Allow limiting the number of GC threads (+RTS -qn<n>)
authorSimon Marlow <marlowsd@gmail.com>
Sat, 9 Apr 2016 19:45:50 +0000 (20:45 +0100)
committerSimon Marlow <smarlow@fb.com>
Wed, 4 May 2016 12:30:30 +0000 (05:30 -0700)
commit76ee260778991367b8dbf07ecf7afd31f826c824
treefbddddf878413dab3c01abf8108c26d2bd20db4c
parentf9d93751126e58fb990335095e02fd81a3595fde
Allow limiting the number of GC threads (+RTS -qn<n>)

This allows the GC to use fewer threads than the number of capabilities.
At each GC, we choose some of the capabilities to be "idle", which means
that the thread running on that capability (if any) will sleep for the
duration of the GC, and the other threads will do its work.  We choose
capabilities that are already idle (if any) to be the idle capabilities.

The idea is that this helps in the following situation:

* We want to use a large -N value so as to make use of hyperthreaded
  cores
* We use a large heap size, so GC is infrequent
* But we don't want to use all -N threads in the GC, because that
  thrashes the memory too much.

See docs for usage.
docs/users_guide/runtime_control.rst
includes/rts/Flags.h
rts/Capability.c
rts/Capability.h
rts/RtsFlags.c
rts/Schedule.c
rts/Task.h
rts/sm/GC.c