 1.2, some or all of them will fail (and the UDAPL
BTL will print warnings for each failure).

On Solaris, where UDAPL *is* the high performance network, if there
are any problems with dat.conf, users will want to know -- they will
want to see the warnings from the UDAPL BTL. But on Linux, you
likely don't care about these warnings because you don't care about
UDAPL anyway (because you almost certainly want to be using the
openib/verbs BTL).

Terry, George, and I went through a bunch of different possible
scenarios to fix this dichotomy, and concluded that the one that was
the least evil was simply to disable building the udapl BTL on Linux
by default -- you can override this default by specifying --with-
udapl on the configure command line. This solution has the following
properties:

1. Most importantly, the default configure/build/run on Solaris and
Linux/OFED clusters works -- it follows the Law of Least Astonishment.

2. Avoids schitzophrenia in the UDAPL BTL trying to divine when a
user would care about the warning messages or not.

If anyone *wants* the UDAPL BTL build on Linux, they'll likely
disagree that we follow the Law of Least Astonishment, but I suspect
that that is a fairly small group of people. We'll add something to
the FAQ about this issue so that at least the solution is a simple
Google search away.

-- 
Jeff Squyres
Cisco Systems