On Tue, Mar 25, 2014 at 1:05 PM, Igor Stasenko <siguctua@gmail.com> wrote:

On 24 March 2014 22:54, 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]. 

I would not recommend you to use this method for anything.

This method heavily relies on how process scheduler works, and in case of any changes, it may break everything.

For the sake of programming, one shall never assume there is a way to "stop the world while i busy doing something".

If you reshape the world, it makes sense. I was looking at how classes were migrated, that's why I found it. 

And all of the new Pharo way of doing these things.

Hey, it is becoming really cool down there. Martin and Camille have been hard at work. Kudos!

migrateClasses: old to: new using: anInstanceModification

instanceModification := anInstanceModification.

old ifEmpty:  [ ^ self ].

[

1 to: old size do: [ :index |

self updateClass: (old at: index) to: (new at: index)].

old elementsForwardIdentityTo: new.

" Garbage collect away the zombie instances left behind in garbage memory in #updateInstancesFrom: "

" If we don't clean up this garbage, a second update would revive them with a wrong layout! "

" (newClass rather than oldClass, since they are now both newClass) "

Smalltalk garbageCollect.

] valueUnpreemptively

The global GC here is pretty unfortunate.  It is there because the VM used to leave old instances lying around.  It works like this:

we want to reshape instances of class C, e.g. by adding an inst var, and so

1. create C', which is C plus an inst var

2. create a parallel set of instances of class C', one for each instance of class C

3. for each corresponding pair of instances copy state from the instance of C to the instance of C'

4. forward-become the instances of C to the instances of C' (now no references to the instances of C remain)

5. become C to C' (now C' is the new C)

The bug is that the old instances of C are still in the heap.  Because of the become in 5. they look like instances of the new C, but are the wrong size; they lack space for the new inst var.  They're not reachable (4. replaced all references to them with references to the instances of C') but they can be resurrected through allInstances (someInstance,nextInstance) which works not by following references from the roots (Smalltalk and the activeProcess) but by scanning objects in the heap.

However, this was "fixed" in