Building Pharo from PharoKernel in 10 seconds

Hi. Last week I was playing with Pharo kernel images and I thought it could be interesting to document here what we (I am not alone!) were doing. First, the context:

Context of Pharo, PharoCore and PharoKernel

In older versions of Pharo, there were always 2 distributions: Pharo (a.k.a PharoDev) and PharoCore. The latter was a small core with just the basic stuff and very few development tools. It was perfect for deployment purposes for example. Then, on top of such core, we build PharoDev which added several packages for development: OmniBrowser, code completion, Shout, refactorings, etc, etc, etc. Since Pharo 1.4 (and now in Pharo 2.0) we now have only one image which is more similar to what we know as PharoDev. Of course the image can also be shrinked and get the core.

As said, the PharoCore was great for deployment because it was small. However, Pavel Krivanek was not satisfied, and hence started to work on “KernelImage”. The image is now known as PharoKernel. Such image contains a real small core and it is only about 2 MB. The image has to run headless. Can you imagine Seaside running in 4.6 MB? Just try Seaside on PharoKernel.

The first challenge was to be able to really shrink a Pharo image to a Kernel image. This does not happen in one afternoon. Pavel has done *a lot* of work improving the modularization of the system to reduce the dependencies between packages (to be able to correctly unload packages). So PharoKernel is the result of a HUGE work.

The second step was how to take a PharoKernel and to reload and *re initialize* everything so that it works. You know…unloading Morphic (remember PharoKernel is headless) and correctly load back and initialize it is not easy. In summary, it is a really complex process.

So far, PharoKernel was working. But there was something that we didn’t like: we still needed the Compiler in PharoKernel since to load code back we needed the source and therefore the compiler.

FuelPackageLoader arrived

So…Pavel didn’t give up and continue his efforts. Fuel is a general-purpose binary serializer but in addition, we have packages like FuelPackageLoader that let us export classes and packages in a binary way. This is what I used in the post I show you how to export and import Seaside. For more details of Fuel, FuelMetalevel and FuelPackageLoader read this link. The idea is that know we can export in a binary way. So when we are serializing a class, we serialize the class itself with the method dictionary, compiled methods, subclasses, instance variables, etc.  And to import we do not need the Compiler! we just materialize 😉   In addition, the export and import is much faster.

Wanna try it yourself?

The following are the needed steps if you want to try yourself:

1) Create a working directory where you will place all your stuff.

2) Download a Pharo 1.4 image and a PharoKernel 1.4 image and move them to your working directory.

3) Download the scripts to do this experiment. The scripts are in GIT together with all the scripts used by the Pharo Jenkins. The easiest way is to clone the repository:

git clone

4) Move the files (,, and from /XXX/pharo-build/scripts/pharo/Kernel-2.0/FuelPackageLoader to your working directory.

If you follow the steps correctly you should have something like this:

;-) ls -la
total 93112
drwxr-xr-x  13 mariano  staff       442 Apr 27 23:07 .
drwxr-xr-x@ 17 mariano  staff       578 Apr 27 23:06 ..
-rw-r--r--@  1 mariano  staff  10772559 Apr 27 20:35 Pharo-1.4.changes
-rw-r--r--   1 mariano  staff  15863104 Apr 26 12:05 Pharo-1.4.image
-rw-r--r--@  1 mariano  staff   2488141 Apr 26 12:17 PharoKernel-1.4.changes
-rw-r--r--@  1 mariano  staff   2272532 Apr 26 12:17 PharoKernel-1.4.image
-rw-r--r--@  1 mariano  staff  16235372 Oct 19  2009 PharoV10.sources
-rw-r--r--   1 mariano  staff      4023 Apr 27 20:34
-rw-r--r--   1 mariano  staff      9215 Apr 27 20:29
-rwxr-xr-x   1 mariano  staff       151 Apr 27 20:29
-rw-r--r--   1 mariano  staff      3548 Apr 27 20:29
drwxr-xr-x  14 mariano  staff       476 Apr 26 19:27 pharo-build

5) Now we take the Pharo image and we export almost all core (except what is already present in PharoKernel) using Fuel. The script first downloads Fuel. Then we use Fuel for both things: export some class variables and fonts, but also the code (packages). If you are lazy you can see the files online. So for example, these lines export fonts and a class variable:

FLSerializer serialize: (TextStyle named: 'Bitmap DejaVu Sans') toFileNamed: 'dejavu.fuel'.
FLSerializer serialize: (UCSTable classPool at: #JISX0208Table) toFileNamed: '#jisX0208Table.fuel'.

And this is how we export packages:

packageNames := 'Ring-Core-Containers
Ring-Core-Kernel' lines.

FileStream forceNewFileNamed: 'ring.fuel' do: [:aStream |
aStream binary.
FLPackageStore new storeOn: aStream packages: packageNames.].

In this example we are exporting two packages (Ring-Core-Containers and Ring-Core-Kernel) with the same stream (‘ring.fuel’ file). If you want to only export one package you can use instead the message #storeOn:packageNamed:.

So its time to take the Pharo image and export everything. To do that we need to run the image from command line and send the file as argument.

/Users/mariano/Pharo/VM/ /Users/mariano/PhD/Marea/Fuel/PharoKernelExperiments/blog/Pharo-1.4.image

6) Once we have exported, we should have several .fuel files in our working directory. The biggest file is pharo-core.fuel and it is about 5 MB. The export should have taken approx. 10 seconds (considering also the time to download Fuel).

7) As you can see in, the way to import a package with Fuel is:

FileStream readOnlyFileNamed: 'ring.fuel' do: [:aStream |
aStream binary.
FLPackageLoader new loadFrom: aStream contents readStream].

Now we take a PharoKernel image and we load the files.

/Users/mariano/Pharo/VM/ /Users/mariano/PhD/Marea/Fuel/PharoKernelExperiments/blog/PharoKernel-1.4.image

This step should also take less than 10 seconds. You can notice that now PharoKernel is not 2MB anymore but more about 13MB 😉

8) Once packages has been loaded, we have to correctly initialize the system. FuelPackageLoader has a setting to send class side #initialize or not. For most cases it works. But in other cases (like the case of PharoKernel or any Smalltalk boostrap), the initialize has to be done in a careful order. Therefore, the initialize is done manually in a script called That scripts not only sends the #initialize to classes but also performs all the necessary actions to get back a headfull (not headless) working Pharo image. So the step now is to run:

/Users/mariano/Pharo/VM/ /Users/mariano/PhD/Marea/Fuel/PharoKernelExperiments/blog/PharoKernel-1.4.image

9) Finally!!!! If everything was fine, we should now have a working Pharo image built from a PharoKernel. You can just open PharoKernel-1.4.image and give it a try 🙂


  • It is possible to have a minimal image without compiler and boostrap from there a bigger image.
  • The performance seems quite good so far (of course we still need to add lots of things)
  • FuelMetalevel (the package to serialize and materialize classes) is working really well since it could serialize and materialize almost all classes and traits from Pharo.

Known limitations

For this experiment of exporting and importing packages we are using FuePackageLoader. This is a prototype and we still have lots of missing features. In fact, that’s why Martin is now as a student in the GSoC project 😉  The current limitations are:

  • We are not exporting source code, timestamp, class comments, etc.
  • We are not doing all needed validations nor recompiling in those cases that may be necessary.
  • We are not updating instances if the classes already existed in the image.
  • and more…. (read here for more details).

So, that’s all.  I hope you had fun. See you

3 thoughts on “Building Pharo from PharoKernel in 10 seconds

    1. Indeed. What people don’t see is that it is not only about Fuel. Of course, we need to Fuel to be able to export and import packages. However, this won’t be possible if the system woudn’t have all the work in modularity and dependencies. If you see the issue tracker you will see there was/is a LOT of work made by Pavel, Guillermo, etc., to break unwanted dependencies between packages and things like that. Only after that work we are able to cleanly unload packages, boostrap or generate kernel or smaller images. And yes, those smaller images may be useful for iPhone and other mobile platforms.

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