The Boost Statechart Library - Performance

###  | # The Boost Statechart Library ## Performance

Speed versus scalability tradeoffs
Memory management customization
RTTI customization
Resource usage

## Speed versus scalability tradeoffs

Quite a bit of effort has gone into making the library fast for small simple machines and scaleable at the same time (this applies only to `state_machine<>`, there still is some room for optimizing `fifo_scheduler<>`, especially for multi-threaded builds). While I believe it should perform reasonably in most applications, the scalability does not come for free. Small, carefully handcrafted state machines will thus easily outperform equivalent Boost.Statechart machines. To get a picture of how big the gap is, I implemented a simple benchmark in the BitMachine example. The Handcrafted example is a handcrafted variant of the 1-bit-BitMachine implementing the same benchmark.

I tried to create a fair but somewhat unrealistic worst-case scenario:

- For both machines exactly one object of the only event is allocated before starting the test. This same object is then sent to the machines over and over
- The Handcrafted machine employs GOF-visitor double dispatch. The states are preallocated so that event dispatch & transition amounts to nothing more than two virtual calls and one pointer assignment

The Benchmarks - running on a 3.2GHz Intel Pentium 4 - produced the following dispatch and transition times per event:

- Handcrafted:
  - MSVC7.1: 10ns
  - GCC3.4.2: 20ns
  - Intel9.0: 20ns
- 1-bit-BitMachine with customized memory management:
  - MSVC7.1: 150ns
  - GCC3.4.2: 320ns
  - Intel9.0: 170ns

Although this is a big difference I still think it will not be noticeable in most real-world applications. No matter whether an application uses handcrafted or Boost.Statechart machines it will...

- almost never run into a situation where a state machine is swamped with as many events as in the benchmarks. A state machine will almost always spend a good deal of time waiting for events (which typically come from a human operator, from machinery or from electronic devices over often comparatively slow I/O channels). Parsers are just about the only application of FSMs where this is not the case. However, parser FSMs are usually not directly specified on the state machine level but on a higher one that is better suited for the task. Examples of such higher levels are: Boost.Regex, Boost.Spirit, XML Schemas, etc. Moreover, the nature of parsers allows for a number of optimizations that are not possible in a general-purpose FSM framework.
  Bottom line: While it is possible to implement a parser with this library, it is almost never advisable to do so because other approaches lead to better performing and more expressive code
- often run state machines in their own threads. This adds considerable locking and thread-switching overhead. Performance tests with the PingPong example, where two asynchronous state machines exchange events, gave the following times to process one event and perform the resulting in-state reaction (using the library with `boost::fast_pool_allocator<>`):
  - Single-threaded (no locking and waiting): 840ns / 840ns
  - Multi-threaded with one thread (the scheduler uses mutex locking but never has to wait for events): 6500ns / 4800ns
  - Multi-threaded with two threads (both schedulers use mutex locking and exactly one always waits for an event): 14000ns / 7000ns

  As mentioned above, there definitely is some room to improve the timings for the asynchronous machines. Moreover, these are very crude benchmarks, designed to show the overhead of locking and thread context switching. The overhead in a real-world application will typically be smaller and other operating systems can certainly do better in this area. However, I strongly believe that on most platforms the threading overhead is usually larger than the time that Boost.Statechart spends for event dispatch and transition. Handcrafted machines will inevitably have the same overhead, making raw single-threaded dispatch and transition speed much less important

- almost always allocate events with `new` and destroy them after consumption. This will add a few cycles, even if event memory management is customized
- often use state machines that employ orthogonal states and other advanced features. This forces the handcrafted machines to use a more adequate and more time-consuming book-keeping

Therefore, in real-world applications event dispatch and transition not normally constitutes a bottleneck and the relative gap between handcrafted and Boost.Statechart machines also becomes much smaller than in the worst-case scenario.

### Detailed performance data

In an effort to identify the main performance bottleneck, the example program "Performance" has been written. It measures the time that is spent to process one event in different BitMachine variants. In contrast to the BitMachine example, which uses only transitions, Performance uses a varying number of in-state reactions together with transitions. The only difference between in-state-reactions and transitions is that the former neither enter nor exit any states. Apart from that, the same amount of code needs to be run to dispatch an event and execute the resulting action.

The following diagrams show the average time the library spends to process one event, depending on the percentage of in-state reactions employed. 0% means that all triggered reactions are transitions. 100% means that all triggered reactions are in-state reactions. I draw the following conclusions from these measurements:

- The fairly linear course of the curves suggests that the measurements with a 100% in-state reaction ratio are accurate and not merely a product of optimizations in the compiler. Such optimizations might have been possible due to the fact that in the 100% case it is known at compile-time that the current state will never change
- The data points with 100% in-state reaction ratio and speed optimized RTTI show that modern compilers seem to inline the complex-looking dispatch code so aggressively that dispatch is reduced to little more than it actually is, one virtual function call followed by a linear search for a suitable reaction. For instance, in the case of the 1-bit Bitmachine, Intel9.0 seems to produce dispatch code that is equally efficient like the two virtual function calls in the Handcrafted machine
- On all compilers and in all variants the time spent in event dispatch is dwarfed by the time spent to exit the current state and enter the target state. It is worth noting that BitMachine is a flat and non-orthogonal state machine, representing a close-to-worst case. Real-world machines will often exit and enter multiple states during a transition, what further dwarfs pure dispatch time. This makes the implementation of constant-time dispatch (as requested by a few people during formal review) an undertaking with little merit. Instead, the future optimization effort will concentrate on state-entry and state-exit
- Intel9.0 seems to have problems to optimize/inline code as soon as the amount of code grows over a certain threshold. Unlike with the other two compilers, I needed to compile the tests for the 1, 2, 3 and 4-bit BitMachine into separate executables to get good performance. Even then was the performance overly bad for the 4-bit BitMachine. It was much worse when I compiled all 4 tests into the same executable. This surely looks like a bug in the compiler

#### Out of the box

The library is used as is, without any optimizations/modifications.

#### Native RTTI

The library is used with `BOOST_STATECHART_USE_NATIVE_RTTI` defined.

#### Customized memory-management

The library is used with customized memory management (`boost::fast_pool_allocator`).

### Double dispatch

At the heart of every state machine lies an implementation of double dispatch. This is due to the fact that the incoming event and the active state define exactly which reaction the state machine will produce. For each event dispatch, one virtual call is followed by a linear search for the appropriate reaction, using one RTTI comparison per reaction. The following alternatives were considered but rejected:

- Acyclic visitor: This double-dispatch variant satisfies all scalability requirements but performs badly due to costly inheritance tree cross-casts. Moreover, a state must store one v-pointer for each reaction what slows down construction and makes memory management customization inefficient. In addition, C++ RTTI must inevitably be turned on, with negative effects on executable size. Boost.Statechart originally employed acyclic visitor and was about 4 times slower than it is now (MSVC7.1 on Intel Pentium M). The dispatch speed might be better on other platforms but the other negative effects will remain
- GOF Visitor: The GOF Visitor pattern inevitably makes the whole machine depend upon all events. That is, whenever a new event is added there is no way around recompiling the whole state machine. This is contrary to the scalability requirements
- Single two-dimensional array of function pointers: To satisfy requirement 6, it should be possible to spread a single state machine over several translation units. This however means that the dispatch table must be filled at runtime and the different translation units must somehow make themselves "known", so that their part of the state machine can be added to the table. There simply is no way to do this automatically and portably. The only portable way that a state machine distributed over several translation units could employ table-based double dispatch relies on the user. The programmer(s) would somehow have to manually tie together the various pieces of the state machine. Not only does this scale badly but is also quite error-prone

## Memory management customization

Out of the box, everything (event objects, state objects, internal data, etc.) is allocated through `std::allocator< void >` (the default for the Allocator template parameter). This should be satisfactory for applications meeting the following prerequisites:

- There are no deterministic reaction time (hard real-time) requirements
- The application will never run long enough for heap fragmentation to become a problem. This is of course an issue for all long running programs not only the ones employing this library. However, it should be noted that fragmentation problems could show up earlier than with traditional FSM frameworks

Should an application not meet these prerequisites, Boost.Statechart's memory management can be customized as follows:

- By passing a model of the standard Allocator concept to the class templates that support a corresponding parameter (`event<>`, `state_machine<>`, `asynchronous_state_machine<>`, `fifo_scheduler<>`, `fifo_worker<>`). This redirects all allocations to the passed custom allocator and should satisfy the needs of just about any project
- Additionally, it is possible to separately customize state memory management by overloading `operator new()` and `operator delete()` for all state classes but this is probably only useful under rare circumstances

## RTTI customization

RTTI is used for event dispatch and `state_downcast<>()`. Currently, there are exactly two options:

1. By default, a speed-optimized internal implementation is employed
2. The library can be instructed to use native C++ RTTI instead by defining `BOOST_STATECHART_USE_NATIVE_RTTI`

There are 2 reasons to favor 2:

- When a state machine (or parts of it) are compiled into a DLL, problems could arise from the use of the internal RTTI mechanism (see the FAQ item "How can I compile a state machine into a dynamic link library (DLL)?"). Using option 2 is one way to work around such problems (on some platforms, it seems to be the only way)
- State and event objects need to store one pointer less, meaning that in the best case the memory footprint of a state machine object could shrink by 15% (an empty event is typically 30% smaller, what can be an advantage when there are bursts of events rather than a steady flow). However, on most platforms executable size grows when C++ RTTI is turned on. So, given the small per machine object savings, this only makes sense in applications where both of the following conditions hold:
  - Event dispatch will never become a bottleneck
  - There is a need to reduce the memory allocated at runtime (at the cost of a larger executable)

  Obvious candidates are embedded systems where the executable resides in ROM. Other candidates are applications running a large number of identical state machines where this measure could even reduce the overall memory footprint

## Resource usage

### Memory

On a 32-bit box, one empty active state typically needs less than 50 bytes of memory. Even very complex machines will usually have less than 20 simultaneously active states so just about every machine should run with less than one kilobyte of memory (not counting event queues). Obviously, the per-machine memory footprint is offset by whatever state-local members the user adds.

### Processor cycles

The following ranking should give a rough picture of what feature will consume how many cycles:

1. `state_cast<>()`: By far the most cycle-consuming feature. Searches linearly for a suitable state, using one `dynamic_cast` per visited state
2. State entry and exit: Profiling of the fully optimized 1-bit-BitMachine suggested that roughly 3 quarters of the total event processing time is spent destructing the exited state and constructing the entered state. Obviously, transitions where the innermost common context is "far" from the leaf states and/or with lots of orthogonal states can easily cause the destruction and construction of quite a few states leading to significant amounts of time spent for a transition
3. `state_downcast<>()`: Searches linearly for the requested state, using one virtual call and one RTTI comparison per visited state
4. Deep history: For all innermost states inside a state passing either `has_deep_history` or `has_full_history` to its state base class, a binary search through the (usually small) history map must be performed on each exit. History slot allocation is performed exactly once, at first exit
5. Shallow history: For all direct inner states of a state passing either `has_shallow_history` or `has_full_history` to its state base class, a binary search through the (usually small) history map must be performed on each exit. History slot allocation is performed exactly once, at first exit
6. Event dispatch: One virtual call followed by a linear search for a suitable reaction, using one RTTI comparison per visited reaction
7. Orthogonal states: One additional virtual call for each exited state if there is more than one active leaf state before a transition. It should also be noted that the worst-case event dispatch time is multiplied in the presence of orthogonal states. For example, if two orthogonal leaf states are added to a given state configuration, the worst-case time is tripled

Revised 03 December, 2006

Copyright © 2003-2006 Andreas Huber Dönni

Distributed under the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)