[Haskell-cafe] Threads with high CPU usage

Mads Lindstrøm mads_lindstroem at yahoo.dk
Sun Dec 21 17:16:22 EST 2008

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-Concurrent.html#4 .

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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