[boost] Re: FSM review

From: Jonathan Turkanis (technews_at_[hidden])
Date: 2005-03-21 21:02:03

Andreas Huber wrote:
> Jonathan Turkanis wrote:
>> Andreas Huber wrote:
>>> I think it is
>>> difficult to respond to statements like:
>>>
>>> <quote>
>>> Possibly the joints between parts of machines defined in separate
>>> TUs must be weaker (i.e., less known at compile time) than the
>>> connections between collections of states implemented in a single
>>> TU. </quote>
>>
>> Why should it be easy to respond?
>
> I'm not saying responding should be easy. My point was that such
> statements are not likely to bring up useful answers.

Since you apparently don't know what I was suggesting, and you didn't bother to
ask, I don't see how you can say this.

>> What I found shocking in your documentation was the statement that
>> hand-written machines can be much faster than framework-generated
>> machines.
>
> One could probably produce an equally shocking case for just about any
> non-trivial library out there. My "mistake" was to measure it and
> publish the numbers.

This is ridiculous. Consider, for example, the iterators library, or Andrei and
Dave's smart pointer library. If incrementing an iterator constructed using the
framework could take up to twenty times as long as incrementing a hand-written
iterator, or if copying a framework-generated smart pointer was twenty times as
expensive as copying a hand-written smart pointer, these libraries would be dead
in the water.

A case closer to home is the iostreams library. I conducted measurements showing
that a stringstream constructed using the library was essentially
indistinguishable from Dinkumware stringstream, and that a library-generated
fstream was slightly faster (See
http://lists.boost.org/MailArchives/boost/msg71285.php for caveats) I couldn't
honestly recommend that somebody use a library-generated socket_stream, say, if
it would involve any significant slowdown.

>> To accept a library with such poor performance (even if it
>> is adequate in many cases) requires a very convincing justification,
>> which I believe is obviously absent from the documentation.
>
> That became sufficiently apparent during the review. My hope was that
> people would understand that the provided functionality must have a
> significant overhead. I was obviously wrong and there will be better
> documentation in this direction.
>
>> As a
>> reviewer, I could have done my job simply by pointing to the absence
>> of justification, and left it at that.
>
> Fortunately, there are many other things that matter in a review.

My objection to your library was based on a deficiency in the rationale. I don't
know if this deficiency could be remedied without exposing fundamental problems
in your library. The only way to find out would be to try to fix it.

>> (This is similar to a motion
>> for summary judgment by the defense in an American trial: if the
>> plaintiff hasn't presented evidence, you just point out that fact and
>> you're done.)
>
> This seems to imply that in a review the library author is the
> plaintiff and the reviewers are the defendants.

Yes. Metaphorically, of course.

> I don't think the
> roles in a review regarding evidence are usually as clear-cut as they
> are in a trial.

Hah! You should read a few trial transcripts. But we digress.

>> All my other vague suggestions were attempts to suggest either (i)
>> how the library could have been redesigned to yield better
>> performance or (ii) how you might have documented the impossibility
>> of achieving better performance given your requirements.
>
> This is exactly something that I don't think I will ever be able to
> prove. I can only provide evidence that hints in this direction. New
> C++ techniques and idioms are discovered almost on a monthly basis and
> there's always the possibility that such a technique enables a much
> better implementation (e.g. I wouldn't have imagined that it is
> possible to enumerate overloads like Alexander does or I couldn't
> possibly have conceived the technique that makes possible the FOREACH
> macro). I think it is close to impossible for an individual to be
> proficient in all the currently known techniques, not to mention the
> ones that have yet to be discovered.

Okay, I shouldn't have said "impossibility"; maybe "impossibility, using known
methods" or "unliklihood, using known methods" would have been more accurate.
But seriously, you must know that I'm not insisting you prove that you're
library could not possibly be improved by yet-to-be-discovered-techniques.

>> I'm aware
>> they were not fully fleshed-out proposals. I was hoping you would try
>> to meet me half-way, by trying to figure out what I was getting at
>> and explaing why it wouldn't work.
>
> ... which I did in the cases where I could imagine candidate
> solutions. I the cases where I couldn't see one I said so. Since the
> latter usually didn't lead to a follow-up on your part I assumed that
> you don't have a solution either. There's no meeting half-way in such
> a situation.

This is getting very vague.

>>> Things that I
>>> *consciously* avoided or unsuccessfully tried are described in the
>>> rationale.
>>
>> If all the things you unsuccessfully tried are described in the
>> rationale, then you didn't try very much. I was giving you the
>> beneift of the doubt by assuming you had omitted describing your
>> alternate designs.
>
> What else should I have tried?

Normally, in designing a framework one considers a small handful (say two to
six) of alternate designs which view the subject matter from completely
different perspectives. In your case, what were those alternate designs?

>>> Moreover, during any programmer career a bag of knowledge
>>> is acquired that is unconsciously applied to every programming job.
>>> Naturally, this bag of knowledge is not the same for everyone
>>
>> This is obviously true.
>>
>>> which is
>>> probably the main reason why the rationale seems to be sufficient
>>> for some people while others find it unsatisfying.
>>
>> This is just funny. People who find your rationale satisfying could
>> be people who don't care about performance as long as it's good
>> enough for their application or people who failed to consider the
>> same design alternatives you failed to consider.
>
> Which alternatives did I fail to consider?

Note the subjunctive mood.

Let me make one last attempt to explain what should be in the rationale. Look at
section 16.2 of "The C++ Programming Language", 3d ed. Here Stroustrup gives two
design criteria for a containers library, and then discusses three designs:

  "Specialized containers and iterators" - satisfies the first criterion
  "Based Containers" - satisfies the second criterion
  "STL" - satifies both criteria

I'm not suggesting that you give such a detailed treatement as Stroustrup.
However, if you were to give a similar overview of the landscape of possible FSM
frameworks, and thoughtfully discuss the tradeoffs in flexibility and performace
which each involves, reviewers might have some confidence that you are aware of
the techniques which might be used to construct a fast, flexible FSM framework,
and have carefully considered the design.

This type of overview would probably be a useful component of any library's
documentation; in your case it is made absolutely essential by the performance
figures you cite.

Jonathan