[GHC] #14781: Incorrect CPU core counts in virtualized environments

[GHC]

#14781: Incorrect CPU core counts in virtualized environments
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           Reporter:  rtfeldman      |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Core           |           Version:  8.2.2
  Libraries                          |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Incorrect result
  Unknown/Multiple                   |  at runtime
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 To reproduce this bug:

 1. Write a program that prints `numCapabilities` (the # of CPU cores the
 program thinks are available for parallelization) to stdout
 2. Run this program in a virtualized environment such as Travis CI to see
 what it prints

 Expected output: it prints the number of CPU cores available to the
 process, which in the case of Travis's virtualized environment is 2.

 Actual output: it prints the number of *physical* cores on the machine,
 which in the case of Travis's servers is 32.

 The real-world consequences of this bug manifest for the compiler for the
 Elm programming language, which is written in Haskell and compiled with
 GHC. By default, `elm-make` runs extremely slowly on Travis and Circle CI
 because it's trying to parallelize across 32 cores when only 2 are
 actually available to it.

 For a proof-of-concept for how to correctly detect the number of available
 cores (as opposed to physical), here is the source code to a Rust library
 which does so correctly:
 https://github.com/seanmonstar/num_cpus/blob/master/src/lib.rs - the
 library distinguishes between "number of CPUs" and "number of physical
 CPUs," and on Travis it correctly reports 2 for CPUs and 32 for physical.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14781>
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