[Haskell-cafe] Memory and Threads - MVars or TVars

[Job Vranish]

Atomic operations are special operations where you don't have to worry about
some other process messing with things while the operation is taking place.

For a simple example of why atomic operations are important:
(taken from: http://en.wikipedia.org/wiki/Linearizability#Non-atomic)

The naive, non-atomic implementation:

   1. reads the value in the memory location;
   2. adds one to the value;
   3. writes the new value back into the memory location.

Now, imagine two processes are running incrementing a single, shared memory
location:

   1. the first process reads the value in memory location;
   2. the first process adds one to the value;

but before it can write the new value back to the memory location it is
suspended, and the second process is allowed to run:

   1. the second process reads the value in memory location, the *same* value
   that the first process read;
   2. the second process adds one to the value;
   3. the second process writes the new value into the memory location.

The second process is suspended and the first process allowed to run again:

   1. the first process writes a now-wrong value into the memory location,
   unaware that the other process has already updated the value in the memory
   location.


Atomic operations fix this problem by preventing (STM is a little fancier
and doesn't actually _prevent_, but you can pretend that it does) any other
process from writing to the memory in question until the computation is
finished and the result is written back.


For many simple cases something like atomicModifyIORef is all you really
need. However, if you have cases where you need to make sure _multiple_
IORefs/MVars/TVars/etc.. are not written to until you're finished then you
really need something like STMs 'atomically' function. Which runs a block of
STM operations "atomically".

<http://en.wikipedia.org/wiki/Linearizability#Non-atomic>Hope that helps,

- Job

On Wed, Jul 28, 2010 at 8:23 PM, Eitan Goldshtrom <thesourceofx at gmail.com>wrote:

>  Hi everyone. I was wondering if someone could just guide me toward some
> good information, but if anyone wants to help with a personal explanation I
> welcome it. I'm trying to write a threaded program and I'm not sure how to
> manage my memory. I read up on MVars and they make a lot of sense. My real
> question is what is "atomic" and how does it apply to TVars? I don't
> understand what atomic transactions are and I can't seem to find a concise
> explanation. I also saw some stuff about TMVars? But I can't find much on
> them either. Any help would be appreciated.
>
> -Eitan
>
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