On 2008-02-06 21:04-0500 Brad King wrote:
Alan W. Irwin wrote:
On 2007-12-14 09:53-0800 Alan W. Irwin wrote:
On 2007-12-14 10:32-0500 Brad King wrote:
CMake employs a 2-level make recursion system that is independent of
the
directory structure. The first level never builds anything...it just
evaluates target-level dependencies with phony targets. That
determines
the order in which targets must be built. The second level is the
build.make for each target. This is where file-level dependencies are
evaluated.
In your example the file1...fileN rules are showing up in target1's
build.make and target2's build.make but they should never be evaluated
in the second target. They are pulled in through the additional_file
rule's dependencies on them (see below), but they should always be up
to
date if target2 doesn't build until after target1 finishes. Then only
the additional_file rule will be invoked. However if there is no
dependency from target2->target1 then both build.make files may be
built
simultaneously and you get race conditions causing the double
evaluations.
CMake traces through the dependencies of custom commands in each
target.
When it is constructing target2 it doesn't know that target1 will also
provide rules for the files. If you place the targets in different
directories it would not be able to make this extra connection, but
then
the build would not work correctly unless you add the target-level
dependency. Any further explanation here will just duplicate my
previous message so I'll stop.
That's fine. Your combined explanation now makes sense and completely
confirms my working hypothesis that the make recursion system of CMake
is
responsible for the parallel build issues I was encountering. I hope I
can
work around these PLplot parallel build issues (note the double copy
issue
was only the most obvious one) by using extra target dependencies. The
problem is that parallel build issues tend to appear and disappear
depending
on load, the N level (for -j N), and hardware. Thus, even if a whole
flock
of PLplot developers confirm success for parallel builds, there could be
some subtle dependency issue left that we have missed, and some user
down
the road is going to come up with a combination of load, N, and hardware
that triggers the parallel build problem because of that dependency
issue.
As a PLplot developer, I don't like being in such an uncertain position!
I thought it important to resurrect this two-month old thread because
today
I _finally_ got success (at least no obvious issues, see comment below)
with
parallel builds of PLplot on my particular platform. That's the good
news.
The bad news is it took so much effort. Plplot is not that big a piece of
software, but there are a large number of different components with
complex
dependencies between them. Therefore I had several tries in the two
months
to get parallel builds to work that failed miserably. This last
successful
effort of getting "make -J N" to work for many different N values took at
least several days of isolating the problem by enabling/disabling various
PLplot components until I was finally able to find and fix the last two
dependency issues that showed up on my system.
Even worse news is I caught the last problem only by accident. That
problem
only showed up intermittently for N = 4 for a very specific PLplot
configuration. N=2 and N=8 never showed any problems for that
configuration
for my two-processor hardware! So from that experience it is unlikely I
caught all issues.
To help to sort out such difficult dependency issues with CMake (which
affect parallel builds on Unix system and I understand also certain kinds
of
builds on Windows), I have a feature request I would like to discuss here
before I make a formal feature request on the kitware bug system.
I already made one for this:
http://public.kitware.com/Bug/view.php?id=6285
That is great that you are considering automatically putting in target
depends if two targets depend on the same file. That new feature would
address the original issue that started this thread, and I am all in favour
of this feature for that reason.
However, during my dependency hell I discovered other issues with the PLplot
depends such as missing dependencies between custom commands. Those missing
dependencies didn't matter for the non-parallel build case because the order
of the custom commands was deliberately chosen (back in our autotools days
and simply copied to our CMake build system without much thought) so that
the files were built in the correct order, but of course that doesn't happen
for parallel builds. So some sort of output that emphasizes targets or
files without many depends (which mean they are suspects for missing
dependencies) is needed as well. Bill's idea of adding file depends to the
graphviz output file would probably satisfy that need since isolated
files/targets would really stand out.
Alan
__________________________
Alan W. Irwin
Astronomical research affiliation with Department of Physics and Astronomy,
University of Victoria (astrowww.phys.uvic.ca).
Programming affiliations with the FreeEOS equation-of-state implementation
for stellar interiors (freeeos.sf.net); PLplot scientific plotting software
package (plplot.org); the libLASi project (unifont.org/lasi); the Loads of
Linux Links project (loll.sf.net); and the Linux Brochure Project
(lbproject.sf.net).
__________________________
Linux-powered Science
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