[GHC] #8224: Excessive system time -- new IO manager problem?

[GHC]

#8224: Excessive system time -- new IO manager problem?
-------------------------------------+-------------------------------------
        Reporter:  rrnewton          |                Owner:
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:  8.2.1
       Component:  Runtime System    |              Version:  7.7
      Resolution:                    |             Keywords:  IO Manager,
                                     |  System Time
Operating System:  Linux             |         Architecture:  x86_64
 Type of failure:  Runtime           |  (amd64)
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #9221             |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by alkar):

 Here is a difference between outputs for `+RTS -s` and  `+RTS -N -qg -s`:
 https://i.imgur.com/8S5brky.png

 In my grep-like example, the first step is single-threaded (in the sense
 that I don't do any forkIO):

 > 1. Read all lines from patterns.txt into a set.

 And the pattern file can be quite large (this is why I'm doing it in a
 single process instead of multiprocessing), so the less penalty I get by
 using `-N` the better.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8224#comment:36>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler