Allow programs to change the number of capabilities

[NeilMitchell]

It would be useful for a program to choose the number of capabilities it has, perhaps in response to a flag. i.e. I'd like users to type hlint -j3 rather than hlint +RTS -N3.

To support this change one option would be to allow increasing the number of capabilities after a program has started. Another option would be to provide a primitive restartWithCapabilities :: Int -> IO () that aborted the program, and restarted main but with the required settings.

Using RTS flags is unsuitable for people who use Haskell programs but aren't really Haskell developers. As more programs go multi-core, this feature will probably become more desirable. However, it's not too important and can always be solved by a little shell script.

[simonmar]

commit 92e7d6c92fdd14de424524564376d3522f2a40cc

Author: Simon Marlow <marlowsd@gmail.com>
Date:   Tue Dec 6 15:12:07 2011 +0000

    Allow the number of capabilities to be increased at runtime (#3729)
    
    At present the number of capabilities can only be *increased*, not
    decreased.  The latter presents a few more challenges!

 docs/users_guide/using.xml |   14 +++-
 includes/rts/Threads.h     |   10 +++
 rts/Capability.c           |   66 ++++++++++++-----
 rts/Capability.h           |    6 ++-
 rts/Linker.c               |    1 +
 rts/Schedule.c             |  172 ++++++++++++++++++++++++++++++++++++--------
 rts/Stats.c                |    2 +-
 rts/Task.c                 |   28 +++++++
 rts/Task.h                 |    5 ++
 rts/Trace.c                |    7 ++
 rts/Trace.h                |    1 +
 rts/eventlog/EventLog.c    |   27 ++++++--
 rts/eventlog/EventLog.h    |    1 +
 rts/sm/GC.c                |   50 ++++++++-----
 rts/sm/GC.h                |    2 +-
 rts/sm/Storage.c           |   70 +++++++++++--------
 rts/sm/Storage.h           |    4 +
 17 files changed, 358 insertions(+), 108 deletions(-)

[simonmar]

I think we now handle the use case given in this ticket, and we also have #3729 which covers the remaining issues, so I'll close this one.