[Haskell-cafe] Using tiny (atomic) mutables between multiple threads

Bulat Ziganshin bulat.ziganshin at gmail.com
Sun Sep 13 02:58:54 EDT 2009

Hello Belka,

Sunday, September 13, 2009, 10:45:35 AM, you wrote:

i suggest you to use IORef Bool instead - as it was said once by
SimonM, it's safe to use in m/t environment, of course without all
fancy features of MVar locking

if you need to be as fast as possible, IOUArray (1,1) may be used -
this avoids boxing (array of one element is equivalent to IOURef type
lacking in std libs)

> Hello, Haskell Cafe!

> I used an MVar to signalize to many threads, when it's time to finish their
> business (I called it a LoopBreaker). Recently I realized, that it might be
> too expensive (to use MVar) for cases when threads are many and all of them
> read my LoopBreaker intensively. This assumption appeared in a case, where I
> widely (in many threads) used my stopableThreadDelay, which checks
> LoopBreaker every d = 100 milliseconds.

> So I decided that I don't really need all the great features, that MVar
> provides, and that a simpler memory usage concept might be applied here. In
> a most (machinely) reduced view, all I need is a mutable byte. It would be
> thread safe, since reading and writing are atomic operations. I then wrote a
> simple experimental module (my first experience with Ptr in Haskell):
> -----------------
> import Control.Monad
> import Foreign.Marshal.Utils
> import Foreign.Ptr
> import Foreign.Storable

> newtype MyVar a = MyVar { mvPtr :: Ptr a }

> newMyVar :: Storable a => a -> IO (MyVar a)
> newMyVar val = MyVar `liftM` new val

> readMyVar :: Storable a => (MyVar a) -> IO a
> readMyVar val = peek $ mvPtr val

> writeMyVar :: Storable a => (MyVar a) -> a -> IO ()
> writeMyVar var val = poke (mvPtr var) val
> -----------------

> Now, please, help me to answer few questions about all it:
> 1. Might readMVar really be computationally expensive under heavy load,
> (with all it's wonderful blocking features)? How much (approximately) more
> expensive, comparing to a assembler's "mov"?
> 2. Are the above readMyVar and writeMyVar really atomic? Or are they atomic
> only if I apply them to <MyVar Word8> type?
> 3. Are the above readMyVar and writeMyVar safe against asynchronous
> exceptions? Or again, only if I use <MyVar Word8> type?

> Belka

Best regards,
 Bulat                            mailto:Bulat.Ziganshin at gmail.com

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