Array.ST is not being nice to me

[Simon Marlow]

Robert van Herk wrote:
> Hi all,
> 
> 
>  > >> So my theory now is:
>  > >> I do a large number of lookups.
>  > >
>  > > Try using Data.Array.Base.unsafeRead (and maybe
>  > > ata.Array.Base.unsafeWrite). These avoid the bounds checking on the
>  > > index each time you lookup something in the array.
>  >
>  > Right.  Also keep an eye on the GC time (+RTS -sstderr) if you're using
>  > boxed mutable arrays - we don't do card-marking, so GC is pretty
>  > sub-optimal when it comes to large mutable boxed arrays.  Decreasing the
>  > number of GCs with +RTS -A<big> can help if you're suffereing from 
> this -
>  > but always try with and without, sometimes it makes things worse by 
> making
>  > less effective use of the L2 cache.
> 
> Thanks for you advice.
> 
> I changed all the reads to unsafeReads and all the writes to 
> unsafeWrites. That indeed sped up things a bit (about 10 percent on the 
> total running time).
> 
> I also checked the GC time, but that is (only) 30% of the total running 
> time.
> 
> So still, the program with ST array is about 3 times slower than the 
> program with Data.Map.
> 
> Also, the function lookupEq that looks up the states of the equations 
> from the array, is responsible for 20% of the running time, and 19% of 
> the allocated memory. I would say that is should allocate almost no memory:
> 
> lookupEq :: Int -> MyMonad (Maybe EquationState)
> lookupEq cid =
>   do s <- get
>      mEs <- lift $ unsafeRead s cid
>      return mEs
> 
> type MyMonad a = forall s2. StateT (Eqns s2) (ST s2) a
> type Eqns s2 = STArray s2 Int (Maybe EquationState)
> 
> data EquationState = EQS {cDefinition::Equation, usedBy::Map.Map Int (), 
> ...}

Perhaps your monad is not being optimised away?  Look at the -ddump-simpl 
code for lookupEq to figure out why it is allocating memory.

Cheers,
	Simon