[CMake] Parallel builds do not work correctly when using "cmake -E copy" to copy files

[Alan W. Irwin]

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.

Could we have a cmake command-line option to evaluate/diagnose the complete
list of file and target dependencies as understood by cmake? You could start
with a print out of complete target dependency chains and file dependency
chains as cmake understands them.  As part of that printout it would be
useful to highlight files or targets that are built with few dependencies
since that might be a sign of missing dependencies.  And also highlight
chains of file depends that include files that are part of other chains of
file depends. You could put in some error analysis as well (in case two
targets which do not target-depend on each other file-depend on the same
file, for example.)

Anyhow, as I went through this dependency hell for PLplot I kept wishing for
such a diagnostic tool, and I think it would be useful for others as well
that are dealing with projects like PLplot with complex dependency chains
spread over quite a few different directories.

What do you think?

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).
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Linux-powered Science
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