Re: [Haskell-cafe] Threads with high CPU usage

21 Dec 2008

      Hi Peter,

Peter Verswyvelen wrote:
...
Would forkOS instead of forkIO help in this case?
No, according to
http://www.haskell.org/ghc/docs/latest/html/libraries/base/Control-Concurren... :

"Using forkOS instead of forkIO makes no difference at all to the
scheduling behaviour of the Haskell runtime system. It is a common
misconception that you need to use forkOS instead of forkIO to avoid
blocking all the Haskell threads when making a foreign call; this isn't
the case. To allow foreign calls to be made without blocking all the
Haskell threads (with GHC), it is only necessary to use the -threaded
option when linking your program, and to make sure the foreign import is
not marked unsafe."

/Mads
...
On Sun, Dec 21, 2008 at 11:16 PM, Mads Lindstrøm
 wrote:
        Hi Günter
Günther Schmidt wrote:
        > Hi,
        >
        > in an application of mine I start a long-running operation
        in a thread via
        > forkIO so that the UI process doesn't get blocked.
        > It just so happens, that the long-running process also takes
        the CPU to
        > nearly 100% while it runs.
        >
        > During that time the run-time system does *not* switch back
        and forth
        > between the UI-process and the long-running task, it seems
        that the UI
        > process only gets woken up *after* the high CPU thread
        finishes completely.
        >
        > To the effect of course that it makes no difference at all
        to the UIs
        > responsiveness whether I use forkIO or not.
        >
        > The long running process is pretty atomic, it's a single
        query to the
        > database which takes up to a minute to complete so I don't
        see a chance to
        > squeeze a "mainIteration" in there.
It could be the database library, as it may use unsafe foreign
        calls.
        Unsafe foreign calls blocks all other threads, even if you
        compile with
        the -threaded option. See
        http://www.haskell.org/ghc/docs/latest/html/libraries/base/Control-Concurren... .
I cannot claim to know how all Haskell database libraries are
        implemented, but at least some of them use unsafe foreign
        calls. So
        which database library is you using?
>
        > What can I do?
If the problem has to do with unsafe foreign calls, then you
        can
        implement the database calls in a separate process. Not the
        easiest
        options, but I can think of no other.
>
        > Günther
        >
/Mads Lindstrøm
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