[Vm-dev] [squeak-dev] re: Image Segment semantics and weakness

karl ramberg karlramberg at gmail.com
Sun Oct 26 09:30:37 UTC 2014

One aspect with Etoys projects is that they can not extend the system. It
works nicely if you just use Etoys tile scripting. But if you introduce a
new class in a project, loading the project in a system that do not have
that class will fail. So the use of project as a distribution system of
applications will be limited to a certain version of images.


On Wed, Oct 22, 2014 at 9:53 PM, Eliot Miranda <eliot.miranda at gmail.com>

> Hi Stephane, Hi All,
>     let me talk a little about the ParcPlace experience, which led to
> David Leibs' parcels, whose architecture Fuel uses.
> In the late 80's 90's Peter Deutsch write BOSS (Binary Object Storage
> System), a traditional interpretive pickling system defined by a little
> bytecoded language. Think of a bytecode as something like "What follows is
> an object definition, which is its id followed by size info followed by the
> definitions or ids of its sub-parts, including its class", or "What follows
> is the id of an already defined object".  So the loading interpreter looks
> at the next byte in the stream and that tells it what to do.  So the
> storage is a recursive definition of a graph, much like a recursive grammar
> for a programming language.
> This approach is slow (its a bytecode interpreter) and fragile (structures
> in the process of being built aren't valid yet, imagine trying to take the
> hash of a Set that is only half-way through being materialized).  But this
> architecture was very common at the time (I wrote something very similar).
> The advantage BOSS had was a clumsy hack for versioning.  One could specify
> blocks that were supplied with the version and state of older objects, and
> these blocks could effect shape change etc to bring loaded instances
> up-to-date.
> David Leibs has an epiphany as, in the early 90's, ParcPlae was trying to
> decompose the VW image (chainsaw was the code name of the VW 2.5 release).
> If one groups instances by class, one can instantiate in bulk, creating all
> the instances of a particular class in one go, followed by all the
> instances of a different class, etc.  Then the arc information (the
> pointers to objects to be stored in the loaded objects inst vars) can
> follow the instance information.  So now the file looks like header, names
> of classes that are referenced (not defined), definitions of classes,
> definitions of instances (essentially class id, count pairs), arc
> information.  And materializing means finding the classes in the image,
> creating the classes in the file, creating the instances, stitching the
> graph together, and then performing any post-load actions (rehashing
> instances, etc).
> Within months we merged with Digitalk (to form DarcPlace-Dodgytalk) and
> were introduced to TeamV's loading model which was very much like
> ImageSegments, being based on the VM's object format.  Because an
> ImageSegment also has imports (references to classes and globals taken from
> the host system, not defined in the file) performance doesn't just depend
> on loading the segment into memorty.  It also depends on how long it takes
> to search the system to find imports, etc.  In practice we found that a)
> Parcels were 4 times faster than BOSS, and b) they were no slower than
> Digitalk's image segments.  But being independent of the VM's heap format
> Parcels had BOSS's flexibility and could support shape change on load,
> something ImageSegments *cannot do*.  I went on to extend parcels with
> support for shape change, plus support for partial loading of code, but I
> won't describe that here.  Too detailed, even thought its very important.
> Mariano spent time talking with me and Fuel's basic architecture is that
> of parcels, but reimplemented to be nicer, more flexible etc.  But
> essentially Parcels and Fuel are at their core David Leibs' invention.  He
> came up with the ideas of a) grouping objects by class and b) separating
> the arcs from the nodes.
> Now, where ImageSegments are faster than Parcels is *not* loading.  Our
> experience with VW vs TeamV showed us that.  But they are faster in
> collecting the graph of objects to be included.  ImageSegments are dead
> simple.  So IMO the right architecture is to use Parcels' segregation, and
> Parcels' "abstract" format (independent of the heap object format) with
> ImageSegment's computation of the object graph.  Igor Stasenko has
> suggested providing the tracing part of ImageSegments (Dan Ingalls' cool
> invention of mark the segment root objects, then mark the heap, leaving the
> objects to be stored unmarked in the shadow of the marked segment roots) as
> a separate primitive.  Then this can be quickly partitioned by class and
> then written by Smalltalk code.
> The loader can then materialize objects using Smalltalk code, can deal
> with shape change, and not be significantly slower than image segments.
> Crucially this means that one has a portable, long-lived object storage
> format; freeing the VM to evolve its object format without breaking image
> segments with every change to the object format.
> I'd be happy to help people working on Fuel by providing that primitive
> for anyone who wants to try and reimplement the ImageSegment functonality
> (project saving, class faulting, etc) above Fuel.
> On Wed, Oct 22, 2014 at 11:56 AM, Stéphane Ducasse <
> stephane.ducasse at inria.fr> wrote:
>> What I can tell you is that instability raised by just having one single
>> pointer not in the root objects
>> pointing to an element in the segment and the implication of this pointer
>> on reloaded segments, (yes I do not want to have two objects in memory
>> after loading) makes sure that we will not use IS primitive in Pharo in any
>> future. For us this is a non feature.
>> IS was a nice trick but since having a pointer to an object is so cheap
>> and the basis of our computational model
>> so this is lead fo unpredictable side effects. We saw that when mariano
>> worked during the first year of his PhD (which is a kind of LOOM revisit).
>> Stef
> --
> best,
> Eliot
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