[MacPorts] #38766: Building atlas with clang 3.3 needs excessive memory

[MacPorts]

#38766: Building atlas with clang 3.3 needs excessive memory
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  Reporter:  bgschaid@…  |      Owner:  vince@…
      Type:  defect      |     Status:  new
  Priority:  Normal      |  Milestone:
 Component:  ports       |    Version:  2.1.3
Resolution:              |   Keywords:
      Port:  atlas       |
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Comment (by bgschaid@…):

 Replying to [comment:1 ryandesign@…]:
 > It is intentional that atlas now defaults to clang; see r104549.

 OK. But judging from the version number Clang 3.3 is still in development.
 Wouldn't it make sense to only use stable versions (Clang 3.2) as the
 default? Maybe atlas compiled with reasonable memory usage with a previous
 version of 3.3. I guess you're not testing every dependent package when a
 new version of clang 3.3 is uploaded. So that would be an extremely moving
 target

 > Yes, it's known that clang can use much more memory than gcc in some
 circumstances.
 >
 > MacPorts usually starts multiple compiler processes, and by default it
 limits this to one process per CPU core or 1 process per GB of memory,
 whichever is less. But these limits were decided upon before we started
 using clang. Perhaps we should decrease this to one process per 2 GB of
 memory when clang is in use.
 >
 > Individual ports can override this e.g. using `use_parallel_build no` to
 turn off parallel building entirely, and to my surprise, the atlas
 portfile already does this. So either the atlas build system is taking
 matters into its own hands about how many jobs to start, in which case it
 should be disabused of that notion, or a single clang process is taking
 that much memory, in which case that's very unfortunate.

 That was something I noticed: that in the beginning the build only used
 one CPU (my first thought: "somebody is feeling uneasy about CLang").
 Later the CPU-indicator was fully "filled". But its hard to tell whether
 this was accurate with the machine being almost unresponsive

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Ticket URL: <https://trac.macports.org/ticket/38766#comment:3>
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