[GHC] #12181: Multi-threaded code on ARM64 GHC runtime doesn't use all available cores

[GHC]

#12181: Multi-threaded code on ARM64 GHC runtime doesn't use all available cores
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        Reporter:  varosi                   |                Owner:
            Type:  bug                      |               Status:  new
        Priority:  normal                   |            Milestone:
       Component:  Runtime System           |              Version:  7.10.3
      Resolution:                           |             Keywords:
Operating System:  Unknown/Multiple         |         Architecture:  arm
 Type of failure:  Runtime performance bug  |            Test Case:
      Blocked By:                           |             Blocking:
 Related Tickets:                           |  Differential Rev(s):
       Wiki Page:                           |
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Comment (by thomie):

 varosi: you are reporting an issue with `+RTS -N`, correct?

 When compiling the following program with `-threaded`, and running it with
 `./Main +RTS -N`, it prints `2` instead of `4` on your machine:
 {{{
 import Control.Concurrent

 main = getNumCapabilities >>= print
 }}}

 Here's the code that gets the number of processors when using `+RTS -N`,
 from `rts/posix/OSThreads.c`:
 {{{#!C
 uint32_t
 getNumberOfProcessors (void)
 {
     static uint32_t nproc = 0;

     if (nproc == 0) {
 #if defined(HAVE_SYSCONF) && defined(_SC_NPROCESSORS_ONLN)
         nproc = sysconf(_SC_NPROCESSORS_ONLN);
 #elif defined(HAVE_SYSCONF) && defined(_SC_NPROCESSORS_CONF)
         nproc = sysconf(_SC_NPROCESSORS_CONF);
 #elif defined(darwin_HOST_OS)
         size_t size = sizeof(uint32_t);
         if(sysctlbyname("hw.logicalcpu",&nproc,&size,NULL,0) != 0) {
             if(sysctlbyname("hw.ncpu",&nproc,&size,NULL,0) != 0)
                 nproc = 1;
         }
 #elif defined(freebsd_HOST_OS)
         size_t size = sizeof(uint32_t);
         if(sysctlbyname("hw.ncpu",&nproc,&size,NULL,0) != 0)
             nproc = 1;
 #else
         nproc = 1;
 #endif
     }

     return nproc;
 }
 }}}

 From `man sysconf`:
 {{{
         - _SC_NPROCESSORS_CONF
               The number of processors configured.

         - _SC_NPROCESSORS_ONLN
               The number of processors currently online (available).
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12181#comment:1>
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