Finally, this release mostly entails maintenance work **on the model front**: a bug was fixed when using ptasks on multicore hosts, and the legacy
stochastic generator of external load has been reintroduced.
-Version 3.33 (not released yet)
--------------------------------
+Version 3.33 (never released)
+-----------------------------
+
+This version was overdue for more than 6 months, so it was skipped to not hinder our process of deprecating old code.
+
+Version 3.34 (June 26. 2023)
+----------------------------
-**On the maintainance front,** we removed the ancient MSG interface which end-of-life was scheduled for 2020, the Java bindings
+**On the maintenance front,** we removed the ancient MSG interface which end-of-life was scheduled for 2020, the Java bindings
that was MSG-only, support for native builds on Windows (WSL is now required) and support for 32 bits platforms. Keeping SimGrid
alive while adding new features require to remove old, unused stuff. The very rare users impacted by these removals are urged to
move to the new API and systems.
-We also conducted many internal refactorings to remove any occurence of "surf" and "simix". SimGrid v3.12 used a layered design
+We also conducted many internal refactorings to remove any occurrence of "surf" and "simix". SimGrid v3.12 used a layered design
where simix was providing synchronizations to actors, on top of surf which was computing the models. These features are now
provided in modules, not layers. Surf became the kernel::{lmm, resource, routing, timer, xml} modules while simix became
the kernel::{activity, actor, context} modules.
development of its replacement (the *bmf solver*) is still ongoing. However, this combination of I/Os and
communications seemed easier as these activities share the same unit (bytes).
-After a few tentatives, we opted for a simple, slightly unperfect, yet convenient way to implement such I/O streams
-at the kernel level. It doesn't require a new model, just that the default HostModels implements a new function which
-creates a classical NetworkAction, but add some I/O-related constraints to it. A couple little hacks here and there,
-and done! A single activity mixing I/Os and communications can be created whose progress is limited by the resource
-(Disk or Link) of least bandwidth value.
+After a few tentatives, we opted for a simple, slightly imperfect, yet convenient way to implement such I/O streams at the
+kernel level. It doesn't require a new model, just that the default HostModels implements a new function which creates a
+classical NetworkAction, but add some I/O-related constraints to it. A couple little hacks here and there, and done! A single
+activity mixing I/Os and communications can be created whose progress is limited by the resource (Disk or Link) of least
+bandwidth value. As a result, a new :cpp:func:`Io::streamto()` function has been added to send data between arbitrary disks or
+hosts. The user can specify a ``src_disk`` on a ``src_host`` and a ``dst_disk`` on a ``dst_host`` to stream data of a
+given ``size``. Note that disks are optional, allowing users to simulate some kind of "disk-to-memory" or "memory-to-disk" I/O
+streams. It's highly inspired by the existing :cpp:func:`Comm::sendto` that can be used to send data between arbitrary hosts.
-We also modified the Wi-Fi model so that the total capacity of a link depends on the amout of flows on that link, accordingly to
+We also modified the Wi-Fi model so that the total capacity of a link depends on the amount of flows on that link, accordingly to
the result of some ns-3 experiments. This model can be more accurate for congestioned Wi-Fi links, but its calibration is more
demanding, as shown in the `example
<https://framagit.org/simgrid/simgrid/tree/master/teshsuite/models/wifi_usage_decay/wifi_usage_decay.cpp>`_ and in the `research
We also worked on the usability of our models, by actually writing the long overdue documentation of our TCP models and by renaming
some options for clarity (old names are still accepted as aliases). A new function ``s4u::Engine::flatify_platform()`` dumps an
-XML representation that is inefficient (all zones are flatified) but easier to read (routes are explicitely defined). You should
+XML representation that is inefficient (all zones are flatified) but easier to read (routes are explicitly defined). You should
not use the output as a regular input file, but it will prove useful to double-check the your platform.
**On the interface front**, some functions were deprecated and will be removed in 4 versions, while some old deprecated functions
Expressing your application as a DAG or a workflow is even more integrated than before. We added a new tutorial on simulating
DAGs and a DAG loader for workflows using the `wfcommons formalism <https://wfcommons.org/>`_. Starting an activity is now
-properly delayed until after all its dependencies are fullfiled.
-
-The new :cpp:func:`Io::streamto()` function has been inspired by the existing ``Comm::sendto()`` function (which also derives from the
-ptask model). The user can specify a ``src_disk`` on a ``src_host`` and a ``dst_disk`` on a ``dst_host`` to stream data of a
-given ``size``. Note that disks are optional, allowing users to simulate some kind of "disk-to-memory" or "memory-to-disk" I/O
-streams.
-
-Two new useful plugins were added: The :ref:`battery plugin<plugin_battery>` can be used to create batteries that get discharged
-by the energy consumption of a given host, while the :ref:`photovoltaic plugin <plugin_photovoltaic>` can be used to create
-photovoltaic panels which energy production depends on the solar irradiance. These plugins could probably be better integrated
+properly delayed until after all its dependencies are fulfilled. We also added a notion of :ref:`Task <API_s4u_Tasks>`, a sort
+of activity that can be fired several time. It's very useful to represent complex workflows. We added a ``on_this`` variant of
+:ref:`every signal <s4u_API_signals>`, to react to the signals emitted by one object instance only. This is sometimes easier than
+reacting to every signals of a class, and then filtering on the object you want. Activity signals (veto, suspend, resume,
+completion) are now specialized by activity class. That is, callbacks registered in Exec::on_suspend_cb will not be fired for
+Comms nor Ios
+
+Three new useful plugins were added: The :ref:`battery plugin<plugin_battery>` can be used to create batteries that get discharged
+by the energy consumption of a given host, the :ref:`solar panel plugin <plugin_solar_panel>` can be used to create
+solar panels which energy production depends on the solar irradiance and the :ref:`chiller plugin <plugin_chiller>` can be used to
+create chillers and compensate the heat generated by hosts. These plugins could probably be better integrated
in the framework, but our goal is to include in SimGrid the building blocks upon which everybody would agree, while the model
elements that are more arguable are provided as plugins, in the hope that the users will carefully assess the plugins and adapt
them to their specific needs before usage. Here for example, there is several models of batteries (the one provided does not
so that the model-checker only reads the memory of the application for state equality (used for liveness checking) and for
:ref:`stateful checking <cfg=model-check/checkpoint>`. Instead, the network protocol is used to retrieve the information and the
application is simply forked to explore new execution branches. The code is now easier to read and to understand. Even better,
-the verification of safety properties is now enabled by default on every platforms since it does depend on advanced OS
+the verification of safety properties is now enabled by default on every platforms since it does not depend on advanced OS
mechanisms anymore. You can even run the verified application in valgrind in that case. On the other hand, liveness checking
still needs to be enabled at compile time if you need it. Tbh, this part of the framework is not very well maintained nowadays.
We should introduce more testing of the liveness verification at some point to fix this situation.
exploration, but this somewhat hinders the reduction quality (as we don't miss branches anymore). Some scenarios which could be
exhaustively explored earlier (with our buggy algorithm) are now too large for our (correct) exploration algorithm. But that's
not a problem because we implemented several mechanism to improve the performance of the verification. First, we implemented
-source sets in DPOR, to blacklist transitions that are redundent with previously explored ones. Then, we implemented several new
+source sets in DPOR, to blacklist transitions that are redundant with previously explored ones. Then, we implemented several new
DPOR variants. SDPOR and ODPOR are very efficient algorithms described in the paper "Source Sets: A Foundation for Optimal
Dynamic Partial Order Reduction" by Abdulla et al in 2017. We also have an experimental implementation of UPDOR, described in
the paper "Unfolding-based Partial Order Reduction" by Rodriguez et al in 2015, but it's not completely functional yet. We hope
IPC and synchronization. Check `the examples <https://framagit.org/simgrid/simgrid/tree/master/examples/sthread>`_. In addition,
sthread can now also check concurrent accesses to a given collection, loosely inspired from `this paper
<https://www.microsoft.com/en-us/research/publication/efficient-and-scalable-thread-safety-violation-detection-finding-thousands-of-concurrency-bugs-during-testing>`_.
-This feature is not very usable yet, as you have to manually annotate your code, but we hope to improve it in the near future.
+This feature is not very usable yet, as you have to manually annotate your code, but we hope to improve it in the future.
+
+Version 3.35 (November 23. 2023)
+--------------------------------
+
+**On the performance front**, we did some profiling and optimisation for this release. We saved some memory in simulation
+mixing MPI applications and S4U actors, and we greatly improved the performance of simulation exchanging many messages. We even
+introduced a new abstraction called MessageQueue and associated Mess simulated object to represent control messages in a very
+efficient way. When using MessageQueue and Mess instead of Mailboxes and Comms, information is automagically transported over
+thin air between producer and consumer in no simulated time. The implementation is much simpler, yielding much better
+performance. Indeed, this abstraction solves a scalability issue observed in the WRENCH framework, which is heavily based on
+control messages.
+
+
+**On the interface front**, we introduced a new abstraction called ActivitySets. It makes it easy to interact with a bag of
+mixed activities, waiting for the next occurring one, or for the completion of the whole set. This feature already existed in
+SimGrid, but was implemented in a crude way with vectors of activities and static functions. It is also much easier than earlier
+to mix several kinds of activities in activity sets.
+
+We introduced a new plugin called JBOD (just a bunch of disks), that proves useful to represent a sort of hosts gathering many
+disks. We also revamped the battery, photovoltaic and chiller plugins introduced in previous release to make it even easier to
+study green computing scenarios. On a similar topic, we eased the expression of vertical scaling with the :ref:`Task
+<API_s4u_Tasks>`, the repeatable activities introduced in the previous release that can be used to represent microservices
+applications.
+
+We not only added new abstractions and plugins, but also polished the existing interfaces. For example, the declaration of
+multi-zoned platforms was greatly simplified by adding methods with fewer parameters to cover the common cases, leaving the
+complete methods for the more advanced use cases (see the ChangeLog for details). Another difficulty in the earlier interface
+was related to :ref:`Mailbox::get_async()` which used to require the user to store the payload somewhere on her side. Instead,
+it is now possible to retrieve the payload from the communication object once it's over with :ref:`Comm::get_payload()`.
+
+Finally on the SMPI front, we introduced :ref:`SMPI_app_instance_join()` to wait for the completion of a started MPI instance.
+This enables further mixture of MPI codes and controlled by S4U applications and plugins. We are currently considering
+implementing some MPI4 calls, but nothing happened so far.
+
+**On the model-checking front**, the first big news is that we completely removed the liveness checker from the code base. This
+is unfortunate, but the truth is that this code was very fragile and not really tested.
+
+For the context, liveness checking is the part of model checking that can determine whether the studied system always terminates
+(absence of infinite loops, called non-progression loops in this context), or whether the system can reach a desirable state in
+a finite time (for example, when you press the button, you want the elevator to come eventually). This relies on the ability to
+detect loops in the execution, most often through the detection that this system state was already explored earlier. SimGrid
+relied on tricks and heuristics to detect such state equality by leveraging debug information meant for gdb or valgrind. It
+kinda worked, but was very fragile because neither this information nor the compilation process are meant for state equality
+evaluation. Not zeroing the memory induces many crufty bits, for example in the padding bytes of the data structures or on the
+stack. This can be solved by only comparing the relevant bits (as instructed by the debug information), but this process was
+rather slow. Detecting equality in the heap was even more hackish, as we usually don't have any debug information about the
+memory blocks retrieved from malloc(). This prevents any introspection into these blocks, which is problematic because the order
+of malloc calls will create states that are syntactically different (the blocks are not in the same location in memory) but
+semantically equivalent (the data meaning rarely depends on the block location itself). The heuristics we used here were so
+crude that I don't want to detail them here.
+
+If liveness checking were to be re-implemented nowadays, I would go for a compiler-aided approach. I would maybe use a compiler
+plugin to save the type of malloced blocks as in `Compiler-aided type tracking for correctness checking of MPI applications
+<https://scholar.google.com/citations?view_op=view_citation&citation_for_view=dRhZFBsAAAAJ:9yKSN-GCB0IC>`_ and zero the stack on
+call returns to ease introspection. We could even rely on a complete introspection mechanism such as `MetaCPP
+<https://github.com/mlomb/MetaCPP>`_ or similar. We had a great piece of code to checkpoint millions of states in a
+memory-efficient way. It was able to detect the common memory pages between states, and only save the modified ones. I guess
+that this would need to be exhumed from the git when reimplementing liveness checking in SimGrid. But I doubt that we will
+happen anytime soon, as we concentrate our scarce manpower on other parts. In particular, safety checking is about searching for
+failed assertions by inspecting each state. A counter example to a safety property is simply a state where the assertion failed
+/ the property is violated. A counter example to a liveness property is an infinite execution path that violates the property.
+In some sense, safety checking is much easier than liveness checking, but it's already very powerful to exhaustively test an
+application.
+
+In this release, we made several interesting progress on the safety side of model checking. First, we ironed out many bugs in
+the ODPOR exploration algorithm (and dependency and reversible race theorems on which it relies). ODPOR should now be usable,
+and it's much faster than our previous DPOR reduction. We also extended sthread a bit, by adding many tests from the McMini tool
+and by implementing pthread barriers and conditionals. It seems to work rather well with C codes using pthreads and with C++
+codes using the standard library. You can even use sthread on a process running in valgrind or gdb.
+
+Unfortunately, conditionals cannot be verified so far by the model checker, because our implementation of condition variables is
+still synchronous. Our reduction algorithms are optimized because we know that all transitions of our computation model are
+persistent: once a transition gets enabled, it remains so until it's actually fired. This enables to build upon previous
+computations, while everything must be recomputed in each state when previously enabled transitions can get disabled by an
+external event. Unfortunately, this requires that every activity is written in an asynchronous way. You first declare your
+intent to lock that mutex in an asynchronous manner, and then wait for the ongoing_acquisition object. Once a given acquisition
+is granted, it will always remain so even if another actor creates another acquisition on the same mutex. This is the equivalent
+of asynchronous communications that are common in MPI, but for mutex locks, barriers and semaphores. The thing that is missing
+to verify pthread_cond in sthread is that I didn't manage to write an asynchronous version of the condition variables. We have an
+almost working code lying around, but it fails for timed waits on condition variables. This will probably be part of the next
+release.
+
+Version 3.36 (TBD)
+------------------
+
.. |br| raw:: html
