Here is the better documentation I found online about it when I learnt it:

http://wiki.squeak.org/squeak/382

Afterwards... reading the vm and playing was the hard way to learn it...

Then it seems there is an ongoing chapter in the topic

https://ci.inria.fr/pharo-contribution/job/PharoForTheEnterprise/lastSuccessfulBuild/artifact/ConcurrencyBasics/ConcurrencyBasics.pier.html

On Mon, Mar 24, 2014 at 10:54 PM, phil@highoctane.be <phil@highoctane.be> wrote:

On Mon, Mar 24, 2014 at 8:23 PM, Alexandre Bergel <alexandre.bergel@me.com> wrote:

>> I am working on a memory model for expandable collection in Pharo. Currently, OrderedCollection, Dictionary and other expandable collections use a internal array to store their data. My new collection library recycle these array instead of letting the garbage collector dispose them. I simply insert the arrays in an ordered collection when an array is not necessary anymore. And I remove one when I need one.
>
> Hm, is that really going to be worth the trouble?

This technique reduces the consumption of about 15% of memory.

>> At the end, #add:  and #remove: are performed on these polls of arrays. I haven’t been able to spot any problem regarding concurrency and I made no effort in preventing them. I have a simple global collection and each call site of "OrderedCollection new” can pick an element of my global collection.
>>
>> I have the impression that I simply need to guard the access to the global poll, which is basically guarding #add:  #remove: and #includes:
>
> One of the AtomicCollections might be the right things for you?

I will have a look at it.

>> What is funny, is that I did not care at all about multi-threading and concurrency, and I have not spotted any problem so far.
>
> There isn’t any ‘multi-threading’ like in Java, you got a much more control version: cooperative on the same priority, preemptive between priorities.
> So, I am not surprised. And well, these operations are likely not to be problematic when they are racy, except when the underling data structure could get into an inconsistent state itself. The overall operations (adding/removing/searing) are racy on the application level anyway.
>
> However, much more interesting would be to know what kind of benefit do you see for such reuse?
> And especially, with Spur around the corner, will it still pay off then? Or is it an application-specific optimization?

I am exploring a new design of the collection library of Pharo. Not all the (academic) ideas will be worth porting into the mainstream of Pharo. But some of them yes.

Thanks for all your help guys! You’re great!

Cheers,
Alexandre

--
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Alexandre Bergel  http://www.bergel.eu
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An interesting method I stumbled upon which may help in understanding how these things do work. 

BlockClosure>>valueUnpreemptively

"Evaluate the receiver (block), without the possibility of preemption by higher priority processes. Use this facility VERY sparingly!"

"Think about using Block>>valueUninterruptably first, and think about using Semaphore>>critical: before that, and think about redesigning your application even before that! 

After you've done all that thinking, go right ahead and use it..."

| activeProcess oldPriority result semaphore |

activeProcess := Processor activeProcess.

oldPriority := activeProcess priority.

activeProcess priority: Processor highestPriority.

result := self ensure: [activeProcess priority: oldPriority].